Automotive Glossary

Automotive Glossary

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The common descriptive title for paints that require the addition of a hardener (without which they will not cure).



Anti-lock braking system.


American Petroleum Institute.

A.P.I. (C)

American Petroleum Institute classification for oils suitable for diesel (compression ignition engines).

A.P.I. (S)

American Petroleum Institute classification for oils suitable for gasoline (spark ignition) engines.


Engine lubricating oil classification suitable for basic petrol and diesel engines.


Engine lubricating oil classification suitable for modern engines.


Air Conditioning

A/C - Air Conditioning

Air conditioning system that uses a refrigerant to remove heat from the air entering the passenger compartment.

A/D Converter

An electronic device used to convert analog values to digital signals for electronic control units and instruments.


Air/Fuel Ratio


Automatic transmission.


The term used when a test cannot be completed because the NT considers it unsafe to continue or because it becomes apparent during the test that certain items cannot be satisfactorily inspected. An appropriate fee may be charged for the test.


The term used when a test cannot be completed because of a problem with the VTS’s/DC’s testing equipment or the NT. No fee may be charged for the test.


Wearing away of a surface by some external force, such as sanding.


Substance used to wear away or smooth a surface by friction.


Substances used to wear away a surface by friction.


Antilock braking system.

Absolute Pressure

The pressure that exists when the forces of gravity are excluded from the actual pressure. Absolute pressure is equal to gauge pressure plus atmospheric pressure.


The penetration of a solid substance by a liquid as by capillary, osmotic, solvent, or chemical action.


Of heat: Takes in heat energy. Of sound: Takes in sound energy.


Air Bypass Solenoid Valve


Alternating Current

AC - Alternating Current

An electrical current that flows in forward and reverse bias alternately.


To speed up a process.


Increasing velocity or speed.


A substance that, when added to a paint, will speed up the rate of cure.

Accelerator Pump

Carburettor pumping system that introduces flow of fuel into the carburettor choke (venturi) on acceleration, particularly when accelerating from low speeds.


A hydraulic device that dampens the pulses and retains a residual pressure in a hydraulic circuit.


How close the data given is to the true value.


Association des Constructeurs Europeens d'Automobiles. The European organisation that sets specifications for lubricants. There are three categories for engine oils. A for gasoline/petrol engines. B for light vehicle diesel engines. C for commercial vehicle diesel engines.


A very fast evaporating solvent with high solvency for certain types of compounds and resins. Has a characteristic ether-like odour.


An inorganic or organic compound that (1) reacts with metals to yield hydrogen; (2) reacts with a base to form a salt. They are corrosive to human tissue and should be handled with care. Examples are hydrochloric acid, sulphuric acid, and nitric acid.

Acid Rain

Forms when pollution is combined with water in the atmosphere. When sulphur dioxide and nitrogen oxides mix with water molecules, they form weak sulphuric and nitric acids. As the water evaporates, the acids gain strength and can etch the finish of any vehicle upon which they have been deposited.

Acidic Compounds

Harmful chemicals that are formed during combustion and can contaminate engine oil. Sulphur in fuel can form sulphuric acid which can cause corrosion, sludging and lubricating oil defects.

Ackerman Principle

System of steering that produces wheel turning axes around a common point.


A coating based on a polymer containing short chain esters of acrylic and methacrylic acid. One-pack Acrylics were once widely used as automotive topcoats, but have generally been replaced by water-borne base colours overcoated with two-pack Acrylic clear coats

Acrylic Urethane

A coating based on urethane chemistry which also includes acrylic chemistry as part of the cross-linked polymer backbone.


Air conditioning switch.

ACT - Air Charge Temperature

Air charge temperature sensor measures the intake air temperature on fuel injection systems.


A necessary component used to provide a chemical reaction to cure paints, resins and adhesives.


A system that takes action all the time as opposed to a passive system that waits for a specific event before reacting.


Air Charge Temperature Sensor


Electronic components that receive signals from an electronic control unit (ECU) and operate to make a system work.


General term used to describe a device that allows one component to fit with another.

Adaptive Memory

A computer memory process that allows for changes in an open-loop mode.


A chemical added to improve the properties or performance of something.


Chemical substances added to paint products in relatively small amounts to impart or improve desirable properties. Examples include UV screeners, flow agents, and fish eye eliminators.


The phenomenon by which one material is attached to another by means of surface attraction.


General term used to describe a component that changes the adjustment of something (brake shoes or brake bands for example).


Guidance offered by someone.


Authorised Examiner – the organisation that operates and manages one or more VTSs and is responsible for controlling the quality of testing carried out. Except in the case of a ‘sole trader’ the AE is not a person but a legal entity (e.g. a company or partnership).


Air mixture in a hydraulic liquid that reduces the performance of a hydraulic system.

Aesthetic Value

A value put on how nice or appealing something looks.


Air Flow Control


Air Flow Meter


Air Flow Sensor


Clumps of pigment particles which have formed loose clusters. Usually undesirable in paint, since agglomerates tend to settle out and exhibit poor colour or 'seeds', which are small granular deposits, other than dust, found in paint, varnish or lacquer colour.


A paint stirrer.

Agitator Cup

Paint cup used with high metallics and pearls to keep the pigment particles in suspension by continually mixing the paint. This ensures better colour uniformity.

Air Bleed Valve

An automatic device used on fuel injection systems to provide fast idle during cold start and warm up.

Air Cap

Fitted in front of a spray gun fluid nozzle to direct compressed air against the paint to form and shape an atomised cloud of droplets.

Air Charge

Description of the amount of air taken into an engine on the induction stroke.

Air Cleaners/Filters

Canisters holding paper, plastic foam or gauze elements to trap dirt and grit.

Air Dry

The ability of a coating to dry or cure to its ultimate hardness under normal atmospheric conditions, without baking or force-drying. Measurement of time required must state conditions such as temperature and humidity.

Air Flow

Movement of a quantity of air past a point in a set time.

Air Flow Meter

Provides information to fuel injection system electronic control unit (ECU) on engine load. Measures the mass of air entering an engine.

Air Flow Sensor Plate

A plate used to sense the movement of air and therefore control fuel quantity.

Air Gap

The gap between two parts of a component, such as the spark plug electrodes, an inductive sensor or an armature and field shoes of a motor or generator.

Air Horn

The air inlet side of a carburettor or throttle body assembly.

Air Injection

An emission control device that adds air to the exhaust to add oxygen for the conversion of hydrocarbons and carbon monoxide to water and carbon dioxide. Usually fitted upstream of a catalytic converter.

Air Intake Duct

A pipe or similar used as part of the intake manifold or general system.

Air Line

A pipe that delivers compressed air.

Air Lock

A bubble of air in a hydraulic system that can be compressed and therefore reduces the performance or causes failure of the system.

Air Ratchet

A ratchet wrench driven by compressed air.

Air Resistance

Friction force caused by, for example, a vehicle moving through the air.

Air Spray

A system of applying paint in the form of tiny droplets. The paint is broken into droplets (atomised) by a spray gun as a result of being forced into a high velocity air stream. The shape and paint density of the resulting droplet cloud can be controlled by variations in air pressure, paint viscosity and gun tip geometry.

Air Temperature Sensor

Electronic device that provides information for engine management systems and frost warning indicator.

Air-fuel Ratio

The air and fuel mixture strength ratio.

Airflow Meter

Provides information to fuel injection system electronic control unit (ECU) on engine load. Measures the mass of air entering an engine.

Airflow Sensor Plate

Mechanical fuel injection component that moves in proportion to the air intake and sets fuelling accordingly.

Airless Spray

A system of applying paint in which the paint, under extremely high pressure, is passed through a nozzle and breaks up into droplets as it enters the lower pressure region outside the gun tip.


A pipe that delivers compressed air.


Assembly Line Communications Link


Assembly line communication or diagnostic link. A multisocket connector for attaching a diagnostic tool to a vehicle electronic system.

Alcoholics Anonymous

A worldwide group of men and women who meet in order to help one another stop drinking alcohol and remain sober.


Assembly Line Data Link


Bringing into line.


A chemical 'base' (loosely, the opposite of an acid). Certain types of alkalis have been used as fuel cell electrolytes.


A coating based on a polyester binder. Such polyesters are chemical combinations of molecules that contain more than one acid or alcohol group.


A name used to describe a hexagonal key.


A mixture of metals and other elements to form a metal with specific properties such as phosphor bronze used for bushes.

Alternative fuel

A fuel used in vehicles that comes from a source other than petroleum, such as ethanol made from corn and biodiesel made from vegetable oil.


An electrical generator attached to and driven by a motor vehicle engine.

Alternator Brushes

Soft carbon brushes that conduct electricity to the field windings via the slip rings.


Non-ferrous metal used as a substrate or a pigment.

Aluminium Oxide

Sharp and hard abrasive - used in dry-flatting Production papers

Ambient Pressure

Air pressure in the surrounding area.

Ambient Temperature

Temperature of the air surrounding an object.


Instrument for measuring electrical current.


A compound of nitrogen and hydrogen with the formula NH3. It is normally encountered as a gas with a characteristic pungent odour but can be included in a water based form into cleaning compounds for example.


Strength of an electrical current in amperes.

Ampere (Amp)

Electrical unit for measuring the flow of electricity in a circuit.


Discharge rate used to describe what a battery will produce. For example, a 40Ah battery will produce 2A for 20Hrs. The 20hrs is an agreed value used to allow comparisons to be made.


An increased electrical voltage and/or current compared to say the original signal.


An electronic device that increases or allows an increased electrical voltage and/or current to flow.


The maximum distance moved by a point on a vibrating body or wave measured from its equilibrium position. It is equal to one-half the length of the vibration path i.e. half the peak to peak value of a wave.


Short for Ampere (unit of electrical current)


Used to describe a sealant that dries when air or oxygen is excluded.


A measuring process that uses a variable electrical current to provide a signal for computers and instruments.

Analog Gauge

An instrument readout that displays using a moving needle or similar method.


A measuring process that uses a variable electrical current to provide a signal for computers and instruments.

Analogue Gauge

An instrument readout that displays using a moving needle or similar method.

Anionic Electrodeposition

One of the electro-coating methods in which the body is charged positively and completely immersed in the negatively charged paint. Frequently used for OEM primer application, the resultant coating is known as 'E-coat'

Annual Income

Amount of money you earn each year.


The outer gear with internal teeth on an epicyclic gear train.


A positive connection.

Anodised Aluminium

A surface treatment for aluminium that adds an oxidised coating to harden the surface of the component.


An electrolytic surface treatment for aluminium which builds up an aluminium oxide coating, to provide better adhesion.


Protective coating applied on metal surfaces to prevent corrosion.


A chemical such as ethylene glycol, added to the cooling water of an engine to lower the freezing point for winter operation.


Usually used to refer to an additive that is added to fuel, which makes the burning process more controllable, resulting in reduced tendency to knock.

Anti-roll Bars

Suspension component that reduces vehicle roll.

Anti-skinning Agents

Chemicals added to a paint to help prevent the formation of a surface film during storage.

Anti-trap Function

Facility to prevent electric windows closing and trapping something - your fingers for example! See also Bounce Back.


Additive put in the cooling system to prevent freezing.


A metal, which is added to lead, to improve its mechanical strength. Batteries were traditionally constructed with lead-antimony.

Antivirus Software

A software package that prevents computer viruses from damaging or destroying the system.


Another name for a computer program such as Microsoft Word.

Applied Solids

Solids that remain on the substrate being coated or painted.


Absolute Pressure Sensor

APS - Absolute Pressure Sensor

A barometric pressure sensor used to compare atmospheric and manifold pressures in electric fuel injection systems.


The effect when a vehicle's tyres tend to skid on top of water on the road.


Describes a water-based solution or suspension (see 'waterborne').


Conduction of electricity across a gap. An arc is caused as contacts open.


Length x width


Electrical windings around the core fixed to the spindle of a motor or dynamo.


The smell of something such as perfume or worse...


A type of solvent based on benzene ring molecules.


Fibrous silicate material that can be woven into an incombustible fabric. Requires special handling as it is a hazardous substance.


Airbag System Diagnostic Module

Aspect Ratio

Ratio of two axes at right angles to each other such as the long and short sides of a rectangle. When applied to low profile tyres it refers to the width and depth and is shown as a percentage value in the tyre size marking.


Breathing or induction process of an engine. Non-turbocharged engines may be described as naturally aspirated.


The teacher or tutor, who assesses the portfolio of evidence.


A description of objects that are different on each side of their centre line.


After Top Dead Centre

ATDC - After Top Dead Center

ATDC - Before Top Dead Centre.

ATDC - After Top Dead Centre

The position of the crankshaft when the piston has passed the top of the cylinder and is on the way down.


Automatic Transmission Fluid.


Automated Test Lane – These are lanes authorised by VOSA of use in the automated mode.


The air that surrounds the Earth's surface.

Atmospheric Pressure

Pressure in the atmosphere produced as a result of the Earth's gravity. Measured at sea level it has a value of 1 Bar, 14.7 psi or 100 kPa.


Smallest particle in a chemical element that can take part in a chemical reaction. A single particle of an element.


Relating to the atom...

Atomic clock

This is a type of clock that uses an atomic resonance frequency standard as its timekeeping element. It is very accurate.


The formation of tiny droplets of liquid as in the paint spraying process. Atomisation is usually achieved by turbulence in an air stream, or a sudden drop in pressure.


The stage in which the metered air/fuel emulsion is drawn in to the airstream in the form of tiny droplets.


Breaking up of fuel into fine particles during induction into an engine.


The stage in which the metered air/fuel emulsion is drawn in to the airstream in the form of tiny droplets.


Air Temperature Sensor


Of or relating to the ear or to the sense of hearing - aural diagnostics mean listening for faults such as a bearing noise or a misfire


See Automatic Transmission

Automatic Adjusters

Brake components that automatically take up wear to keep brake shoes or pads close to the drum or rotor.

Automatic Choke

A carburettor component using a bimetallic strip that operates at low engine temperatures to improve cold start and idle performance. Operates independently of the driver.

Automatic Transmission

A vehicle gearbox with automatic gear shifts based on engine speed and load conditions. Does not require individual gear shifts by the driver.

Auxiliary Air Flow

Used to describe a flow of air that bypasses the normal route. It is used to control idle speed or idle mixture.

Auxiliary Air Valve

A valve used to control auxiliary air flow.

Auxiliary Components

Components that are outside a specific system. For example, when working on the engine, an alternator would be described as auxiliary.

Auxiliary Lights

Extra, non-essential lights such as front spotlights or driving lights.

Auxiliary shafts

Shafts that are used on some engines for balance or to drive other components.

Avalanche Diode

A diode that conducts when a value voltage is applied. Used to protect circuits from overload.


The sum divided by the number of items.

Awarding body

There are several awarding bodies City and Guilds: AQA, Edexcel and OCR for example.


Transmission system that uses all four wheels of a vehicle to provide a drive. This term is usually used on vehicles with a permanent four wheel drive that transfers drive to another axle when some loss of drive occurs on the main drive axle. The term 4WD or 4x4 is used for used for vehicles with selectable four wheel drive.

AWD - All Wheel Drive

Transmission system that uses all four wheels of a vehicle to provide a drive. This term is usually used on vehicles with a permanent four wheel drive that transfers drive to another axle when some loss of drive occurs on the main drive axle. The term 4WD or 4x4 is used for vehicles with selectable four wheel drive.

Axial Loading

Along the axis of a shaft.

Axial Motion

Movement along or parallel to the axis of a shaft.


The centre line through a shaft or axle. The centre about which an object rotates or where the object is in equilibrium.


Horizontal transverse shaft or beam with spindles on which road wheels are mounted.

Axle Loading

Term used to describe the forces acting on an axle.

Axle Mounting

Method of fixing the axle to the body or springs.

Axle Stands

Vehicle support equipment manufactured with a rated weight capacity. Used to support a vehicle when wheels are removed or when access to the underside is needed. Also known as jack stands.

Axle Windup

A condition in an axle where the driven component locks and the axle casing attempts to rotate on its mountings. Usually caused when a differential lock or limited slip device is used and wheel spin is not possible such as on a dry road.



Body CAN or Basic CAN


British Association old standard size of threads."

B.S. Kite Mark

British Standards Kite Mark. The symbol of the British Standards Institute consisting of a triangle on its point with a capital B on its side above the triangle and a capital S inside the triangle.


British Standard Fine size of thread."


Battery Positive Voltage

Babbitt Metal

An alloy of tin, copper, antimony and lead used as a coating for plain steel bearings.

Back EMF

A voltage produced that tends to oppose the voltage that created it.

Back Plate

(1) Non-rotating plate carrying the shoes of a drum brake. (2) Metal member carrying friction material of the disc brake pad.

Back Pressure

The pressure build-up in the exhaust caused by the resistance to exhaust flow by pipe diameters and muffler baffles. Used in design to balance an exhaust to an engine.


Technique of sanding a surface to taper the paint film away from the metal repaired area.


Combustion of fuel in the intake or exhaust resulting in a loud explosive noise.


The base material of flatting paper onto which abrasive grains are coated. For masking tape, the backing is the paper section without the adhesive.

Backing Pad

A supporting pad for abrasive disc and/or polishing pads that is attached to a polishing mop or a DA sander. In the compounding, glazing or polishing operation, a foam or wool polishing or glazing pad is attached on top of the backup pad. In dry sanding operations an abrasive disc is attached to the face of the pad.


The clearance between two components, usually used to measure the running clearance of gear teeth.


A second copy of work in case the original is damaged or destroyed. Should be stored away from the computer.


A part fitted behind the air cap in a spray gun to direct the air stream and reduce air turbulence.


Application of heat to cure and dry a coating. In automotive refinishing, baking is used to speed up the drying of air-drying finishes and is sometimes called force-drying. The metal temperature in refinish baking usually does not exceed 80°C (180°F).

Balance Shaft

A rotating shaft geared to another shaft, usually the crankshaft that carries counterbalance weights to bring the shafts into harmonic balance.

Balance Valve

A hydraulic device that controls the pressure in different parts of a hydraulic circuit. Used to balance front and rear brake actuation.

Balance Weights

Small weights used to balance a shaft or a wheel.


Additive used to balance colour formulae in proportion to size and pigment-to-binder ratio.

Balk Ring

A component of a synchromesh gear hub that provides a cone clutch and guidance of the outer hub to the gear dog teeth.

Ball Bearing

A bearing consisting of inner and outer tracks with a series of ball bearings in between. A single surface hardened steel ball.

Ball Flange

Flexible exhaust connection.

Ball Joints

Joints that allow movement for suspension and steering.

Ball Pivot

A pivot that allows movement in several directions.

Ball Valve

A simple valve that is closed off as a ball moves.

Ballast Resistor

Electrical resistor used in low tension circuit to regulate ignition coil output when the engine is running and is by-passed during starting to give an increased spark voltage for cold starting.


The maximum amount of information (bits/second) that can be transmitted along a channel.

Banjo Joint

A hydraulic joint consisting of a hollow through bolt and a hose connector that looks like a banjo and from which it gets its name.


Measurement of pressure - one atmosphere.


Barometric Pressure

BARO (Barometric Pressure)

Air or gas pressure measured in a standard unit. (Pascal, psi, bar)

Barrier Cream

Used to help prevent skin damage when working, for example, with oils.


Terminal of a transistor that becomes conductive to the emitter when energised. The trigger terminal of transistors and thyristors.


A colour coat requiring final protection in the form of a sprayed clearcoat. The basecoat provides colour effects and appearance, while the clearcoat provides gloss as well as UV and chemical resistance.

Basic skills

Reading, writing, speaking in English (or Welsh) and using numbers sufficiently well to be able to function in society and at work. Key skills and basic skills overlap at levels 1 and 2.



Bath tub

An old type of combustion chamber looking like a bath tub when the cylinder head is turned over.


A chemical device that will give out an electrical current when connected into a circuit.

Battery Carrier

A tray to hold the battery, usually under the bonnet.

Battery Grease

Petroleum jelly used on battery terminals.

Battery module

A generic term meaning a collection of battery cells and control systems. Often relates to EVs or HEVs.

Battery Posts

The main connections on a battery.

Battery terminal

See Battery Post

Battery Volts

A term used to describe the voltage of the particular system. This is often 12V - but could be 14V when the engine is running.


Body Control Module


Bottom dead centre. The position of the crankshaft when the piston is at its lowest position in the cylinder. The crankshaft is at 180° of rotation from the top dead centre 0° position.


Part of the tyre.

Beam Axle

Type of non-independent suspension.

Beam Setter

Equipment used to check the alignment of vehicle headlights.

Bearing Cap

Half-round parts that holds the big end and main bearings into place either on the con rods or engine block.

Bearing Clearance

The freeplay between a bearing shell and journal.

Bearing Nip or Crush

Applies to the applied grip of a split shell bearing when the two halves are tightened together.


Components used to support a shaft and allow rotation.

Bell Housing

Cover over the clutch assembly. Usually joins the gearbox to the engine.


A benign condition is one that is not usually serious or harmful. Zero emissions for example.


Hydrocarbon fuel additive and octane improver.

Bevel Gears

Gears cut at an angle from the periphery so that they can transmit a drive through an angle. An example is the final drive crown wheel and pinion.


Brake Horse Power

Bi-Metal Spring

A spring made of two metals that expand differently with temperature change. This changes the tension as temperature changes.


Towards one side or one polarity.


Favouring one thing over another.

Big End

End of the con rod that fits to the crankshaft.

Big End Bearings

The journals and shell bearings on a crankshaft that carry the big ends of the connecting rods.

Bimetal Strip

Two metals bonded together so that when heated, the different rates of expansion cause the strip to bend. The amount of bend is proportional to the temperature.

Bimetal Thermostatic Spring

A spring that changes tension with temperature.

Bimetalic Valve

See Bi-metalic Valve.

Bimetallic Valve

A valve that operates when a temperature change moves a bimetal strip.


A system of numbering using 0 and 1 and used in computer operating systems.

Binary Counter

An electronic device that counts in binary (1s and 0s) by using logic gates and basic memory devices.

Binary digit

A binary digit or bit is the basic unit of information in computing and telecommunications; it is the amount of information that can be stored by a device or other physical system that can normally exist in only two distinct states - 1 or 0, on or off for example.


The component in a paint material that forms the film. So-called because it binds the pigment and any additives present into a solid durable film.


Term used to describe one component sticking or rubbing against another.


The capacity of a material to decompose over time as a result of biological activity, especially to be broken down by microorganisms.


A binary digit or bit is the basic unit of information in computing and telecommunications; it is the amount of information that can be stored by a device or other physical system that can normally exist in only two distinct states - 1 or 0, on or off for example.

Black box

Equipment that records information about the performance of an aircraft during flight – or a vehicle during its normal use.

Blade Connector

An electrical cable connector consisting of a blade and spring sided socket.

Blanking Plugs

Screwed plugs used to blank off or block off a test port or adjustment point.


The act of removing air from a liquid system.

Bleed Nipple

A small nipple valve that allows connection of a pipe and is operated by loosening.


A defect in which pigment from a lower coat of paint diffuses into an upper coat and discolours the latter. A non-bleeding colour is one that is not soluble in materials used over it and consequently does not work up into succeeding layers. Body filler may also cause bleeding if an excessive amount of hardener is added.


Repainting of panels or parts of panels and fading out the new colour onto adjacent prepared panel work to render the repair invisible. Normally only carried out on basecoat-and- clear finishes, as it is very difficult to blend a direct gloss paint finish.

Blending Flaps

Flaps used to mix hot/cold and fresh/recirculated air in a heater system.

Blipped Throttle

Description of a 'quick press' of the throttle to make the engine rev up.


The development of hollow bubbles or water droplets in a paint film. Blistering is usually caused by the expansion of air or moisture trapped beneath the film. It can occur rapidly or any time after application as the paint film ages. Blistering may also occur from the presence of unreacted acid within the paint film.

Block diagram

A diagram of a system, in which the main parts or functions are represented by blocks connected by lines, which show the relationships between the blocks.

Blocker Bar

General term used to describe a bar that prevents an action of some kind.


A powder-like deposit which sometimes forms on the surface of a paint. It is often the result of a solvent component partly dissolving the pigment(s) and then depositing them on the surface when the temperature is below the 'dew point',( i.e. the temperature at which moisture will condense on the surface)

Blow Back

Ignition of fuel in the inlet manifold causing a flame to rise out of the air intake through the carburettor.

Blower Motor

A motor that uses a fan to assist in moving air.


Wireless Technology. A chip is responsible for the transmission of data between a wide range of devices (mobile phone and hands-free system) through short range digital two-way radio.


Trade name for 'blue' headlight bulbs produced by Philips.


The appearance of whitish or "cloudy" areas in a paint finish, caused by absorption and retention of moisture in a drying paint film.


Barometric/Manifold Absolute Pressure Sensor

Bodied Vehicles

A bodied vehicle is defined as ‘a vehicle with a structure consisting/comprising of a floor pan and panels which, with a roof fully encloses the vehicle occupants and without a roof encloses them other than where the roof would be’. As a guide, if the driver sits in the vehicle with surrounding structure it would be classed as bodied.


Main portion of motor car.

Body Filler

A heavy-bodied 2K plastic filler material which cures very hard and is used to fill small dents in metal and plastic panel work.


Thickening of paint in the can, usually due to evaporation of solvents or volatile material because of poor fitting lids or exposure to heat or air.


Attraction between two things such as to cause them to stick together.

Bonding Cables

Cables or wires used to connect different panels together.

Bonding Resin

Adhesive often used on friction linings.


Brake on/off switch

Boost Pressure

The amount of increase above atmospheric pressure provided by a supercharger.

Booster Fan

See blower motor.


Usually made of rubber, a cover for a moving component such as a track rod end.


A rubber (or similar) boot used to cover working components. It is used to keep dirt out and in some cases lubrication in place. Also slang for alligator...

Bore (cylinder)

The internal part of a hollow cylinder in which a piston is located.

Boring Bar

Used to cut cylinder bores.

Boron steel

Boron steels possess hardenability equivalent to that of much higher carbon steels and more expensive low alloy heat treatable steels. Tempering toughens boron steels. The addition of only 0.001-0.003% soluble boron to a suitably protected base steel can produce an increased hardenability compared other steels.


Robert Bosch GmbH is a technology-based corporation which was founded by Robert Bosch in Stuttgart, Germany in 1886.


A reinforcing section around a cylindrical bore in a component.

Bottom Dead Centre

The position of the crankshaft when the piston is at its lowest position in the cylinder.


Vehicle body suspension movement when hitting bumps.

Bounce Test

A manual test of suspension dampers (shock absorbers) by pushing down on the suspension and observing the oscillations when released.


Movement of an electric window as it hits an obstruction - and bounces back (opens again).

Boundary lubrication

This form of lubrication is essentially a breakdown of hydrodynamic action. At high loads or low speeds, the pressure of the oil film cannot prevent metal-to-metal contact. So the opposing surfaces partially ride on an oil film and partially rub together as surface high points come in contact. Lubrication is provided by lubricant decomposition products or surface-active additives which form a thin, soft, solid film on the metal surfaces and prevent metal-junction adhesion.

Bourdon Tube

A U shaped tube used to measure vacuum.

Box Sections

Parts of the vehicle body - chassis and structural members.

Boxer Engine

A horizontally opposed engine.

BP Sensor

A barometric pressure sensor.


Back pressure Sensor


Back Pressure Transducer


(1) Device to retard the motion of a vehicle or to prevent inadvertent movement when parked. (2) Dynamometer for measuring the power of an engine, originally a friction brake by which torque could be measured. Also brake dynamometer (informal).

Brake Accumulator

A pressure container fitted into pumped hydraulic systems to maintain a reserve of pressure if the pump fails. They also smooth out the pump pulses in the system.

Brake Adjuster

A mechanical device for taking up the free play in brake components. Can be either manually or automatically operated.

Brake Balance

The splitting of brake effort to each axle in relation to the optimum that the axle can provide.

Brake Band

A friction material lined strap around a epicyclic gear annulus. Operated by hydraulic pressure to hold the annulus for gear ratio selection.

Brake Booster

See brake servo.

Brake Caliper

The fixed component of a disc brake holding the pistons that apply an inward force to press the brake pads onto the rotor.

Brake Cylinder

The wheel cylinders of hydraulic brakes in the brake unit.

Brake Drum

Rotating drum inside which brake shoes operate.

Brake Fade

Reduction of braking effort resulting from overheating of friction surfaces, or other transient effect.

Brake Fluid

A special liquid used in hydraulic brake systems. Always check data because some types should not be mixed.

Brake Horsepower (bhp)

A measure of the power output from an engine when measured with a dynamometer using a brake applied to the engine flywheel. The engine effort to overcome the braking force gives the bhp value.

Brake Lines

The hydraulic pipes and hoses of the brake system.

Brake Lining

The friction material attached to brake shoes and pads.

Brake Linkage

A general term to describe components that link different parts of the brake system together.

Brake Pad

A steel plate with a bonded friction lining used on disc brakes. Similar to the brake shoe of a drum brake.

Brake Rollers

Rollers used to drive the wheels of a vehicle for brake testing purposes.

Brake Servo

A mechanism (vacuum or hydraulic) that increases the braking effort applied by the driver.

Brake Shoes

Part of drum brake carrying linings.

Braking Effort

The force applied to the brakes.

Braking Thrust

Forces that are caused by braking.

Breakout Box

A terminal box for carrying out diagnostic tasks on electrical and electronic systems.

Bridge Rectifier

A type of rectifier using four or more diodes to give full wave rectification.


A characteristic of undercoat performance that occurs when a scratch or surface imperfection is not completely filled, leaving a cavity underneath. It i caused by under-reducing the primer or using too fast a solvent and can lead to the appearance of sand scratches in the finished repair.


The formation of a metallic-appearing haze on a paint film.

Brush End Plate

A plate used to hold brushes in contact with a commutator.


Pieces of carbon or metal ending in wires or strips. Contacts onto rotating slip-rings or generator/motor commutators.


Before Top Dead Centre

BTDC - Before Top Dead Center

BTDC - Before Top Dead Centre.

BTDC - Before Top Dead centre

The position of the crankshaft before the piston has passed the top of the cylinder and is still on the way up.


British Thermal Units


A technique used to polish an area to remove sanding marks or surface imperfections (see Polishing).

Buffing Compound

Soft paste containing various grades of fine abrasives suspended in a neutral medium, used to eliminate fine scratches and polish the topcoat.


The amount of paint film deposited on a substrate (the depth or thickness of which is measured in microns).


Substantial structural panel of vehicle body in front of passenger compartment. Also known as a firewall.

Bump Cap

A hard hat.

Bump Steering

Steering action that results from hitting a bump.

Bump Stop

A rubber spring that limits and dampens the movement of a spring to prevent excessive deflection and spring damage.


The ability to float in liquid or in air.


Term used to describe rough edges on metal components.


A motor vehicle which is constructed or adapted to carry more than eight seated passengers (see also Minibus).


Cylindrical sleeve forming a bearing surface for a shaft or pin.

Bushes (rubber)

A metal and rubber composite bearing that provide both a limited range of movement and insulation from road vibration.

Buss Bar

A conductor in a fuse box or other electrical distribution device.


Synthetic rubber impervious to oxygen.

Butyl Acetate

A medium-evaporating solvent used in paint.


A secondary route in a system that allows gas, liquid or electricity to flow outside of the main route.

Bypass Valve

In a lubrication system a valve that allows oil to bypass a filter.




C Classification (SAE)

Engine oil suitable for diesel engines. The second letter in the classification denotes the quality and minimum conditions that the oil must meet.


Cold filter plugging point. Applies to diesel fuel with a cold weather additive."


Computer Controlled Coil Ignition

Cadence braking

An emergency braking procedure using rhythmic application and release of the brakes to strike a compromise between slowing down with locked wheels, so retaining the ability to steer.


A metal used in the manufacture of durable but expensive red and yellow pigments.


A soft grey metallic element used a component in driers and pigments.

Calcium Carbonate

An extender pigment, also known as "whiting."


The act of making sure a piece of measuring equipment is accurate. Often this is done by checking it against equipment that is known to be good.


This means that a piece of measuring equipment is accurate. Often this is done by checking it against equipment that is known to be good.

Calibrated Aperture

An accurate size hole to control flow.

Calibrated Float

The float used in a hydrometer. It is designed to work accurately (calibrated) usually at 15 degrees Celsius.

Calibration Resistor

A variable resistance that is used to set the basic position of a circuit.


In a disc brake system, the mechanism that brings brake pads to bear on the disc by a clamping or pinching action, similar to that of the jaws of a Vernier caliper.

Caliper Piston

A slave cylinder in a brake caliper.


Calibration Pack


(1) A shaped or profiled component that produces linear or angular motion or lift of a follower. Except in the steering box, most automotive cams are in the form of non-circular wheels which impart motion to the follower by their rotation. (2) A camshaft (informal). Misleading usage, as for instance twin cam to describe an engine with two camshafts.

Cam Belt

Belt, usually made of reinforced rubber, which drives the cam from the crank.

Cam Linkage

A cam shaped (egg shaped) part.

Cam Lobe

High point of a cam which operates components.

Cam Plate

A plate with one or more raised areas in the shape of a cam. Used in rotary diesel injection pumps to supply high pressure injection.

Cam Ring

A shaped ring with a cam shape.


(1) Convex arched curvature of a (usually horizontal) surface. (2) Average curvature of the chord wise section of an aerofoil. (3) Mildly arched profile of a road or pavement. (4) Inclination of the plane of a wheel to the vertical plane of symmetry of a vehicle. Camber is considered positive if the wheel leans out towards the top, and negative if it slopes inward.


Shaft on which suitably phased cams are mounted, as for example to operate intake and exhaust valves of an engine. In four-stroke engines the camshaft rotates at half crankshaft speed.


Controller Area Network. A protocol for a multiplexed system of control and communication on a vehicle.


The actual circuit on which the CAN signal/message is transmitted. It is usually a pair of wires but fibre optics have been used.

CAN message

The information transmitted on a data bus from one node to another.


Canister Purge Solenoid Valve

Cap (Tire Valve)

Air Valve. Secondary air valve for an inflated tire.


An ability to store an electrical charge in two conductors separated by an insulator.


Electrical/electronic device consisting of two plates separated by an insulator. It is capable of holding a charge.

Capacity (engine)

Applied to the sum total of the swept volumes for all cylinders in an engine. Also known as 'total displacement'.

Carbon Black

A black pigment manufactured by collecting the carbon resulting from incomplete combustion of natural gas.

Carbon Monoxide (CO)

Polluting, odourless, toxic (poisonous) gas produced by incomplete combustion of fuel.


See Carburettor.


Device for Vaporizing liquid fuel and mixing it in appropriate proportions with a stream of air, prior to combustion in an engine.

Cardan Joint

A drive shaft universal joint consisting of two yokes and a cross bearing. Also Hooke's joint.


Crank Angle Sensor


A metal forming process where molten metal is poured or injected into a mould.


(1) Originally an undercarriage member with a vertical pivot axis behind which the wheel trailed, so that the wheel aligned itself to its direction of motion. (2) Steering geometry, parallel to the direction of travel, in which the steering pivot is inclined away from the vertical in order to give the effect of castor as described in 1. Also castor offset and castor trail. Small pivoting wheel attached to piece of furniture to facilitate movement.

Castor Angle

Angle in side elevation between the steering and vertical axis.


A substance that causes a chemical reaction when it is mixed with another substance.

Catalytic Converter

Uses platinum, palladium and rhodium to change pollutants in exhaust gases to harmless substances.

Catch Up

The process by which testing test data recorded while testing under Emergency Testing is subsequently entered via the VTS Device.


A negative connection.

Cationic electrodeposition

One of the electro-coating (E-coat) methods in which the body is charged negatively and the paint positively. The cationic method is superior to the anionic method in terms of corrosion resistance. Frequently used for OEM primer application.


Sealing used in joints to prevent the passage of fluid (commonly moisture) or gas.


The formation of cavities in a structure or of gas bubbles in a liquid, or of a vacuum or partial vacuum as between a body moving in a fluid and the fluid.


Cubic Centimetres


Cold cranking amps (a rating for batteries)


Comite des Constructeurs d'Automobile du Marche Commun. The European organisation that set standards for engine lubricants. This organisation has been replaced by ACEA (Association des Constructeurs Europeens d'Automobiles).


Compact Disk Read Only Memory. Stores up to 800Mb of data. The data is 'read only' which means that you cannot change or overwrite it.


Capacitor Discharge Ignition

CDI (Ignition)

Capacitor discharge ignition system that uses a capacitor to produce the high tension for arcing across the electrodes of the spark plug.


Cold Engine Air Bleed


Central Electronic Control Unit


That part of a battery made from two dissimilar metals and an acid or alkali solution. A cell stores chemical energy for use later as electrical energy.

Cell Caps

Removable covers for a battery cell.

Center Bearings

See Centre Bearings.

Center of Gravity

See Centre of Gravity.

Centre Bearings

Bearings usually used to support a rotating shaft - at its centre. Many RWD propshafts have a centre bearing for example.

Centre Link

The centre track rod of a three-part steering system which connects the steering box drop arm to the idler and the two outer track rods.

Centre of Gravity

The point about which an object would balance in all directions. Also the point about which it can be said that gravity is acting on something.

Centre Point Steer

Steering geometry, parallel with the vehicle axis, in which wheel camber and steering swivel axes meet at the centre line of a steered wheel at ground level.

Centrifugal Force

The outward movement felt as a car turns a sharp corner.

Centrifugal Unit

Mechanical system using weights attached to a shaft. These move out against spring force as the shaft speed increases. Senses shaft speed for ignition advance and diesel pump speed.


Cold Enrichment Rod


Cold Engine Sensor Switch

Cetane Number

Applies to diesel fuels as a measure of the delay in ignition after injection. A higher number indicates a shorter delay.

Cetane Rating

See cetane number.


Central Fuel Injection

CFI (Fuel Injection)

Usually centre point fuel injection system but sometimes used for continuous fuel injection.


Cubic Feet Per Minute


Used to describe the unintentional action of one component rubbing against another.


The degradation of a paint film by gradual erosion of the binder, typically due to weathering. It is common in ageing red finishes and is characterized by a pinkish haze and loose pigment particles on the surface of the paint


A soft suede leather formerly from the skin of the chamois antelope but now more often made from sheepskin.

Charcoal Canister

A part of a fuel evaporative (EVAP) system. The charcoal absorbs petrol vapour during fuel tank venting and returns the fuel to a reverse air flow and into the engine under specific conditions.

Charge Current

See charge rate.


Process of building up the level of charge in a battery or capacitor.

Charging Rate

The recommended rate of charge for a battery. Usually one tenth of the amps in the Amp hour rate for one and a half times the hours for a fully discharged battery.


Structural lower part of a vehicle to which the running gear and body are attached. The true chassis is now evident only in larger vehicles.

Check Valve

A one way valve, using a plate or ball and spring, that permits fluid flow in one direction only. Used in hydraulic systems to retain a residual pressure.


A type of paint film defect in which cracks in the film begin at the surface and progress downward. The result is usually a straight v-shaped crack which is narrower at the bottom than the top.


A checksum is a count of the number of bits in a transmission unit that is included with the unit so that the receiver can check to see whether the same number of bits arrived. If the counts match, then one can assume that the complete transmission was received.

Chemical Bond

A reaction between atoms that causes a force such as to create a bond.

Chemical Cartridge Respirator

A respirator which uses a cartridge containing various chemical substances to purify inhaled air of certain gases and vapours.

Chemical Staining

Spotty discolouration of the topcoat sometimes caused by environmental conditions (acid rain, tree sap, etc.) It can also be caused by improper activation of body filler which leads to staining in the subsequent layers of topcoat.

Chemical Treatment

A process that uses chemical to bring about a change in something.


Description of an integrated circuit where the circuit is built on a silicon chip.


Small flakes of a finish losing adhesion from the substrate. Usually caused by the impact of stones or hard objects.


A restriction in a carburettor. The term is used to describe either the venturi as in single or twin choke carburettor, or for the cold start device.

Choke Flap

The butterfly type valve that restricts the flow of air.

Chopper Module

An electronic circuit or unit that switches a supply on and off rapidly.


The level of saturation or intensity and richness of a colour. De-saturated or "dirty" colours have less chroma; saturated or "clean" colours have more chroma.


A chemical compound used as a component in primers to provide corrosion protection to metal.

Chromate Rinse

Newly manufactured vehicle bodies are completely immersed in a chromic acid solution after zinc or iron phosphating to passivate the metal at the base of cracks and pinholes in the phosphate. Chromate rinsing, as used in modern mass-production paint preparation, will greatly increase corrosion resistance.


cylinder identification sensor


A complete conducting path.

Circuit Board

A board used to construct an electronic circuit.

Circuit Breaker

A switch to turn off the current flow in a circuit. Can be manually or automatically operated.

Circuit diagram

A diagram using symbols for electrical and electronic components that represents a real circuit.


The boundary line of a circle.


Continuous Injection System


Crankshaft Position Sensor

CKP Sensor

Abbreviation for the crankshaft position sensor.

Clamp Plate

A simple plate to hold another item securely.


To make clear.


Clay is a naturally occurring material composed primarily of fine-grained minerals. It is used in some cases to act as a very fine abrasive for improving paintwork.


Material used to remove dirt and other impurities from a substrate prior to painting.

Clear Coat

The transparent, non-pigmented top coat that is applied over a coloured base coat paint. Most modern cars have factory-applied clear coat paints. Clear coat paint increases paint durability, gloss, and resistance to harmful environmental effects such as ultra-violet radiation.


A gap between two components.

Clearance Fit

The relationship between the dimensions of two components where one fits inside the other. In this instance the fit is one where the inner dimension of the outer component is greater than the outer dimension of the inner component.

Clearance Size

A drilling size slightly larger than the nominal size of a thread.

Clearance Volume

The space above a piston at top dead centre in a cylinder. Includes the combustion chamber and any other volume formed by the thickness of gaskets, etc.


A temporary area used to store copied information.

Clock spring

These are generally made of rectangular section material, flat strip, with round edges. They are general used as brush springs in D.C. motors. Also known as spiral springs


Condition where abrasive paper becomes clogged by the abraded surface coating.

Closed Circuit

Applied to an electrical circuit when the switch is 'on' or the circuit is continuous.

Closed Loop

A system where a change in the output causes a changed input which in turn changes the output and so on...

Closed Loop System

Electronically controlled system, which adjusts settings by feedback from sensor signals that are monitoring the performance of actuators in the system and comparison with the pre-programmed map.


The formation or presence of a haze in a liquid such as a paint film.


A device to allow drive to be connected and disconnected. In a starter motor a clutch is used to only allow drive to pass one way - starter to engine, NOT engine to starter.

Clutch (Transmission)

Disengages / Engages drive from the engine.

Clutch Fork

See Release Fork.

Clutch Linkage

Mechanism to connect the pedal to the clutch release bearing. May be cable, electrically or hydraulically operated.

Clutch Pack

A set of friction plates and pressure plates in a multi-plate clutch.

Clutch Plate

The friction plate used as part of a clutch.


Camshaft Position Sensor

CMP Sensor

Abbreviation for the camshaft position sensor.


Carbon Monoxide


Carbon Dioxide


A motor vehicle which is constructed or adapted t carry more than 16 seated passengers, with a DGW of more than 7,500kg and a maximum speed in excess of 60mph.


Coaching is a method of directing, instructing and training a person or group of people, with the aim to achieve some goal or develop specific skills.


The fusing or flowing together of liquid particles. The term is frequently applied to waterborne coatings, which require coalescence to form a continuous film.


Driving on very light throttle at a constant speed.


The act of applying paint or the actual film left on the substrate by an applied paint. A single coat may be produced by two passes of a spray gun, one overlapping the other in half steps. A double coat is two single coats with little or no flash-off time between them.


The tendency of sprayed paint to form strings or strands rather than droplets as it leaves the gun. May be caused by too fast or improper solvent, paint too thick or air pressure too low.

Coefficient of Friction

A value between 0 and 1 that indicates the ability of two materials to grip each other when sliding or rolling in contact with other.

Coil (Ignition)

An electrical transformer that increases the battery voltage to a high tension capable of arcing across the gap of spark plug electrodes.

Coil Spring

A spring that consists of a single length of bar wound into a helix (coil).

Cold Cranking Rating

Applies to a battery's ability to provide a minimum current for a minimum time for starting a cold engine.

Cold Start Enrichment

An increase in the amount of petrol mixed with the air under cold starting conditions.

Cold Start Valve

An extra valve used only when starting from cold.

Cold Starting

Cranking the engine so that it will start and run under cold conditions.


Cracking of a paint film resulting from a sudden drop in temperature, or repeated cycling from high to low to high temperatures.

Cold-start Valve

An extra valve used only when starting from cold.


Terminal of a transistor that becomes conductive to the emitter when the base is energised.


Small half-round and tapered components used to fix a valve stem to its cap.


The visual appearance of an object that can be described in terms of hue, value, and chroma. Colours are seen differently by different people and under different lighting conditions.

Colour Coat

The single-stage or basecoat that provides the visible colour of a coating system.

Colour Match

Achieved when the applied colour duplicates all aspects of the original colour's appearance in terms of hue, value, and chroma.

Colour Retention

The ability of a paint to maintain colour under exposure to light, heat, moisture or other conditions of use.

Colour Sanding

The sanding of a cured or dried paint film to prepare it for polishing or recoating.


The burning of fuel in air.

Combustion Chamber

Area above the piston when it is at TDC where the combustion takes place.

Combustion Knock

Advanced or quick detonation of fuel in a cylinder where the resultant pressure rise causes a shock wave through the piston, crankshaft and engine block.

Combustion Stroke

The third stroke of the four stroke cycle when the fuel is burnt to produce heat energy, expansion of the air/fuel charge and power on top of the piston. Also known as the power stroke.


This is a process of transferring information from one entity to another. An exchange of thoughts and information if human.


Rotating contacts for electrical windings on a generator or motor armature.


A strategy that involves adjusting something up or down to more closely approximate the required or ideal value.


Provides equal force to mechanically braked wheels.

Compliance Coating

A coating whose volatile organic compound (VOC) content does not exceed that allowed by regulations. Compliance coatings may be waterborne, low solvent, or powder.

Composite Materials

Material made up of constituents that remain recognisable.


Place where cars are taken when they die, or dogs when they get lost...A chemical where the two or more elements they contain combine at the molecular level - salt for example is a compound of sodium and chlorine (sodium chloride).

Compound Gear Train

Two or more pairs of gears.


The use of an abrasive material, either by hand or machine, to smooth and enhance the gloss of the applied topcoat.


The reduction in volume of a gas when it is squashed. The gas pressure increases and the temperature rises.

Compression Ignition (CI)

Fuel ignited by heated air (diesel).

Compression Ratio

The ratio between the TOTAL volume of a cylinder (swept volume plus clearance volume) and the CLEARANCE volume (volume above the piston at TDC - the combustion chamber).

Compression Ring

A piston ring that helps to contain combustion pressure.

Compression Spring

A spring that is designed to provide force when compressed.

Compression Stroke

The second stroke of the four stroke cycle when air or the fuel/air mixture is compressed. Compression is necessary for clean and efficient combustion of the fuel.


Electrically or mechanically driven pump used to increase pressure of a gas or liquid.

Compressor Wheel

Part of a turbocharger that pumps extra air into the intake and therefore the engine.


Must be completed.

Computer Application

Programs such as Word, Excel and PowerPoint.

Computer Crash

An event that causes the computer to become inactive. This can often result in the loss of unsaved work.

Con Rod

Short for connecting rod, this joins the pistons to the crankshaft.


A term that describes objects that revolve or are fitted around a common centre.


Expressing a lot but in few words.


1. Part of an air conditioning system positioning in the outside air stream. It gives off heat to atmosphere as a refrigerant condenses. 2. Alternative name for a capacitor in an ignition system.

Condenser (A/C)

A heat exchanger in an air conditioning system that is similar to, and fitted in front of, the coolant radiator where it passes heat energy to the air stream passing through it. Operates by condensation of the refrigerant from gas to liquid by pressure and heat loss.

Condenser (Ignition)

An electrical capacitor that is used in a contact breaker ignition system.


Transfer of electrical or heat energy along material.


A material that allows free movement of electricity or heat through it.

Conductor Foil

General term to describe a type of 'flat' wiring harness.

Cone Clutch

A simple clutch shaped like a cone. Used as part of a synchromesh mechanism.

Cone Pulley

A pulley shaped like a cone such as to allow different ratios to be set by the position of a drive belt.

Confidential Information

Information that is private and should be protected.

Connecting Rod

The engine component that joins the piston to the crankshaft.

Constant Velocity Joint

A drive shaft joint that transmits drive through an angle without changes in speed.

Construction and Use

The UK Construction and Use Regulations that cover the legal requirements on manufacturers and users of road vehicles.

Constructive feedback

A way of giving feedback to somebody so that they feel valued and that they can improve what they did based on your comments.


The rate at which a vehicle uses fuel.

Contact Breakers

Mechanical spring-loaded switch located in the distributor of an ignition system and actuated by a rotating cam. It makes or breaks the ignition circuit to control the coil output and delivery of a spark to the spark plug.


Something which contaminates; an impurity; foreign matter


Any polish, wax, tree sap, tar, oil, etc., that would damage the paint film or spoil the adhesion of a new paint film.


The condition of an electrical circuit when a current will flow.

Continuous Injection

Injectors that inject or 'squirt' all the time.

Continuous Injection Systems

Injection systems where the supply to the inlets is active all the time but may only 'spray' on the intake stroke due to the lower pressure.


(see Mapping)


Agreement between parties enforceable in law.


A decrease in size. For example, when a metal rod is cooled it decreases in length and diameter. Opposite of expansion.

Control Flap

A flap that is used to control something! Air temperature for example.

Control Plunger

Part of fuel injection system. Regulates fuel delivery against air flow.

Control Pump

A pump that provides a set pressure.


Travel of heat by movement in heated fluids due to a reduction in density and consequent rise of the hotter material.

Conversion Coating

Conversion coatings are used on metal parts (such as vehicle bodies) to be painted. They are acidic chemical products used before paint to convert the metal substrate to an iron or zinc phosphate surface. The chemical reaction modifies the metal surface to enhance paint adhesion and corrosion protection

Conversion Factor

Used to make it easier when converting from one form of ‘measurement' to another.


To change one expression to another. For example, expressing miles in kilometres.


An additive used to provide a chemical reaction to cure paint. Also used with some primers to convert them to 'non-sanding' versions, by changing their properties and making them suitable for wet-on-wet procedures.


Liquid made up of water, anti-freeze and inhibitors used to cool an engine in a sealed but circulated liquid system.


Coil on plug, where a different ignition coil is used for each spark plug on an engine. Also known as direct ignition.


A difficult metal substrate to paint. Also used in the manufacture of special decorative pigments and synthetic driers.

Core Plug

A steel plate fitted into a cylinder block or cylinder head casting after the casting sand has been removed in order to produce a fully sealed component.


The decomposition of a metal in contact with its environment - a chemical reaction of oxygen and moisture or corrosive materials on a metal surface. Also called oxidation or rusting (on ferrous metals).


An acid or other chemical that will chemically react or attack another material.


The Control of Substances Hazardous to Health Regulations 2002 is a United Kingdom Statutory Instrument that stipulates general requirements on employers to protect employees and other persons from the hazards of substances used at work by risk assessment, control of exposure

Cost of production

The actual cost to the manufacturer of producing a vehicle (it does not include mark-up).

Cotter Pin

Tapered pin used to hold shafts into housings. May also be used to describe a split pin.

Counter Gear

See countershaft assembly.

Counter Shaft

An alternative term used to describe the layshaft in a manual gearbox. The shaft in a manual gearbox carrying the gear cluster and meshing with the constant mesh gears.


An alternative term used to describe the layshaft in a manual gearbox. The shaft in a manual gearbox carrying the gear cluster and meshing with the constant mesh gears.


Steering force that acts such as to prevent the wheels moving off their intended line.


A pair of equal parallel forces that are opposite in direction. The effect of a couple is to produce or prevent the turning of a body.

Coupling Point

The point or speed at which a torque converter takes up drive.

Courtesy Lights

Lights for the vehicle occupants to use.


The amount of area a volume of paint will cover at a certain thickness. Theoretical coverage is described as the number of square metres a coating will cover at 1 mm film thickness.


General term used to describe a cover.


See single point injection.


Crankshaft position and speed sensor.


Central processing unit. This relates to the main component in an ECU or computer.


Splitting of a paint film. Cracking usually appears as straight lines which penetrate the entire film thickness and can be caused by over-baking or by application of excessive film builds.


A term used to describe turning the engine by the starter motor.


The part of the engine that houses the crankshaft and the bearings in which it runs. It can be either a separate part or be integral with the engine block.

Crankcase Ventilation

Usually a positive crankcase ventilation (PCV) system that allows any crankcase pressure and oil vapour to be passed into the combustion chamber so that it does not escape into the atmosphere as a pollutant. Early crankcase ventilation consisted of an open vent or breather tube that allowed crankcase gases to be passed out below the vehicle.


Description of making the engine rotate on the starter motor.


The process of turning over an engine using a starter motor (or a handle on earlier models!).


Major rotary component of engine that converts piston movement into rotary movement.

Crankshaft Damper

Usually part of the front pulley. A rubber mounted mass reduced torsional vibration.

Crankshaft Oil Seal

Usually radial lip type seals at the front and rear of the crankshaft.

Crankshaft Pulley

Main drive pulley on the end of the crankshaft.

Crankshaft Shell

See Shell Bearings.

Crankshaft Web

Counterbalance weights that are opposite the big end bearing journals.

Cratering (silicone cratering, see

'Fish Eyes')">Small round depressions in a paint film which may or may not expose the underlying surface. Frequently caused by contamination of the substrate by wax, oils, etc.


The tendency of a wet paint film to recede from certain areas of a painted surface. Often caused by the presence of contaminants on the surface.


The formation of surface cracks, often as a fine network, which do not penetrate to the underlying surface of a paint film. Crazing is sometimes caused by the softening effect of solvents from excessive paint coats as in cellulose or acrylic lacquer repair operations.


A cyclic redundancy check is a method of checking for errors in data that has been transmitted on a communications link.

Crescent Pump

A positive displacement pump made from one externally and one internally toothed gear rotating eccentrically in a housing. Inside the housing, and in the space formed where the teeth are out of mesh, is a crescent shaped block. This divides the gear teeth to create the pumping and carrying chambers of the pump.


Act of squeezing a connection such as a terminal onto a wire.

Crimped Joint

Termination of cables by squashing tabs on the terminal around the cable.


The process of compressing materials to make a join. Usually refers to squeezing a terminal to secure it to a wire.

Crocodile Clip

Hand operated clip that, like a crocodile, bites under spring pressure to make a contact. Also alligator clip.

Cross Member

A chassis or support member that runs across a vehicle frame or chassis.

Cross-Coat (double-header)

A method of paint application often used with high solids paint. The technique involves a side-to-side application followed by a top-to-bottom application with very little flash-off time. Also used by some refinishers when they apply 'drop coats' in more than one direction to eliminate striping in metallic and pearlescent finishes.

Cross-draft Booth

A spray booth in which air movement is horizontal from end to end, i.e. fresh (may be heated) air enters at the front of the booth and is extracted at the rear Not considered to be as efficient as downdraught extraction

Cross-pin (Hooke's) Joint

Type of universal joint with a cross pin and two yokes.


Form of tyre construction in which the casing plies are laid diagonally so that each ply lies at an included angle, usually of more than 40 degrees, to the adjacent ply.


The area exposed if a cut were to be made through the centre of an object.


General term used to describe shafts which are fitted across units rather than along or axially.

Crown Wheel

Bevel gear wheel in which the teeth are set around the periphery, giving the wheel the appearance of a crown. The larger wheel of the crown wheel and pinion bevel pair.

Cruise Control

A system that keeps the vehicle at a set speed.


Driving at a constant speed under light throttle conditions.

Crumple Zone

Safety feature of vehicle body, absorbs impacts.


The arrangement or bonding of atoms in solid materials which form during cooling to give regular structural patterns.

Crystalline Structure

The arrangement or bonding of atoms in solid materials which form during cooling to give regular structural patterns.


Cold Start Spark Advance


Cold Start Spark Hold


Closed throttle.


Coolant Temperature Sensor

CTS Sensor

Abbreviation for the engine coolant temperature sensor or switch.


Choke Thermal Vacuum Switch

CTX Transmission

Constantly Variable TransaXle.

Cup Seal

A hydraulic seal with a lip that contains pressure against the lip but will allow a flow in the opposite direction. Used in the cylinders of brake systems.

Curb Idle

Rotational speed of an engine on no-load and minimum throttle setting. Also tick over and idle.


The process, usually a chemical change, by which paint is converted from the liquid to the solid state.

Cured Paint

Paints that have completed the curing process. Cured paints include all factory-applied paints and refinish paints that have air-dried for more than 30 days. Wax application is recommended only for fully cured paints.


Movement of electrically charged particles. Current flow is measured in Amperes.

Current Regulator

An electrical or electronic device used in generator circuits to control the output current.

Current Relay

A device that usually incorporates a reed switch. It is activated by current flow through a series winding.

Custom Painting

Unique painting, frequently with special effects or designs, normally designed by owner of vehicle and/or the specialist who will be painting the vehicle.


Refers to both the dissolving of solid material in a solvent and the reducing of the viscosity of a liquid by the addition of a thinner.


Process where fuel supply can be stopped, for example when a vehicle is coasting down.


See Cut-off.

CV (Constant Velocity) Joint

A drive shaft joint that transmits drive through an angle without changes in speed.

CV Joint

A drive shaft joint that transmits drive through an angle without changes in speed.


Control Vacuum Regulator

CVT (Transmission)

Constantly variable transmission that uses two conical drums and a drive belt between them. The effective diameter of each drum is varied hydraulically in order to change the drive ratio.

Cylinder (Master)

(1) Primary source of pressure in a hydraulic system such as brake or clutch system, containing the piston by which pressure is applied and connected to a source of hydraulic fluid. (2) Primary unit for dispensing hydraulic fluid under pressure in a hydraulic system.

Cylinder (Slave)

Cylinder and piston that, under hydraulic or pneumatic pressure from a master cylinder, actuates mechanical components. For example, the brakes or a clutch.

Cylinder Balance

A test carried out with diagnostic equipment to make sure each cylinder of the engine is working to its full potential.

Cylinder Block

The part of an engine containing the cylinders. The cylinder block may also incorporate the water cooling jackets and provisions for the valve gear.

Cylinder Bore

The diameter of the cylinder.

Cylinder Charging

Process of filling a cylinder during the induction stroke.

Cylinder Head

Encloses the top of the cylinder block.


Cylindrical or tubular chambers in which the pistons of a reciprocating engine or pump are fitted. They are contained in the cylinder block.


DA (Dual Action)

Refers to a power sander used for flatting substrates.

DA (Dual Action) Sander

A machine used for random orbital sanding and buffing. The pad of a DA sander travels in a randomized orbital pattern, rather than taking a simple circular path. DA sanders minimize the swirl marks that commonly result from rotary sanding and greatly reduce the time necessary to produce feather-edges.


A device to reduce oscillations or vibrations. A kind of damper.


Component that reduces the oscillations of a spring in a suspension system. A device that smoothes out fluctuations.


The reduction of oscillations or smoothing out of fluctuations.

Damping Chamber

An air space in which a damper flap moves that acts to reduce oscillations.

Darlington Circuit

Two transistors connected so that the first switches the second. Used to switch high currents.


A damper used on some variable venturi carburettors.


In computer science, data is anything in a form suitable for use with a computer. Data is often distinguished from programs. A program is a set of instructions that detail a task for the computer to perform. In this sense, data is thus everything that is not program code.

Data Link

General term for a connection to a scanner or similar diagnostic tool.

Data rate

The actual speed at which data is transmitted.

Databank link

A connection to an electronic source of data.




Direct Current

DC Brushless Motor

A DC Brushless Motor uses a permanent magnet external rotor, three phases of driving coils, one or more Hall effect devices to sense the position of the rotor, and the associated drive electronics.

DC Direct Current

An electrical current that flows in one direction only. Conventionally from the positive pole to the negative pole.

DC-DC converter

This is an electronic circuit which converts a source of direct current (DC) from one voltage level to another. It actually converts DC to AC to a different voltage AC and then back to DC!

de Dion suspension

A de Dion suspension uses universal joints at both the wheel hubs and differential, and uses a solid tubular beam to hold the opposite wheels in parallel. Unlike an anti-roll bar, a de Dion tube is not directly connected to the chassis nor is it intended to flex.


Switched off - usually refers to a coil or relay.


Used to describe pure, neutral water used for topping up batteries.

Dead Axle

A non-driving axle. Supports the vehicle and carries freely rotating wheels.

Dead centre

Top or bottom piston positions in the cylinder when the crankshaft is at 0° or 180° of rotation.


Opposite of acceleration. Reducing velocity or speed. Slowing down.


Brake Meter for recording rate of deceleration during braking for test purposes.


A number system that uses a base of 10.

Decimal Place

The position of numbers after (to the right of) the decimal point.

Deck (Block)

The top of the engine block forming the mating face for the cylinder head.

Dedicated Test Equipment

Equipment made to work specifically with one system or one manufacturer's vehicle range.

Deep Socket

Same as a normal socket but longer to allow it to be used on a component such as a spark plug.


Untrue and harmful information.


Digital Electronic Fuel Injection


Applied to springs to indicate the amount of movement away from the original position.


The gradual or rapid disintegration of a paint film, normally due to weathering. May be simply fading/loss of gloss or can lead to total paint film breakdown


Cleaning a bare or previously painted substrate by removing greases, oils, and other surface contaminants.

Degrees of Advance

Refers to crankshaft rotation degrees as linked to ignition timing.


The loss of adhesion between two or more layers of paint, causing material to separate from the painted surface or substrate.

Delta Wound Stator

An alternator stator winding with the three-phase coils connected at the ends in order to give a large current output.


An expression of the mass of a substance for a given volume, e.g. water has a density of 1kg per litre

Desiccant Bag

Chemical used to absorb moisture. Often used in AC systems to remove moisture from the refrigerant.


Careful, in-depth cleaning and polishing of a vehicle's surface finish and/or interior surfaces.


A position holding device consisting of a spring and locking piece that locates in a groove. A locating device for holding sliding gears in mesh in a manual gearbox.


A material intended to assist cleaning. The term is sometimes used to differentiate between soap and other surfactants used for cleaning.


Ignition of fuel in an engine that is too early or too rapid. Produces a knocking or pinking sound.


Deceleration Fuel Shutoff


Design Gross Weight – i.e. the maximum gross weight that the vehicle was designed to operate at by the manufacturer. This is normally found on the manufacturer’s plate fixed to the vehicle, or in the case of older or heavier vehicles by a ‘Ministry Plate’. Such plates are not required on all vehicles.


Direct injection of fuel, usually diesel, into the combustion chamber.

Diacetone Alcohol

A slow-evaporating solvent for paints.


The process of determining the fault in a system based on observed or measures symptoms.

Dial Gauge

A gauge used to indicate lateral movement of a component, on a clock (dial) type display.

Dial Test Indicator

A gauge used to indicate lateral movement of a component, on a clock (dial) type display.


The line that goes through the centre of the circle.


Flexible disc or membrane which deflects under pneumatic or hydraulic pressure and, in so doing, imparts linear motion to a centrally-located rod or other form of mechanical actuator, as in an air brake or carburettor.

Diaphragm Spring

Dished clutch spring applying clamping force and disengagement linkage.


A book containing a list of words in alphabetical order. Each word has information given about it (i.e., the definition).


Tool used to cut an external thread.


Used in a capacitor as an insulator between the two conductive plates.


Heavy fuel oil for diesel engines. Compression ignition engine.

Diesel 'Knock'

Engine noise produced by very rapid rise in cylinder pressure following injection and ignition of the fuel in a diesel engine.


A condition in a petrol engine when it continues to run after the ignition has been turned off. Also known as running on or run on and usually occurs in a very hot carburettor engine where the fuel cannot be fully shut off.


Relating to or using a difference. Operating on differences in pressure. Bevel gear set that allows two wheels to rotate at different speeds.

Differential Pressure Valve

A valve that sets its position depending on the difference between two pressures, for example, two fuel pressures or fuel pressure and manifold pressure.

Differential Pressure Valves

Valves that operate under the control of TWO external pressures or the difference (differential) between them.

Digilal Converters

See Digital Converter.


A number.


A signal current with either an 'on' or 'off' voltage. Digital instruments which show a numerical value.

Digital Converters

Electronic devices that convert analogue signals to digital or digital signals to analogue.


A liquid which increases the capacity of a solvent for the binder. Diluents cannot dissolve the binder themselves. They are usually used to control viscosity or flash-off times.

Dim-dip relay

A switching relay that results in a resistor being connected in series with dipped lights when ignition and sidelights are switched on. This safety feature as used on some vehicles – but not all.


An electronic one way valve with polarity bias manufactured from a semiconductor material.

Diode Pack

A collection of diodes that act as a rectifier. A minimum of six diodes in the pack are needed for a three phase alternator.

Dip Switch

A switch used to select main beam or dipped beam headlights. Sometimes called a dimmer switch.

Dipped Beam

The lower of the two headlamp beams, dipped to avoid blinding the drivers of oncoming vehicles.


To apply paint to an article by immersing it in a container of the paint and then withdrawing the article and allowing the excess paint to drain from the part. Particularly useful for small parts, such as nuts, bolts, brackets, etc.


A stick used to dip in a fluid and check its level!

Direct Injection

Diesel engine design where fuel is injected directly into the cylinder.

Dirt Nibs

Small specks of foreign material in a dried paint film. They can be usually be removed by careful sanding and polishing.


Distributorless ignition system that uses an ignition coil or coils directly connected to the spark plugs. The ignition is triggered by the ignition module on signals from the ECM (Engine control module).

Disc Brake

Brake in which external friction pads are brought to bear on the faces of a rotor or disc, usually by the clamping action of a caliper.

Disc Sander

Power sanding tool used for grinding, sanding, and rough polishing of repaired metal areas.

Discard Dimension

The lowest safe value for a component before it needs to be replaced. Used for brake components such as drums and rotors.

Discharge Time

Refers usually to how long a capacitor will take to release all its stored energy. It can be calculated by multiplying the capacitance by resistance and by five (t = 5CR).


Change in colour of paint due to chalking, dirt collection, fading, darkening or tonal changes.


Unfair or unequal treatment of a person due to their age, sex, disability, race, religion etc.


The act of removing a connection between two gears of through a clutch.

Disk Drives

The primary data storage device used by computers. It stores and retrieves data.

Dispersion Coatings

A type of paint in which the binder molecules are present as colloidal particles. Characterized by a higher percent sprayable solids than is possible with solution-bases paints.


The swept volume of a cylinder between the top and bottom positions of the top of a piston.

Displacer Unit

A hydraulic suspension component that contains gas under pressure and a fluid.


Metallic particles from the basecoat surface in the clearcoat. If severe, the effect can alter the tone and exaggerate the metallic appearance.


Purified by evaporation and condensation.


Distillation is a method of separating mixtures based on differences in their boiling points in a liquid mixture.

Distinctness of Image

A measurement of the accuracy of a reflection in a paint film, which assists the painter in determining the quality of the surface gloss.


A component in the ignition system that houses the primary circuit switching device and the secondary circuit distribution components.

Diverter Valve

Used on air injection systems to control the air feed according to the engine running conditions. Also known as a 'dump valve'.


A digital circuit that, for example, reacts to only every second input hence dividing it in two.


Data link connector for connecting a diagnostic tool to a vehicle's electronic systems.

Dog Clutch

Pegs or similar used to transfer drive rotation from one component to another.

Dog Drives

Pegs or similar used to transfer drive rotation from one component to another.


Dual overhead camshaft. Sometimes double overhead camshaft.

Donut Coupling

A rubber universal joint shaped like a donut.

Double-Cardan Universal Joint

Two Hooke's or Cardan type universal joints close coupled to increase the effective angle of turn before inducing vibration.


A technique that was necessary before the use of synchromesh mechanisms. During a gear change the clutch is lifted as the gear mechanism moves to neutral, the engine is revved (if changing down), the clutch is now depressed and the new gear selected.


A locating pin between two mating components. Can be solid or hollow.

Down Draft

A carburettor where the choke tube (venturi) is fitted vertically.

Downdraft Booth

A spray booth in which the air movement is from the ceiling (input air) through the floor (extracted air).

Downshift Valve

Automatic transmission valve to give a downshift under specific drive conditions.


Diesel Particulate Filter

Drag Link

Steering linkage between the steering box drop arm (Pitman arm) and the drag arm on the steering swivel.

Drain Plug

Removable plug used to let fluid out (engine oil for example).

Drain Tap

A simple valve used to allow a fluid to be removed.

Drain Tube

A tube used to, for example, remove moisture from an air conditioning evaporator.


A chemical which, when added to an enamel, paint, varnish, or oil, hastens the drying or hardening of the film through evaporation.

Drive Axle

An axle that transmits a drive to the wheels. Also known as a live axle.

Drive Belt

Traditionally called the fan belt. A Vee or multi-vee belt to take drive from the engine to auxiliary components such as an alternator or water pump.

Drive Flange

A flange that is used to transmit drive from one component to another. The flange on the end of a propshaft is a good example.

Drive Gear

A toothed wheel to pass drive to another toothed wheel.

Drive Joints

General term used to describe any joint in a drive system such as a CV or universal joint.

Drive Line

Rotary shafts carrying drive to the wheels.

Drive Motor

A motor that supplies a driving force. Term usually refers to an electric motor.

Drive Pinion

A small gear that drives another gear.

Drive Plate

A plate that takes the place of the flywheel on an engine such as to transfer drive to a torque converter.

Drive Pulleys

Pulleys that transfer drive from one shaft to another.

Drive Ratios

Ratio between gears. For example 2:1 or 1.3:1.

Drive Shafts

Open shafts carrying the drive from the final drive to the driven wheels.


Transmission components in the line between the engine and wheels.

Driveline Wind-up

Description of how a vehicle in four-wheel drive (with no centre differential) can build up spring tension in its transmission components. This is because the front wheels may rotate at a slightly different speed to the rear wheels.

Driven Plane

The plane (like a flat sheet) through which the drive operates.

Driven Plate

The clutch disc that is driven by the pressure plate.


Open shafts carrying the drive from the final drive to the driven wheels.

Driving Beam

The upper of the two headlamp beams. Also main beam and upper beam.

Driving Thrust

Forces generated by the engine and transmission.

Drop Arm

The output arm on a steering gearbox. Also known as a Pitman arm.

Drop-Coat (Finesse coat)

A paint application method recommended on many Technical Data Sheets to eliminate patchy finishes in metallic and pearlescent basecoats. The atomising air pressure is reduced and the gun is held further from the surface. Providing the spray gun is then moved across the surface at a uniform speed, angle and distance, this technique should result in better orientation of the metallic and mica particles.

Dropping Resistors

Resistors used to reduce voltage to a component.

Drum Brake

Brake in which friction blocks or brake shoes lined with friction material are brought to bear on the periphery of a drum or cylinder. In most vehicle applications the shoes are brought into contact with the inner periphery of the drum.

Dry Clutch

A friction clutch that runs in an open housing. As distinct from a wet clutch that runs in an oil bath.

Dry Coat

A paint having a very low solvent content which produces a gritty film appearance.

Dry Film Thickness

The resultant film thickness of a coating after it has reached its final state of drying or curing. Measured in microns.

Dry Liner/Dry Sleeve

(US: dry sleeve) Hard metal engine cylinder liner or insert that is not exposed to contact with cooling water.

Dry Sanding

A method of abrading the surface by hand or machine without the aid of any water or other lubricant.

Dry Spray

Sprayed paint which loses so much solvent in the air that it becomes too dry to flow out over the surface. This normally occurs when the chosen thinners is too fast for the atmospheric or application conditions.

Dry Sump (Oil Pan)

An engine lubrication system where the reservoir of lubricating oil is held in an oil tank. The sump (oil pan) below the engine is scavenged with a separate pump for return to the oil tank. As distinct from a wet sump (oil pan) where the reservoir of lubricating oil is kept in an oil pan below the engine.


The change from a liquid to a solid which takes place after a paint is deposited on a surface. This involves both the evaporation of the solvents and any chemical changes that may occur.

Drying Oil

A liquid that will change to a solid under normal atmospheric conditions when spread out in a thin film.


Direct shift gearbox


Diagnostic Trouble Code


Dial Test Indicator.

Dual Bed Converter

A three way catalytic converter with one stage coated with platinum and palladium and the other with platinum and rhodium. Converts HC and CO to H and CO2 and NOX to N and O.

Dual Circuit Brake System

A brake system that has two independent hydraulic circuits so that one continues to act if the other fails. The split of dual circuits can be front and rear, diagonal, split front and one rear, and two complete circuits.

Dual mass flywheel

A dual-mass flywheel is used to isolate torsional crankshaft spikes created by diesel engines during the power stroke. Dual mass flywheels separate the mass of the flywheel between the engine and the transmission. This eliminates damage to the transmission components.

Dual Master Cylinder

A brake master cylinder with two separate chambers for a dual circuit hydraulic system.


Pipe, tube or trunking.


The property of a material that allows it to be drawn into wires.


Large pipes or similar. Usually refers to the air intake system components.

Dump Valve

See diverter valve.


Refers to the retention of gloss and performance properties in a paint film during prolonged use or exposure to sunlight and other atmospheric conditions.

Dust Contamination

Contamination due to visible dust particles of various sizes and forms which are embedded or form raised spots in the paint coating.


Condition when a new paint film has dried sufficiently that it will no longer allow dust to penetrate and stick to the finish.

Duty Cycle

The percentage length of time that an actuator or solenoid is energised by an electronic control unit. Refers to the pulse width.


Digital Volt Ohm Meter

Dwell Angle

The angle of distributor cam rotation when the contact breaker points are closed.

Dwell Meter

Measures the dwell angle.

Dwell Period

The angle of distributor cam rotation when the contact breaker points are closed. This term is used when the dwell angle is not a fixed value. Used for electronic ignition systems.


A term that indicates components that are in motion. The opposite of static.


Machine converting mechanical energy into electrical energy. DC.


A measuring device for engine power output. A brake dynamometer measures the output at the engine flywheel to give the term brake horse power (bhp). A rolling road dynamometer measures the output at the vehicle wheels and is used for test and diagnostic purposes.


E.C. (E)

European Commission (Executive).


Electronic Control Unit."


Electronic Control Unit.


Electro Motive Force.


Electrically Erasable Programmable Read Only Memory


Electronic Air Control Valve

Early Fuel Evaporation

Early fuel evaporation system using a heater element below a carburettor to heat the intake air and improve atomisation at cold engine speeds. Improves driveability and reduces harmful exhaust emissions.

Earth (Ground)

The electrical termination for components connected into a vehicle direct current earth return circuit. On mains electricity a safety route for an electrical short circuit used as a means of protecting people or electrical components from electrical overload.(Ground - US term)

Earth (Ground) Cable

The battery cable connecting the return side of a vehicle circuit to a terminal on the battery where a ground return circuit is used. (Ground - US term).

Earth Switching

Used to describe a circuit where the switch breaks the earth side of the circuit instead of the supply side.

Earth Switcing

See Earth Switching!


Electronically controlled automatic transmission.


Electronic Brake Control Module


Electronic Body Module


Electronic Control Assembly


Electronic Control of Automatic Transmission


Off centre. Usually refers to a rotating component with the operational part rotating about the central axis of the shaft. An example is a round cam offset from the shaft axis.


Engine control module. The electronic control module for engine management systems.


Engine coolant temperature sensor or sender.


Electronic Control Unit.


Electronic Diesel Control.


Painting technique where full panels are sprayed without blending into masked off adjacent panelwork, or where panels are removed from the vehicle to be painted


Electronic distributor or distributorless ignition system.


Explanation Demonstration Participation – a reminder of a good way to teach something practical to somebody.


Electronic engine control module.


Electronically Erasable Programmable Read Only Memory


Electronic Fuel Control


Electronic Fuel Control Assembly


Early fuel evaporation system using a heater element below a carburettor to heat the intake air and improve atomisation at cold engine speeds. Improves driveability and reduces harmful exhaust emissions.


A measure of effectiveness given as a ratio, percentage or decimal value (co-efficient) based on actual and possible performance.

Efficiency (energy)

Energy conversion efficiency is the ratio between the useful output of an energy conversion machine and the input, in energy terms. The useful output may be electric power, mechanical work, or heat.


To produce work without wasting time and energy.


A force or action that tries to achieve something.


Electronic fuel injection using sensors, an electronic control unit and fuel injectors to spray fuel into a central throttle body or the inlet ports. Provides very close control of fuel mixture under all operating conditions.


Exhaust Gas Oxygen Sensor. Also HEGO - Heated Exhaust Gas Oxygen Sensor, EOS Exhaust Oxygen Sensor and Lambda sensor. Provides information to the fuel system ECU on the oxygen content in the exhaust gas.


Exhaust gas recirculation. An emission control system that uses an exhaust gas feed into the intake air stream to reduce the mass of oxygen in the air charge and limit the formation of harmful oxides.


EGR Valve Position Sensor


EGR Temperature


Electronic Ignition

Elastic Limit

Amount of deflection in a spring that can be tolerated before permanent deformation occurs.


The property of materials that can deform and return to their original shape without permanent deformation.

Electric Field

Area round an electric charge in which an electric force is exerted.

Electrical Energy

The potential of electricity to do work.

Electro Motive Force (e.m.f.)

Voltage or pressure in a circuit.

Electro Motive Force (emf.)

Electrical energy available to a circuit at the battery or generator.

Electro Plating

Coating with chromium, silver, etc. by electrolysis.

Electro-coating (Electrophoretic coating)

Electro-coating is an organic coating method that uses electrical current to deposit paint onto vehicle bodies, part, or other assembled products.


Usually a metallic conductor which conducts electricity into a liquid, a gas, a vacuum or on to the surface of a solid.


A method by which paint is applied electrically by immersing electrodes and the work to be painted in a water-based paint and applying a direct electrical current to them.


A combination of electrical and hydraulic control systems.


Electroluminescence (EL) is an optical and electrical phenomenon in which a material emits light in response to an electric current passed through it, or to a strong electric field. It is used on in car entertainment systems and instruments on some vehicles.


A liquid that conducts electricity. Battery acid is a good example.


A magnet formed by a soft iron core and an electrical winding about the core. Becomes a magnet when an electrical current flows through the windings.


See electromagnet.

Electromagnetic Oscillation

Changes from one value to another in a circuit. Usually produces a sine wave output or similar.

Electromagnetic Solenoid

A solenoid (coil of wire) that is used to operate an actuator such as a fuel injection.

Electromagnetic Valve

A valve that is operated by a coil of wire, which becomes magnetic when switched on.


Electrons form the outer shell layers of atoms and orbit the protons and neutrons in the nucleus. The electrons have negative polarity and are electrically negative in opposition to the positive charge of the protons.

Electron Shell

Term used to describe the orbit of electrons round a nucleus.


Electrical systems operated or controlled by semi-conductor devices.

Electronic Control

System usually based round an electronic control unit where the operation of the system is pre-programmed.

Electronic Ignition

Petrol ignition system in which switching semi-conductors make and break the low tension circuit.


Describes the electrical state of something as in static (not flowing) electricity.


The basic form of a material.


Electronic Mail. Messages sent from one person to another electronically via a computer.


Electromagnetic compatibility. EMC requirements stipulate that a device shall not cause interference within itself or in other devices, or be susceptible to interference from other devices.

Emergency Brake

A legal requirement for all vehicles provided by a split hydraulic circuit or a mechanically operated parking brake.

Emery Paper

Carborundum grit on a backing sheet.


Electromagnetic Interference

Emission Control

Systems on the vehicle to control (limit) toxic emission.


Systems on the vehicle to control (limit) toxic emission.


Terminal of a transistor that becomes conductive when the base is energised.


A way of expressing emotions in online communication. E.g., :-)


Electronic Module Retard


A suspension of fine particles in a liquid. The dispersed particles may be binder, pigments, or other ingredients.

Emulsion Tube

Combined main and compensating jet tubes in a carburettor with provision for drawing air into the fuel flow to create an emulsion at higher engine speeds, thus preventing over-richness and improving fuel distribution.


EN European Standard e.g. BS EN 50342-2:2007 relates to: Lead-acid starter batteries and is a combined British and European standard (Dimensions of batteries and marking of terminals)


A paint type which forms a film by absorbing oxygen during curing. Characterised by slow drying and having a highly glossy, finished appearance without polishing.

End Float

Longitudinal play in shaft, intentional or otherwise.

End Play

Alternative term for end float. Longitudinal play in shaft, intentional or otherwise.


An ability to perform work. SI unit is the 'Joule'.


The act of making a connection between two gears of through a clutch.

Engaging Pin

A simple pin used to make a drive connection.

Engaging Plunger

General term to describe a plunger that engages something! Often part of a gearbox.

Engine Analyser

Complex test equipment used to check a variety of engine operating functions and conditions. An oscilloscope is usually included.

Engine Analyzer

Complex test equipment used to check a variety of engine operating functions and conditions. An oscilloscope is usually included.

Engine Block

Main part of the engine that contains the cylinders.

Engine Compartment

The space under the bonnet!

Engine Load

How hard the engine is having to work. For example, accelerating uphill creates a high load.

Engine Management

General term used to describe the control of engine operating functions such as ignition and fuelling. Usually related to electronic control.

Engine Map

A three dimensional map held in a microprocessor that provides an optimum set of values for the control of ignition timing, ignition dwell period and fuel quantities for all engine operating conditions.

Engine Speed Sensor

Provides information on engine speed to engine management systems (transducer).

Engine Support Beams

A bar that is used to support an engine, for example, after the transmission has been removed.

Engine Temp. Sensor

Engine temperature sender unit for temperature gauge and engine management systems (thermistor).

Engine Torque

The amount of turning force produced by an engine. Usually quoted at a set speed.


Process of increasing the mixture strength / reducing the air fuel ratio.


European Onboard Diagnostics


EOS Exhaust Oxygen Sensor. Provides information to the fuel system ECU on the oxygen content in the exhaust gas. Also EGO Exhaust Gas Oxygen Sensor, HEGO Heated Exhaust Gas Oxygen Sensor and Lambda sensor.


Environmental Protection Agency

Epicyclic Gear

See Epicyclical Gearing.

Epicyclical Gearing

A gear system, consisting in basic form of an internally toothed annulus or ring gear and a central externally toothed sun wheel, with usually three or four planet wheels or gears meshing with the sun wheel and annulus. The planet wheels are often carried as a unit on a planet carrier or spider. Holding the rotation of the planet carrier, annulus or sun wheel will provide a transmission ratio between the two remaining elements. Planetary gear sets can be coupled to increase the available number of ratios.


An epitrochoid is a shape traced by a point attached to a circle of radius r rolling around the outside of a fixed circle of radius R, where the point is a distance d from the centre of the exterior circle. It is the shape of the cylinder in a Wankel engine.


EGR Valve Position Sensor


A type of paint, adhesive or plastic noted for high mechanical strength, good adhesion and excellent chemical resistance.


Erasable Programmable Read Only Memory.


Exhaust Electronic Pressure Transducer.


Electronic Spark Control


Electronic Spark Timing


A type of organic compound used as active solvents.


An opinion or judgement.


Emergency Testing – the process by which testing may continue when the computerised system is not available due to a wide scale failure affecting a significant number of VTSs.

Etch Primer

Primer with an acid (usually Phosphoric) content. The purpose of the Etch primer in a coating system is to improve the adhesion and corrosion performance of the undercoat system. Their use is recommended when working with ferrous substrates, and essential on non-ferrous substrates


The process of a strong chemical cutting into unprotected parts of a metal.


Ethyl-alcohol produced from starches and sugar plants and used as an octane enhancer.

Ethyl Acetate

A fast evaporating ester solvent.

Ethyl Alcohol

An inexpensive, fast evaporating solvent or diluent.

Ethylene Glycol

Anti-freeze. Chemical used to lower freezing point of coolant.


The European Union (EU) is an economic and political union of 27 (as at 2010) member states, located primarily in Europe. Committed to regional integration, the EU was established by the Treaty of Maastricht on 1 November 1993.

Evaluation Circuit

An electronic circuit used to decide a course of action.


Emission control system that uses a charcoal canister to prevent fuel tank vapours from venting to the atmosphere. The vapours are drawn into the engine during specific running conditions to purge the canister.


The change from liquid to a gas. When solvents leave a wet paint film, they usually do so by evaporation.

Evaporation Rate

The speed at which any liquid evaporates.

Evaporative Canister

A canister usually containing charcoal that prevents emissions from the fuel tank due to evaporation.


A radiator in the air conditioning system fitted in the heater and A/C housing. The refrigerant passes through the pipes in the evaporator and vaporises to absorb heat and therefore cool the air stream flowing into the vehicle passenger compartment.


This is what a candidates needs to produce to prove they have the skills required.


EGR Valve Position Sensor


Electronic vacuum regulator solenoid on EGR system.

Excess Pressure Valve

A pressure relief valve that opens if pressure exceeds a set value. Often for safety reasons.

Exhaust Blow-by

Gases from the exhaust bypassing a component, for example, the piston rings or a valve in a turbocharger.

Exhaust Emissions

Substances vented into the atmosphere from an exhaust system.

Exhaust Gas Analyser

An instrument for measuring the constituent gases in the exhaust.

Exhaust Gas Analyzer

See Exhaust Gas Analyser.

Exhaust Gas Recirculation

An emission control system that uses an exhaust gas feed into the intake air stream to reduce the mass of oxygen in the air charge and limit the formation of harmful oxides.

Exhaust Manifold

Heat resisting ducting that connects the exhaust ports of an engine to an exhaust pipe.

Exhaust Ports

Outlets in the cylinder head for the exhaust after the valve opens. In a two-stroke engine the ports are in the side of the cylinder.

Exhaust Stroke

The last stroke of the four stroke cycle when the burnt gases are expelled to the atmosphere.


An increase in size. For example, when a metal rod is heated it increases in length and diameter. Opposite of contraction.

Expansion Box

Open muffler in the exhaust system to reduce the noise.

Extender Pigment

An inert, usually colourless and semi-transparent pigment used in paints to fortify and lower the price of pigment systems.

Extension Housing

Usually describes an extra housing on the rear of a transmission gearbox.

External assessment

A test set externally to check portfolio evidence.

External Customer

One outside of an organisation.





Fast CAN


Front wheel drive."


The colour perceived by the brain when a painted surface is viewed from an angle of 90° - that being the angle at which the greatest amount of light reflection will occur

Factory-applied Paint

Paint applied to a vehicle at the factory of the original equipment manufacturer (OEM). Factory-applied paint is normally cured by baking.


The gradual loss of colour or gloss of a paint film due to a chemical or physical change, usually due to natural ageing or exposure to harmful environmental conditions

Fail Safe

A device or feature that prevents total failure in the event of a fault occurring.


The process by which testing may continue when the computerised system is not available at a VTS, for example, following the breakdown of the VTS device.

Fallback Review

The process by which test data recorded while testing under Fallback is subsequently confirmed on the MOT computer database.

Fan (fan width)

The spray pattern of paint as it is emitted from a spray gun. Most spray guns have adjustments that can alter the fan width from a small round pattern to an elongated elliptical pattern.

Fan Cowl

A protective shield around a fan.


The unit of capacitance.

Fascia Panel

The panel inside the vehicle below the front windscreen. It carries the instruments, heater ducts and accessories.

Fast Charge

Rapid charge for a battery. 5 times the Ah capacity for about an hour is the recommended maximum.

Fast Idle

The cold engine running speed achieved by a stepped cam on the throttle linkage or by an air bleed valve.


Failure in a material due to excessive and repeated loading usually of an intermittent nature.


The cause of a problem that results in symptoms

Fault Code

A code generated in and output from an ECU. Used for diagnostics.


In engineering, fault-tolerant design, also known as fail-safe design, is a design that enables a system to continue operation, possibly at a reduced level, rather than failing completely, when some part of the system fails.


Feedback Carburettor System


Feedback Carburettor Actuator


Fuel Control Solenoid


Fuel Deceleration Valve

Feather-edge Splitting

Fractures or cracks along the feather-edge which occur during drying or shortly after the topcoat has been applied over primer-surfacer. This problem occurs due to poor preparation, use of excessively fast solvents in primer, improper flash-off time and/or very aggressive solvents in topcoat.


Tapering a paint edge from substrate to topcoat or between layers of paint material by sanding. If feather-edging has been carried out efficiently, it should not be possible to feel the edges with the finger tips.


Term used to describe the way a tyre wears when the tracking is set incorrectly. The tread scuffs up like feathers.

Feed (leather)

The process of treating leather with soaps and creams to improves its lifespan and appearance.

Feed Cable

The main supply wire to a component.


A feature used in electronic control systems to monitor performance by the use of a sensor downstream of an actuator. An example is an exhaust oxygen sensor monitoring the fuel injection quantities.

Feedback Resistor

A variable resistor used to indicate position of a moving component.

Feeler Gauge

Set of accurate blades used to set or measure small gaps (valve clearances for example).


These are chemical compounds, ceramic with iron oxide as their main components. Many of them are magnetic materials and they are used to make permanent magnets, ferrite cores for transformers, and in various other high tech applications.


Describes any metal composed of or containing iron, such as low-carbon steel, stainless steel, etc.


Fuel Injection


Very fine fibres of glass that are supplied as reels of single fibres or spun together as woven matting. It is used in the manufacture of glass-reinforced vehicle bodies, panels, and for component parts and repairs on automobile bodies.

Field (Magnetic)

A magnetic flux about a permanent magnet or live electrical conductor.

Field Circuit

An electrical control circuit for the field coils used to regulate the output from a generator.

Field Coil

An electrical winding round a magnet pole shoe that is used to create a magnetic field when an electrical current flows through the windings. The direction of current flow determines the polarity of the magnetic pole. Used in generators and electrical motors.

Field Coil (Winding)

An electrical winding round a magnet pole shoe that is used to create a magnetic field when an electrical current flows through the windings. The direction of current flow determines the polarity of the magnetic pole. Used in generators and electrical motors.

Filament (Bulb)

The tungsten element in a bulb that glows to produce light.


The tungsten elements in a bulb that glow to produce light.

Filiform Corrosion (Spidering)

A paint defect caused by moisture ingress between the substrate and the paint film, e.g. via an untreated stone chip, that continues to travel in 'spidery' threads.


Commonly, a heavily pigmented sprayed foundation paint used to fill imperfections and pores in a substrate. To avoid confusion with 'Body Filler', should be referred to as 'Primer-Filler' or 'High-Build Filler'

Filler Neck

The connection into the fuel tank for refuelling.

Fillet Radius

A machined curve between two faces of a component. An example is the curve between a crankshaft web and journal.


A very thin continuous sheet of material. Paint forms a film on the surface to which it is applied.

Film Thickness

The thickness or depth of a dry paint film is important. Thin films may appear to be the wrong colour due to their lower opacity, and may also have durability issues. Over-thick films are likely to crack in use. The thickness of paint films on iron or steel surfaces is easily determined with a magnetic gauge.

Film Thickness Gauge

A device used to measure the coating thickness on a substrate. Magnetic devices are used to measure the thickness of ferrous metals; whereas electronic devices are used on non-ferrous substrates.


A fine mesh or paper screen used to remove dirt and other contaminants from a liquid or gas (air).

Final Drive

The gearing immediately before the driven wheels. It provides a split to each driven wheel with a final gear reduction for the transmission. Can also be used to turn the drive through 90° with the use of bevel gears.

Finesse coat

See 'Drop coat'


A protective coating of paint; usually referring to the final appearance of the painted surface. Also, to apply a paint or paint system.

Finishing Film

A fine graded abrasive disc for sanding top coat automotive paints before repainting or compounding.


Substantial structural panel of vehicle body in front of passenger compartment. Also known as a bulkhead.

Firing Order

The sequence for ignition of individual cylinders in an engine.

Fish Eyes

A surface depression or crater in the wet paint film. Fish Eyes are caused by repulsion of the wet paint by a surface contaminant such as oil or silicone. The depression may or may not reveal the surface under the paint, depending on its depth or severity.

Fish-Eye Eliminator

Additive used in paint to prevent the occurrence of fish eyes (craters/silicones) in a freshly painted surface - usually caused by the presence of contaminants such as wax polish.

Fixed Caliper

A disc brake caliper in which the two halves and pistons are made as a single unit attached to the axle assembly.

Fixed Choke Carburettor

A carburettor with a fixed diameter venturi (choke tube).

Fixed Venturi

A narrowed section in a carburettor - the size of which remains fixed.


A decorative pigment consisting of flat particles. Usually mica, aluminium or other bright metals, providing special colour effects to the final paint finish.

Flake Orientation

The appearance of the metallic particles in a paint film during and after drying or curing. Selection of the correct flakes in the colour mixing formula and proper application will lead to good flake orientation and thus to good colour match and appearance.


A serious paint defect characterised by the paint film lifting and eventually falling off of the substrate.

Flame Plug

Cold start device fitted in the inlet manifold on some direct injection diesel engines.

Flame Trap

Incorporated into the air cleaner unit to prevent the spread of fire caused by fuel igniting in the carburettor if blow back occurs.


Flat face to flat face butted fitting. Method used to secure exhaust system.

Flange Bolts

Bolts that secure a flat face to flat face butted fitting.

Flange Fitting

Flat face to flat face butted fitting. Method used to secure exhaust system.

Flare Joint

A method of connecting pipes, particularly steel brake pipes, where the end of the pipe is deformed to give a concave or convex flare which is secured with a union nut.

Flash Rusting

A surface rust occurring from improper cleaning or exposing bare metal to the air and moisture for too long of a period.


The first stage of paint drying where some of the solvent content evaporates. This typically dulls the surface from an exceedingly high gloss to a normal gloss or flat appearance.

Flash-off Time

The time between paint application and consecutive coats.


The temperature at which a liquid will give off a flammable vapour that would ignite when a spark is struck.

Flat (finish)

Lacking in gloss. (See Matting agent)

Flat Seat Valve

A valve that operates by one part operating against a flat component to control flow.

Flat Spot

A temporary drop in power that occurs at the point of initial acceleration when the throttle plate is opened. Occurs as a result of a rise in venturi pressure (drop in vacuum) and a resultant loss of fuel supply into the venturi. Remedied by the addition of an accelerator pump.

Flat-seat Valve

A valve that operates by one part operating against a flat component to control flow.


The rubbing down of repaired or pre-painted surfaces with abrasives to achieve the desired shape, prepare the surface for painting or prior to polishing.

Flex Plate

The mounting plate for a torque converter where it is attached to the crankshaft. Fits in place of the flywheel.


Being able to accept change.

Flexible Additive

A substance added to paint to improve its performance characteristics, such as impact resistance, flexibility, and elasticity when applied to plastic or flexible substrates.

Flexible Coupling

A rubber or fabric reinforced rubber coupling that is used to join two shafts. Usually incorporates two forks that act as a safety device in the event of failure of the rubber component.


Another word for bending.


The change in value, hue, and chroma of a metallic or pearl car finish when it is viewed from different angles.


The colour perceived by the brain when a painted surface is viewed from an acute angle (i.e. less than 90°). - the more acute the angle, the less amount of light reflection occurs, giving an impression that the colour is darker


A longitudinal cut

Flitch Panel

Structural panel of vehicle body on sides of engine.


A buoyant component used to indicate or control fluid levels. In a carburettor used with a needle valve to control the petrol level in the float chamber.

Float Chamber

Part of carburettor holding petrol at a controlled level.


The tendency of pigment particles in a wet paint film to separate from one another and concentrate in particular areas, resulting in a 'patchy' finish

Floating Caliper

A disc brake caliper that has a fixed component attached to the axle assembly and a single piston and caliper that is 'floating' by sliding or pivoting on the fixed component.


Clumping together of pigment particles within the wet paint.


The tendency of pigment particles to rise to the surface during curing and produce a colour variation at the surface and a lack of uniformity in the colour appearance through the film.

Floppy Disk

A portable disk that stores 1.44Mb of information.


The levelling characteristics of a wet paint film.

Flow Meter

A sensor that determines flow of a fluid or gas.


Fluid Level Sensor

Fluid Coupling

A drive coupling that uses a fluid to transmit drive. Also sometimes a viscous coupling.

Fluid Flywheel

A drive coupling that uses a fluid to transmit drive.

Fluid Needle

The part of a spray gun that opens and closes the fluid passage in the fluid tip.

Fluid Pump

A pump for moving or pressurising a fluid.

Fluid Tip

Part of a spray gun that meters and directs the fluid stream.


Weights that act on pivots and move outwards due to centrifugal force.


(1) Component of the clutch. (2) Massive wheel or disc attached, for example, to the crankshaft of an engine, to store energy of rotation and smooth the output from the irregular firing of the cylinders. In many automotive engines, the flywheel incorporates the ring-gear and acts as one friction face of the clutch. (3) Maintains the momentum of the engine.


Fuel Octane Adjustment.


A term used to describe the aeration of an oil.

Focal Length

The distance between the focal point and the reflector.

Focal Point

The point within a reflector from where a light source produces parallel beams.


To the point.

Follower (Cam)

The component in contact with the cam on a camshaft.

Foot Pound

Old unit of torque. Also pounds/feet. Torque is now given in the SI units of Newton metres (Nm).


When applied to tyres describes the area and shape of the part of the tyre in contact with the road.


Exerted to overcome a load.

Force Ratio

The comparison of the force applied to the force produced via some kind of machine or system.


A method of accelerating the drying of paint with the aid of heat.


A component forming process applied to hot metal with a hammer or die either by hand or machine. Also hot stamping.


Part of the door latch mechanism, clutch or gearbox. Fits around another component to hold or move that component.

Fossil fuel

This relates to any fuel made from hydrocarbon deposits such as coal, petroleum, natural gas and, to some extent, peat; these fuels are irreplaceable, and their burning generates the greenhouse gas carbon dioxide.

Four Stroke Cycle

The conventional operating principle for the modern internal combustion engine consisting of the induction, compression, combustion/power and exhaust strokes.

Four Wheel Drive (4WD) (4x4)

Transmission system that uses all four wheels of a vehicle to provide a drive. This term is usually used on vehicles with selectable four wheel drive. The term AWD for all wheel drive is used for vehicles with permanent four wheel drive.

Four Wheel Steering (4WS)

A steering system that applies turning movement to front and rear wheels. The rear wheel steer is initially opposite to the front wheels, neutral and then with the front wheels. This provides steering characteristics that aid manoeuvrability.

Four Wheel Steering 4WS

A steering system that applies turning movement to front and rear wheels. The rear wheel steer is initially opposite to the front wheels, neutral and then with the front wheels. This provides steering characteristics that aid manoeuvrability.


Fuel Pump.


Fuel Pump Relay.


A number of parts out of another number of parts.


This is an authorization to sell a company's goods or services in a particular place.

Free Electrons

The electrons in the valency shell of an atom that can link with and move on from other atoms. A material with free electron flow makes a good electrical or heat conductor.

Free Play

The amount of free movement in or between components that is not transmitted to the next component in the sequence. May also be referred to as 'backlash' in some applications such as when applied to gears.

Free Travel

The free play in a clutch linkage before it operates the clutch mechanism.


The amount of free movement in or between components that is not transmitted to the next component in the sequence. May also be referred to as 'backlash' in some applications such as when applied to gears.


Overrun of a one way clutch. Vehicle movement when the transmission is not engaged to the engine.


The number of oscillations per unit of time. Examples are a sound wave or alternating electrical current. The unit for frequency is the Hertz, which equals one oscillation per second.


Resistance a body encounters in moving over another.

Friction Disc

Term used to refer to a clutch drive plate.

Friction Discs

Component of the clutch. The centre plate that is gripped between the flywheel and pressure plate to transmit a drive to the gearbox input shaft.

Friction Material

A material with consistent performance characteristics used to provide a rubbing surface for brake and clutch components.

Friction Plate

Component of the clutch. The centre plate that is gripped between the flywheel and pressure plate to transmit a drive to the gearbox input shaft.

Front-end Solvent

A fast-evaporating solvent that leaves the paint very soon after application.


Fuel Temperature Sensor.

Foot Pound"

Fuel Distributor

The component in a mechanical injection system that controls and distributes the fuel.

Fuel Efficiency

Performance of engine/vehicle. Rate of fuel usage.

Fuel Filter

A filter to remove contamination from fuel.

Fuel Gauge Sender

Sensor, usually a variable resistor, which supplies information on fuel quantity in the tank.

Fuel Injection

The injection of fuel under pressure into the intake air stream of petrol engines by the use of electrical solenoid valves in place of a carburettor.

Fuel Line

The pipes and hoses that connect the fuel system components.

Fuel Pressure Regulator

In a fuel system the component that keeps the pressure across injectors constant. It consists of a valve, diaphragm and spring.

Fuel Rail

A tube or pipe used to supply fuel to electrical injectors. The quantity of fuel in the rail is relatively large so that the pressure remains constant when the injectors operate.


A pivot for a lever. Usually a supporting point or shaft.

Fulcrum Ring

A ring on which clutch release fingers pivot.

Full Flow Lubrication

An engine lubrication system where all of the oil flows through the oil filter before entering the engine oil galleries.

Full Load

Describes the engine condition when it is working as hard as it is able - for acceleration for example.

Full Load Enrichment

A device in a carburettor that provides additional fuel during full throttle and heavy load conditions.

Full Wave Rectification

Alternating current converted to direct current by reverse flow of the negative bias so that it is added to the forward bias electrical flow. Modern rectifiers using a diode bridge achieve full wave rectification.

Fully Floating Axle

An axle hub arrangement that carries the load on two bearings and therefore does not apply a shear or bending force on the axle shaft.


A safety device in an electrical circuit that will burn and melt to break the circuit when an excess current is applied.

Fuse Box

A central termination block in a vehicle electrical system that carries the fuses.

Fuse Rating

The rated value of a fuse before failure due to an excess current load. The rating can be either a constant or a surge value and is given in amps. Fuses are usually colour coded and marked for the fuse rating.

Fuzzy Logic

A digital control system that makes 'educated' guesses.


Front-Wheel Drive


G or g

Unit of force equivalent to the force of the Earth's gravity.


A force on an object equivalent to the force applied by the Earth's gravity.

G/C Clamp

A clamp (hand tool) shaped like a G/C.


A rubber (or similar) boot used to cover working components. It is used to keep dirt out and in some cases lubrication in place. Also slang for alligator...




See Main Gallery.


A protective process for steel sheet where a thin surface layer of zinc is applied by hot dipping the steel into a bath of molten zinc.

Ganged Switch

An electrical switch that operates two or more circuits from one switching point.


A fluid state of matter when the atoms are free. The least dense state of matter which can have three states, solid, liquid and gas.

Gas Analyser

Electronic equipment that measures exhaust gas constituents.

Gas Analyzer

See Gas Analyser.

Gas Turbine

A type of internal combustion that uses rotating blades in a compressor to provide charging of the combustion chamber. The output gas pressure is used to drive turbine blades which are connected by a shaft to drive the compressor and a transmission gearbox or in the case of aircraft jet engines to provide air propulsion.


A material that is fitted between faces to take up the irregularity of the surface finishes and provide a gas or liquid seal.


A hydrocarbon fuel made up from carbon and hydrogen elements. Used as a fuel for internal combustion engines. Also petrol.


The formation, in the can, of gaseous products in a coating material, due to the reaction between two or more constituents of the coating material or between the latter and the metal of the container. Characterised by a 'Pop' sound when opening paint containers


In transmission systems a device to guide a gear into any one position at any one time. In electronics a logic device for regulating electrical current flow according to input voltage levels.


Governor Control Module.


Gasoline direct injection. Petrol/Gasoline is injected directly into the combustion chamber under high pressure.


A method of providing leverage. Toothed wheel that meshes with another toothed wheel.

Gear Brake

Part of an automatic gearbox, the brake is often a band of friction material.

Gear Cluster

A series of fixed gears on one shaft. Used to provide gear selection by engagement with freely rotating gears on another shaft. The selection is made by locking the freely rotating gear to the shaft. This can be by the use of a sliding hub or a sliding gear splined to the output shaft.

Gear Clutch

A clutch that connects or disconnects drive to a gear.

Gear Cogs

Toothed wheels that mesh with other toothed wheels.

Gear Lash

The clearance between the teeth of two meshing gears. Also known as backlash when applied to gears.

Gear Pump

An oil pump using two meshed gears to provide a positive pumping action. The teeth are of the two gears run in mesh. Where they come out of mesh a small chamber forms to allow oil to flow in, the oil is carried round the periphery of pump in the gear teeth and when the teeth come into mesh the chamber is reduced in size and the oil forced from the pump.

Gear Ratio

Ratio of angular velocities of pairs of meshing gears. Number of teeth on driven gear divided by number of teeth on driver gear.

Gear Reduction

A gear drive that reduces the rotational speed of the output shaft below that of the input shaft.

Gear Train

Two or more gears in mesh that transmit a drive through them.


A housing containing gears to allow different ratios to be selected.

Geared Distributor

A distributor driven by a gear from the engine.


The mechanism for selecting gears in the transmission. Consists of the driver's gear lever, the linkage to the transmission gearbox, and the selectors in the gearbox.


Governor Electronic Module.




Machine for converting mechanical energy into electrical energy.


A common type or something that is used for a variety of tasks.

Gerotor Pump

A type of positive displacement oil pump that uses two gears on eccentric axes. One gear is on a shaft inside another rotating in the housing. The pumping and carrying chambers are formed between the gear teeth.


Reflection from the sun or a light onto the computer screen making it difficult to see properly.


A polish that is safe for use on fresh paints. Some glazes contain a mild abrasive that will remove minor surface imperfections. When a glaze with an abrasive is used, it should be followed by application of wax on cured paint or a hand glaze on fresh paint. A glaze also does not contain silicone.


Person who works with glass.


The ability of a surface to reflect light. Measured by determining the percentage of light reflected from a surface at certain angles.

Gloss Meter

An apparatus for measuring the gloss of paint film.

Glow Plug

Cold start device for diesel engine. Fitted into the combustion chamber and heated by an electrical current so that the injected air and fuel is heated as an aid to ignition when the engine is cold.




Popular Search Engine.


A control device that limits the speed of a unit or controls or senses the operation of a unit. An example is the governor in a diesel injection pump which controls the engine speed.

Governor Pressure

The control pressure in an automatic transmission that senses vehicle speed. It is applied to the hydraulic control unit or valve body to effect gear shifts in relation to vehicle speed.

Governor Valve

The valve in an automatic transmission, which is fitted on the output shaft. The rotational speed acts on a spring-loaded valve to regulate pump pressure. The governor pressure acts on the gear shift control plunger in the valve body.


Grams Per Mile


Global Positioning System: This is a navigational system involving satellites and computers that can determine the latitude and longitude of a receiver on Earth by computing the time difference for signals from different satellites to reach the receiver.

Grab (Brake)

A braking condition where the brakes lock up when light pedal effort is applied. Caused by brake linings digging into the drum or disc on application. Usually cured by forming a chamfer on the leading edge of the lining.


Forming well written, easy to read sentences, paragraphs and documents with the use of punctuation (i.e., full stops, commas etc.).


A device that propels rocks at a painted substrate to measure the resistance the finish has to rock chips.


The attraction that all masses experience as a force towards the earth - or any other planet for that matter!

Gravity-feed Gun

A spray gun which has the paint container on the top of the gun, allowing the paint to flow into the fluid passage by gravity.


A thick sticky lubricant made from a variety of bases, usually derived from crude oils, and used for bearing and chassis lubrication.

Greenhouse gases

Gases that are a natural part of the atmosphere. They absorb and re-radiate the sun's warmth, and maintain the Earth's surface temperature at a level necessary to support life.


Abrading painted or unpainted metal surfaces to remove paint/corrosion or prepare the area for filling.


Refers to the abrasive size used in flatting papers, discs, etc. and is classified using 'P' grades, e.g. P40, P120, P600 and so on. The larger the number, the finer the abrasive


A paint product is said to be gritty when it contains large particles, from insufficient dispersion during mixing or when it contains large hard particles of foreign materials.


A blind grommet is a rubber insert fitted into a hole in a plate to blank off the plate. Other types are used to provide protection for cables or pipes that pass through a hole.

Groove (Tire)

A channel cut in the tread of a tire to facilitate the dispersal of road surface water.

Groove (Tyre)

(1) A channel cut in the tread of a tyre to facilitate the dispersal of road surface water. (2) The machined slot around the piston into which the rings are fitted.

Ground (Earth)

The electrical termination for components connected into a vehicle direct current ground return circuit. On mains electricity a safety route for an electrical short circuit used as a means of protecting people or electrical components from electrical overload.

Ground (Earth) Cable

The battery cable connecting the return side of a vehicle circuit to a terminal on the battery where a ground return circuit is used.

Ground Clearance

The dimension under a vehicle which clears any obstacles in the roadway.

Ground Test

A test to make sure the earth or ground or chassis electrical connection is in good working order.


A safety practice where two objects are interconnected with clamps and bare wire. This equalizes the electrical potential between the objects and helps prevent static sparks that could ignite flammable materials.

Gudgeon Pin

The piston pin that connects a piston to a connecting rod. Fitted at the little end of the connecting rod. Also wrist pin.


Petroleum distillate formed during combustion particularly with poor fuel quality. The gum is deposited on cylinder walls and increases the tendency for glazing.

Gun Body

The main part of the spray gun to which all other component parts are attached.


Gross Vehicle Weight



Health And Safety at Work Act 1974.(UK)"



Half Shafts

Shafts by which power is transmitted from final drive to driven wheels in an enclosed axle.

Hall Generator

An electronic pulse generator using the Hall effect of a semiconductor to provide information to the ignition amplifier or ECU on engine position and speed.

Hall IC

Integrated circuit as part of a Hall sensor.

Hall Sensor

An electronic pulse generator using the Hall effect of a semiconductor to provide information to the ignition amplifier or ECU on engine position and speed. Also used for sensing road speed etc.


A description of a series of gases such as argon.


Hand operated parking brake usually with a mechanical linkage. Also forms the reserve or emergency brake on many vehicles.

Handbrake Cable

A cable that operates the brakes when a lever is pulled.

Handbrake Linkage

Cables or rods used to operate the parking/handbrake.

Hard Disk

A storage device that holds large amounts of data.


A chemical specifically designed to ensure cure of a paint finish, filler, stopper, resin or adhesive. Also, another name for an activator.


The physical components of a computer system.


A physical connection between two or more electrical devices.

Harmonic Balancer

Rotating or oscillating engine balance shaft.


The collection of vehicle cables wound together with an insulation tape. Also known as a 'wiring loom'.


Risky. Potential to harm people or property.


The development of a cloudy area in the paint film or in a clear liquid.


(1) Hydrocarbon. (2) High compression.

Head Gasket

Seal between cylinder block and cylinder head.


Viewing a repaired/re-painted area from an angle that is perpendicular (at 90°) to the car.

Head-up Display

Projection of instruments onto the windscreen so that the road and the instruments can be seen at the same time by the driver.

Header Tank

The expansion tank at the top of a coolant radiator, which can be part of the radiator or a separate tank.


Vehicle lighting that provides illumination of the road for the driver. Usually consists of switched main and dipped beams.

Headway Sensor

A device usually fitted in the front grill of a vehicle that in conjunction with a transmitter senses distance in front of the vehicle. Often part of an active cruise control system.


A form of energy in all materials that can be used to do work.

Heat Exchanger

A device for transferring heat from one system or place to another.

Heat Range

Applies to spark plug construction where the rate of heat loss from the plug nose is controlled by the length of the insulation. Spark plugs are given a rating from hot to cold to classify the heat range and application. The plug tip temperature is controlled to keep the tip and electrodes clean.

Heat Sink

A plate to which electronic components are attached to dissipate heat.

Heater Matrix

Small radiator used to exchange heat from the coolant to the vehicle cabin.


Heated Exhaust Gas Oxygen Sensor. Also EGO Exhaust Gas Oxygen Sensor, EOS Exhaust Oxygen Sensor and Lambda sensor. Provides information to the fuel system ECU on the oxygen content in the exhaust gas.


High Energy Ignition. A distributor ignition system using electronic control of the coil secondary output.

Helical Gear

A gear with teeth cut at an angle.

Helical Rotor

A turbine or compressor in a turbocharger often has vanes that are helically shaped.

Helical Spring

Metal spring used in vehicle suspension. Also known as a coil spring.


A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball - a hemisphere is half a sphere


A fuel used in research tests as a comparison when determining the octane rating of fuels. Octane value of zero.


The number of oscillations per unit of time. The SI unit for frequency is the Hertz which equals one oscillation per second.


Short for hexagonal.



Hide Mallet

Soft hammer made of hide (leather). Plastic types are also used.

Hiding or Hiding Power

(See 'Opacity')

High Boilers

Solvents with boiling points above 100 degrees Celsius.

High Lift Cam

A special cam profile used on performance engines to increase the opening size of a valve.

High resistance

An unwanted resistance in a circuit that causes it to operate incorrectly.

High Solids

Paints are described as having high solids when they contain more than 50-60% solids. High solids paints have lower VOC's.

High Tension

The term used for the high voltage in the secondary circuit of the ignition system.


Hall generator. A sensor using a Hall effect to provide signals to an ECU.


Heated Oxygen Sensor


To lift something or a term used to describe a vehicle lift or ramp.

Hold-in Winding

A winding in a starter motor solenoid that holds the switch contacts together and the drive pinion in mesh during engine starting.


The ability of a surface to keep the topcoat from sinking in and causing a decrease in appearance or gloss.


Uniform in structure or composition throughout. This is usually used to refer to a fuel air mixture in a cylinder that is a 'perfect' mixture of fuel and air.


A light grinding process applied to cylinder bores in preparation for the fitting of new pistons.

Hooke's Joint

A drive shaft universal joint consisting of two yokes and a cross bearing. Also Cardan joint.

Horizontally Opposed

An engine configuration where cylinder pairs are fitted on opposite sides of the crankshaft. Sometimes referred to as 'Boxer' engines.


A measure of power equivalent to 0.746 Kilowatts. Defined by a rate of doing work of 33,000 foot pounds per minute (550 foot pounds per second).


Heated Oxygen Sensor.

Hose Clamp

Simple ring type clamp to seal a hose onto a pipe.

Hot Idle Compensator

A device to change idle or mixture settings when an engine is hot.

Hot Spray

The technique of applying paint at elevated temperatures. The elevated temperature reduces the viscosity so that higher solids materials can be sprayed, and allows application with less solvent.

Hot Stamping

A machine forging process using a series of dies to create the completed shape.

Hot Wire

A small thin wire that is heated an placed in an air flow. The change in temperature is used to determine the rate of air flow.




High Tension

HT Cables

High tension leads (high voltage) used to supply the spark.

HT Leads

High tension leads (high voltage) used to supply the spark.


Centre of a wheel or gear or similar.

Hub Nut

Main nut that secures a wheel hub.


The quality by which colours are distinguished from one another, such as">a red from yellow, blue, green, or purple, and all shades in between.


Uneven running of an engine, usually when the mixture is over rich.


Heating, ventilation and air conditioning.

HVLP (High Volume, Low Pressure)

Describes a paint gun that uses a high volume and low pressure of atomising air to apply material to a surface. This provides high transfer efficiency and lower overspray.


A hybrid vehicle is a vehicle that uses two or more power sources to move the vehicle. The term most commonly refers to hybrid electric vehicles (HEVs), which combine an internal combustion engine and one or more electric motors.

Hybrid vehicle

Vehicles that use both a traditional engine (combustion) and a rechargeable system (typically operated by a battery) to improve mileage, cause less pollution, and decrease need for fossil fuel.


A system for transmitting and modifying force by the use of a liquid. Consists of a pump, control devices, hydraulic lines and slave cylinders.

Hydraulic Brake

Brake actuated only by hydraulic pressure.

Hydraulic Clutch

A clutch, operated by fluid pressure.

Hydraulic Hoses

Hoses that carry fluid under high pressure.

Hydraulic Lifters

Hydraulic cam followers in the valve opening mechanism.

Hydraulic Pedestal

Self-adjusting support for a finger tappet in the valve opening mechanism.

Hydraulic Pressure

Pressure applied by a fluid.

Hydraulic Tappets

Self-adjusting cam followers in valve opening mechanism.

Hydraulic Valves

Valves used to control pressure or flow of a hydraulic fluid.


A compound which contains only carbon and hydrogen, which are distillate by-products of petroleum, natural gas and coal.

Hydrocarbon (HC)

Chemical compound which contains only, or principally, Carbon and Hydrogen. The principal constituent of liquid fossil fuels.

Hydrocarbon Emissions

Fuel vapours released into the atmosphere from a vehicle exhaust or fuel tank.


To break down (a hydrocarbon) by the process of hydrocracking. This is a catalytic cracking process of breaking down large hydrocarbon molecules into smaller ones


Reaction with water. Usually refers to a decomposition process leading to paint failure.


Instrument used for measuring specific gravity/relative density of a liquid (coolant, battery electrolyte).

Hydropneumatic Suspension

A suspension system using a liquid to transfer suspension movements to pneumatic (gas) springs.


Instrument used for measuring relative humidity - in an automotive context it is used to check moisture content of brake fluid.


A paint film or other solid that readily absorbs water.

Hypoid Gearing

Bevel gear with the axes of the driving and driven shafts at right angles, but not in the same plane, giving some sliding action between teeth. Widely used in rear axle final drives.


A characteristic of materials, sensors and sometimes instruments to make their behaviour dependent on the immediate history to which they have been subjected. Typically the final settling point is different when approached from above to when it is approached from below



Independent Front Suspension. Also inertia fuel shut off switch used to switch off an electrical fuel pump or diesel fuel pump solenoid when the vehicle is involved in an accident."


Independent Rear Suspension."


Idle adjust. Idle speed adjust.


Idle Air Control


Intake air temperature sensor used in electronic fuel injection as a correction factor for the quantity of fuel to be delivered.


Intake Air Temperature Sensor


Integrated Circuit


In Car Entertainment system.


Ignition Control Module.


Idle Control Solenoid


Information and Communication Technology.


Inside Diameter


Indirect injection of diesel fuel into a pre-combustion or swirl chamber in the cylinder head.


Term used to describe an engine running at a speed when the throttle has not been depressed. Also used to describe a lazy technician...

Idle Air Control (IAC)

Idle air control. An air valve on fuel injection systems to increase the air flow and idle speed of a cold engine and during warm up.

Idle Control Components

Parts that are designed to keep the engine idle speed at a set level.

Idle Mixture

The fuel strength or air/fuel ratio.

Idle Speed

Rotational speed of an engine on no-load and minimum throttle setting. Also tick over and curb idle.

Idler Arm

A steering arm parallel to the Pitman arm (drop arm) and connected to it by a centre track rod.

Idler Gear

(1) A gear between two others that does not affect the gear ratio. Any gearwheel between the driving and driven gears in a gear train, the shaft of which serves only to bear and locate the gear, and is therefore 'idle'. The idler gear may serve to reverse the rotation of the driven wheel, so that it rotates in the same direction as the driving wheel. (2) An intermediate or tensioning pulley for a belt drive.


Describes an engine running at idle speed (usually 700 to 800 rev/min).


International electrotechnical commission


Indirect Fuel Injection


The insulated gate bipolar transistor or IGBT is a three-terminal power semiconductor device, noted for high efficiency and fast switching.




The part of an air bag unit that sets the inflation process in to action.

Ignition - IGN

Starting of combustion. Setting fire to a flammable substance. In a petrol engine the spark jumping the spark plug electrodes that starts the combustion process.

Ignition Advance

The automatic bringing forward of the ignition spark in relation to the engine speed.

Ignition circuit

The system used to connect components involved in producing a high voltage at the spark plugs.

Ignition Coil

An electrical transformer that increases the battery voltage to a high tension capable of arcing across the gap of spark plug electrodes.

Ignition Coil - IGC

An electrical transformer that increases the battery voltage to a high tension capable of arcing across the gap of spark plug electrodes.

Ignition Distributor

A component in the ignition system that houses the primary circuit switching device and the secondary circuit distribution components.

Ignition Lag

A short delay after injection before the fuel ignites.

Ignition Module

The electronic unit that switches the ignition coil on and off in response to a signal from a pulse generator.

Ignition Switch

The engine ignition and vehicle electrical circuits control switch used by the driver to start and stop the engine.

Ignition Timing

The static and dynamic settings of the ignition distributor or in the engine control module (ECM) that control the ignition coil secondary circuit output and spark for starting the combustion process in the engine.


Lighting up, making bright.


Integrated Motor Assist is Honda's term for their hybrid propulsion system. This includes the electric motor/generator, 144v battery pack, controlling electronics and the wiring that connects them all together. ISG or ISA is used by some manufacturers, Integrated Starter Generator or Alternator.


Taking place without delay.

Immiscible (non-miscible)

Incapable of mixing in or blending with other components.


Usually part of an alarm system. It cuts off the ignition or fuel to prevent a vehicle from being started and stolen.

Impact Switch

A switch that operates when the car impacts something. Used as a safety device to cut off a fuel pump.

Impact Tools

Power tools that use a high frequency hammering action for the removal and installation of nuts and bolts, for drilling and chiselling. Impact tools should only be used with sockets, drills and chisels made for that purpose. The use of normal hand tools form impact is hazardous as these may shatter in use.


Vaned disc that creates flow by centrifugal force (water pump). In a torque converter it is driven from the engine to force liquid onto the turbine.


Old British standard of measurement using yards, feet and inches.

Imperial Measurement

Defined by three measures - the gallon, the yard and the pound.

In-line Engine

A engine fitted along the axis of a motor vehicle.

Inboard Brakes

Brake units fitted on the final drive casing and output shafts rather than on the wheel hub and carrier.


Light given off by a high temperature object. Usually describes a bulb filament.

Included Angle

Camber and steering swivel (king pin) inclinations (angles) added together.

Incoming Telephone Call

To receive a telephone call.

Independent Suspension

Suspension fitted to each wheel."

Indirect Injection

Diesel engine design where fuel is injected into a pre-combustion chamber fitted inside the cylinder head.

Induced Voltage

Naturally occurring voltage in a conductor when subjected a magnetic flux.


Property of a coil such that an increase in current causes an increase in magnetism, which in turn tends to prevent the original increase in current. See also Lens's law.


1. The drawing in of a gas or fluid. 2. Process of causing an electrical potential in a conductor due to relative movement of a magnetic field.

Induction Period

An allotted amount of time upon mixing of components for compatibilization. The process is common among the use of epoxy/polyamide coatings.

Induction Stroke

First stroke of four stroke cycle when air and fuel enter the engine as the piston moves down the cylinder. Also known as the intake stroke.

Inductive Coil

All coils possess inductance. This is the property that resists changes in current flow.

Inductive Generator

A magnetic/electrical pulse generator used as a sensor in electronic control systems. It uses an iron core and electrical winding to induce an electrical current when passed by a toothed wheel or rotor. Also known as a magnetic pulse generator.

Inductive Sensor

A coil and magnet used in conjunction with a trigger wheel. It produces a sine wave output with a frequency proportional to speed.

Industrial Fallout

Chemical compounds present in the air which are deposited on the horizontal surfaces of vehicles and which may damage the finish.

Inert Gas

A non-reactive gas element.


The force that holds a body stationary or in uniform motion.

Inertia Reel

Type of seat belt that holds spare webbing on a reel and locks the reel on impact. This is performed by a swinging weighted ball and lever that locks the belt reel ratchet wheel with a pawl.

Inertia Switch

A switch that operates by the inertia (need to stay at a constant speed or state of rest) of a mass. Usually used to cut off supply to a fuel pump in case of a crash.


The general increase in the price of goods and services.


Light energy that produces radiated heat, used for speeding up the curing of paint, filler, etc.

Infrared Light

That portion of the spectrum responsible for most of the heating effects of the sun's light. Not visible to the human eye.

Infrared Lighting

Infrared light is outside the visible spectrum. When used for lighting, a special detector is also needed.

Infrared Sensor

A sensor that is sensitive and produces a signal in response to infrared light.


Take in by swallowing or absorbing.


Breathing in.


To hinder or to restrain progress.


A chemical added to retard or prevent a particular reaction.

Inhibitor Switch

A simple switch that prevents the operation of a circuit. A good example is the switch in an automatic transmission gearbox that prevents operation of the starter motor when drive or reverse is engaged.


A chemical that restricts or reduces the chemical reactions. In a cooling system an inhibitor is added to prevent corrosion of the engine from the ethylene glycol content of the antifreeze.

Initial diagnostic assessment

This is carried out to find a candidates strengths and weaknesses, current levels of attainment and potential.


A chemical added to help start a chemical reaction such as polymerization or curing.


Injector valve.


Of fuel by pressure into inlet manifolds (petrol) or combustion chamber (diesel).

Injection Lag

A short time delay between actuation and actual operation of an injector.

Injection Timing

Point within the four-stroke cycle when injection occurs. Similar to ignition timing on petrol engines.


Control valve and nozzle used to inject and atomise fuel.

Injector lag

A short time delay between actuation and actual operation of an injector.

Injector Nozzle

The tip of a fuel injector, which is drilled to produce a particular spray pattern.

Injector Pulse Width

When viewed on an oscilloscope, the horizontal width of the trace. This represents the open time of the injector.

Injector Pump

Fuel pump that produces the timed high pressure pulses for the injection and timing of fuel delivery on a diesel engine.

Inlet Manifold

Ducting that connects the carburettor/fuel injection components to the inlet ports in the cylinder head.

Inlet Ports

The way in to the engine for fuel/air mixture.

Inlet Valve

Valve in the inlet port that allows air or air/fuel mixture into an engine.

Inline Pump

Type of diesel fuel injection pump with the pump elements in line.

Inner Races

The inside rings of ball or roller bearings.

Inner Tube

The separate sealed air tube on a tyre that is required with certain types of tyre and wheel rim.

Inner Wing

Metal panel structure under the bonnet forming part of the wing.


Power or data that goes into a mechanical unit or electronic control module.

Input Device

A device that allows you to put information into the computer. Egg, keyboard, mouse.

Input Shaft

Shaft that takes drive from the engine into the gearbox.

Insert (Valve Seat)

A steel insert fitted in aluminium and some iron cylinder heads to form a durable seating for the inlet and exhaust valves.


A feature of air brakes designed to give an initial quick application to make up for the time it takes for air pressure change.

Instrument Lights

Illumination for instruments such as the speedometer.

Instrument Panel

The panel in a vehicle fascia that holds the instruments and warning lights. Also dash panel and dashboard.

Insulated Circuit

A vehicle wiring system that does not use the body and chassis as a ground to form a return electrical circuit to the battery and alternator. A separate insulated set of cables connects all components to complete the circuits. Used in vehicles where there is a high fire risk. (Fuel tankers).

Insulation resistance

The resistance to current flow of the insulating materials between circuits and contacts.


Restricts the flow of electricity or heat through the material.

Intake Manifold

See inlet manifold.

Intake Stroke

Alternative term for the induction stroke. First stroke of four stroke cycle when air and fuel enter the engine as the piston moves down the cylinder.

Integral Body Construction

Modern light vehicle body and chassis construction method which has no separate chassis. The strength is built in by the use of stressed and reinforced panels.


To incorporate something into something else or to unite into a whole.

Integrated Circuit (IC)

An electronic device consisting of transistors, diodes and other components in a single unit and usually having a specific function.


Data corruption refers to errors in computer data that occur during transmission, retrieval, or processing, introducing unintended changes to the original data. Computer storage and transmission systems use a number of measures to provide data integrity, or in other words the lack of errors.

Intelligent device

A component that has some ability to monitor itself and react to changes.


A device which converts low pressure fluid power into higher pressure fluid power (used by some injection systems)

Inter-Quartile Mean

The average of the values in the inter-quartile range.

Inter-Quartile Range

The range of numbers with the upper and lower quartiles removed.


Two-way communication.


An air to air radiator type heat exchanger fitted after a turbocharger in the air intake ducting to cool the intake air before it enters the engine.

Interference Fit

The relationship between the dimensions of two components where one fits inside the other. In this instance the fit is one where the inner dimension of the outer component is less than the outer dimension of the inner component.

Interior Lights

Lights inside a vehicle for illumination purposes.

Interleaf Friction

Friction between spring leaves that produces a damping effect.


Part of a manual gearbox selector mechanism that prevents the engagement of two gears at the same time.

Intermediate Gear

Any gear in a transmission gearbox other than top and bottom or reverse.

Intermediate Shaft

A shaft between the input and output shafts, usually as part of a gearbox.

Internal Combustion

Engine where fuel is burnt inside cylinders.

Internal Customer

One inside of an organisation.

Internal Damping

A feature of some suspension components where friction tends to reduce oscillations.

Internal Resistance

Describes the resistance of a component that is the combination of all its internal parts. Even a battery has some resistance inside it.

Internal Splines

Splines (teeth) inside a shaft or tube.

Internal Taper

A hole that narrows.

Internal verification

The process whereby a centre ensures it operates consistently and to national standards in interpreting and assessing the key skills.

Internal Windings

Windings inside a component.


A worldwide network of computers that allows us to view the World Wide Web.


To understand and be able to explain something.


To make somebody feel uncomfortable, timid or even fearful.


An atom with positive or negative charge due to gain or loss of an electron.


Integrated Power Unit.


All colours that contain aluminium, mica, or other particles that impart a metallic appearance to the colour.

Irradiation Treatment

A process where something is acted upon by radiation from a radioactive substance - it is used to make some foodstuffs last longer - but is a contentious issue!


Inertia Switch.


Idle Speed Control. This term is usually used when a stepper motor or solenoid is used for idle speed increases during engine warm-up and when ancillary systems are in use.


International Standards Organization


A fuel used in research tests as a comparison when determining the octane rating of fuels. Octane value of 100.


A chemical found in the hardening agent used with acrylic urethane and other two component reaction type paints. The introduction of Isocyanates in hardeners led to the requirement for air-fed respirators in the vehicle painting industry


Information Technology


Ignition Timing Control System


Idle Tracking Switch



A mechanical or hydraulic tool for lifting a vehicle. Should always be used with axle stands if under vehicle access is needed.

Jack Stands

Vehicle support equipment manufactured with a rated weight capacity. Used to support a vehicle when wheels are removed or when access to the underside is needed. Also known as axle stands.

Jam (Lock) Nut

A nut that is used to lock another nut or threaded component in position on a threaded rod or stud. Also lock nut - US term.

Jamb Switch

Switch fitted on a door jamb or pillar for operating the courtesy light or vehicle alarm system.


The deliberate radiation or reflection of electromagnetic energy for the purpose of disrupting enemy use of electronic devices or systems. Or informal music creation in a group!


Jet Air System


A calibrated nozzle in a gas or liquid injection system. In a carburettor used to regulate fuel and air flows.

Jet Needle

A small needle that works inside a jet (small hole) to control fuel flow in a carburettor.


A device for locating and holding a piece of work during machining operations.

Jiggle pin

Most thermostats have a Jiggle pin. Its function is to allow trapped air past a closed thermostat as a build-up of air in front of the wax capsule could cause engine damage.


Suspension bump travel due to the wheel hitting an irregularity in the road surface. Also applies when the suspension is pushed down. Also known as bounce.


The surface hardened cylindrical part of a shaft that forms part of a bearing. The journal rotates in round sleeve or split shell bearings.


A control stick that operates a system.


Irregular braking or clutch operation with intermittent snatching.

Jump Leads

Heavy duty electrical cables used to connect a slave battery to a vehicle with a discharged battery for starting. Also jumper cables.

Jump Start

Starting a vehicle engine with the aid of a slave battery and jump leads.

Jumper Cables

Heavy duty electrical cables used to connect a slave battery to a vehicle with a discharged battery for starting. Also jump leads.



See Keep Alive Memory.


Kick Down Switch.


A mechanical injection system from Bosch, but with some electronic control.

Keep Alive Memory (KAM)

Part of a microprocessor RAM that holds temporary data for a specified duration. The KAM requires a constant voltage to retain data. It is cleared when the power is turned off.


A kinetic energy recovery system is an electric generation, storage, and propulsion system used in Formula One (up to 2010 anyway), which generates electricity during slow down, stores energy in batteries or a flywheel, and boosts acceleration with electric drive motors assisting the engine. Many hybrid cars are similar.


A locating device that sits in slots on a shaft and pulley or gear wheel. Also known as Woodruff key.

Key Data

Important, relevant information.


The typewriter-like keys used to input data into a computer. An input device.



Kick Down

A switch of a cable on an automatic transmission signals that the throttle is at least 90% open. Under certain conditions this causes a downshift for improved acceleration.

Kick Down Switch

Switch on an automatic transmission that signals that the throttle is at least 90% open. Under certain conditions causes a downshift for improved acceleration.


The precipitation of dissolved binder or additive from solution as a result of solvent incompatibility.


Road shocks transmitted to the driver through the steering system.


1000 watts


Movement energy.

Kinetic Balance

The balance of a rotating component about the perpendicular to the axis. Wheel balance has both static and dynamic quantities.

Kinetic Energy

Energy in the form of a mass moving.

King Pin Angle

Alternative term for KPI (King Pin Inclination).

King Pin Inclination

The angle that the king pin axis tilts from the vertical. Used to provide centre point steering where the centre of the tyre tread is on the steering pin axis.

King Pins

A type of steering swivel fitted to beam axles.




Knock signal from an engine knock sensor to the ECM.


Advanced or quick detonation of fuel in a cylinder where the resultant pressure rise causes a shock wave through the piston, crankshaft and engine block.

Knock Sensor

A piezoelectric sensor used to provide sensor signals to an ECM when an engine is knocking. A closed loop control retards the ignition to prevent knock.


Used to describe engine knock or a noise produced that sounds like one object hitting another.


Knowledge is defined by the Oxford English Dictionary as (i) expertise, and skills acquired by a person through experience or education; the theoretical or practical understanding of a subject; (ii) what is known in a particular field or in total; facts and information; or (iii) awareness or familiarity gained by experience of a fact or situation.

Knuckle (Steering)

Steering stub axle assembly. Also known as a steering swivel.


''Key On, Engine Cranking''


''Key On, Engine Off''


''Key On, Engine Running''


Kilopascals - unit of pressure.