Automotive Glossary

Automotive Glossary

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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.

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