Automotive Glossary

Automotive Glossary

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Power Bakes


Park and neutral position switch for sending signals to the PCM.


Power Steering Pressure Switch. Also PSP switch.


Pressure Air


The relative speed of progress or change.


A material which, when applied as a liquid to a surface, forms a solid film for the purpose of decoration and/or protection. Generally, any paint contains a binder, solvent, and pigment plus other chemicals or additives giving the paint its special characteristics or properties.

Paint Remover

A chemical, usually a mixture of acid and solvents that breaks down an old paint finish by liquefying it, thus allowing it to be completely removed from the substrate.

Paint Strainer

A disposable fine mesh filter used to clean paint as it is poured into the gun cup.

Paint thickness gauge

Precision instruments to accurately measure the dried film thickness of painted objects by non-destructive testing. On ferrous substrates, they operate by measuring the resistance to the flow of magnetism between two probes, and on non-ferrous surfaces special gauges use the 'Eddy Current' principle


Pulsed Secondary Air Injection


Alternative name for a sump.

Panel Lights

Illumination for instruments and other displays.

Panhard Rod

Rear axle transverse link between the chassis and axle beam. It is used to hold the axle in relation to the body during cornering or for sideways forces.


A petroleum based thin and colourless fuel oil, (kerosene in US English).


The condition that exists when the centre lines of two or more items or components or lines do not converge. The extended lines in both directions will always remain at an equal distance apart.

Parallel circuit

A circuit that splits into two or more branches. A break in one will not stop current flowing in the other(s).

Parallel hybrid

The engine and motor can both be used to drive the vehicle (most hybrid vehicles use this system).

Parasitic draw

An unwanted, usually small, current flow from a battery.

Parking Brake

Mechanical system of braking to hold vehicle stationary.

Parking Gear

The component(s) in an automatic box that lock the drive when 'Park' is selected.

Particle Size

The size of the pigment particles in a dispersion (e.g. paint).


Exhaust pollution consisting of very small particles of matter (soot) that are produced as a by-product of combustion. Seen as smoke in a vehicle exhaust, particularly from diesel engines.


Power Assisted Steering.

Pascal (Pa)

Standard unit of pressure (N/m2) named after a scientist who studied pressure in liquids. Pascal's principle of the transmission of pressure in fluids established that the pressure in a completely filled container is equally applied throughout and any change in pressure is transferred to all parts of the fluid and walls of the container.


The conversion of a metal surface to a less reactive state. A process used by vehicle body manufacturers to reduce the corrosion rate of a metal surface, particularly low-carbon steel.

Passive Restraint

Safety features of a vehicle that act only in an accident. Seat belts, head restraints, air bags and pre-tensioners are examples.


Personal Identification, used in conjunction with a Smart Card to access the MOT database via the VTS device. Can be numbers or letters, or a combination of both.


Part of the ratchet mechanism that holds the ratchet. A pivoted tooth that engages in the teeth of the ratchet wheel to either prevent the pawl lever or the wheel from rotating in one direction only or in some mechanisms to drive the wheel forward.


Powertrain Control Module.


Positive Crankcase Ventilation. A series of pipes and valves connecting the engine crankcase to the inlet manifold or air cleaner in order to reduce oil vapour emission to atmosphere.


Power drive unit

Pearls (Pearlescents)

Finishes which include mica flakes in addition to the pigment and binder. These finishes are highly decorative and often give the impression that the colour alters as the viewing angle is changed.


Power Enrichment Control Valve


The failure of a paint film to adhere to its substrate. Peeling results when contaminated surfaces are painted, when there is an excessive difference between paint and surface polarity or thermal expansion, or when there is solvent incompatibility.


The Pentagon, is a government building with five sides that serves as the headquarters of the United States Department of Defence. A pentagon, is a five sided shape.


The proportion of one part of something to the whole. Usually out of 100 (‘per' means ‘out of' and ‘cent' means ‘hundred').

Percentage Change

Changed amounts divided by the original value, then multiplied by 100.

Perceptual Safety

A reference to how safe the driver and passengers feel.


The sum of all the outside edges of a shape.


Description of how some components, particularly if rubber-based, can decay.


At ninety degrees to another plane or line.


Volatile hydrocarbon fuel for spark ignition engines. Also gasoline.

Petroleum Jelly

A type of grease used for battery terminals. Vaseline is a refined version.


The German word for horsepower. Pferdestärke. 1PS equals 0.986bhp.


Port Fuel Injection.


Pulse Generator.

PH value

A measure of the acidity of a substance in an aqueous solution. Pure water has a pH value of 7 and is classified as neutral. Below 7 is acid and above 7 is known as basic. The scale is from 0 (strong acid) to 14 (strong base).


The time span between the start and end of a single wave moving from zero to positive and negative positions and returning to zero.

Phase Sensor

Sensor that indicates camshaft position and hence where the engine is in its cycle.

Phasing (diesel injection)

There are a number of phases of injection: fill phase, spill phase, injection phase and the pressure reduction phase.

Phosfor Bronze

See Phosphor Bronze.


A process used by vehicle body manufacturers which results in the formation of a layer of zinc, iron, or manganese phosphate crystals on the surface of the part to be painted. In modern vehicle construction plants, phosphating is achieved by complete body immersion.

Phosphor bronze

A material that can absorb oil making it ideal for making bushes/bearings.

Phosphoric Acid

An acid commonly used as a catalyst to speed up the cure of Etch primers and some baking finishes, usually alkyd/nitrogen resin combinations. Also a cleaner used for removing surface corrosion from ferrous metals.


The generation of an electrical current by light shining on certain materials such as some semiconductor devices. Photoelectric cell, photodiode or phototransistor.


A transistor that is operated by light.


A group of words in sequence.

Physical layer

Hardware, actual components and wires as opposed to software.


3.141592 (3.142).

Pick Up Pipe

Oil pipe that picks up oil from the oil pan.


The use of a chemical solution (usually acidic) to prepare a surface for painting or bonding by dissolving away surface oxides and other impurities.


Parameter ID


A special crystal that changes resistance as pressure is applied.


Voltage produced by shock waves on a piezo crystal. Used in engine knock sensors.


Small particles added to paint to influence properties such as colour, corrosion resistance, mechanical strength, etc. Pigments may be coloured, semi-transparent, black, white, or colourless. They must be incorporated into a paint system by very efficient dispersion processes.

Pilot Bearing

Small bearing usually where two shafts meet. See also Spigot.


Noise heard from an engine when the ignition of fuel occurs too early (advanced) in the cycle. Also 'ping'.


A defect consisting of tiny holes in the dried paint surface. Defects such as pin holing, popping, bubbling, and cratering are usually caused by solvent vapours forming from the substrate or the paint and rising to the surface.


A small gear wheel or the driver of a meshing pair of gear wheels.

Pinion Gear

A small gear wheel or the driver of a meshing pair of gear wheels.

Pinion Shaft

A shaft on which a small gear runs.


Noise heard from an engine when the ignition of fuel occurs too early (advanced) in the cycle. Also 'ping'.


Type of injector for an indirect injection engine. Similar to pintle but with additional small injection orifice for low engine speeds.


Type of injector for an indirect injection engine. Uses a spring-loaded needle that is raised by fuel pressure to allow a spray of fuel to enter the pre-combustion chamber.


Profile Ignition Pick Up Signal.

Pipe Union

A joint in a pipe.


Reciprocating component, usually in the form of a cylinder closed at one end, that operates under fluid pressure within a smooth walled cylinder. In a reciprocating engine, gas pressure on the piston crown provides the prime force that is converted into rotating mechanical power by the crankshaft.

Piston Blow

Term used to describe gases blowing past the piston rings.

Piston Crown

The top of the piston.

Piston Ring

Sealing rings usually made of cast iron. Fitted on to a piston to make a gas tight seal.


(1) Distance that two points are apart. Examples are thread dimensions and pitch circle diameters of concentric drillings. (2) Vehicle body movement from back to front or from front to back.

Pitman Arm

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


The appearance of holes or pits in a paint film while it is wet. May also refer to the damage caused by more serious rusting on steel panelwork

Pivot Bolt

A simple bolt on which something pivots (turns).

Pivot Point

The point about which something pivots!

Planet Carrier

Frame with spindles that the planet gears rotate on as part of an epicyclic gear set.

Planetary Gears

Gears that rotate around sun gears (differential, epicyclic gearing, etc.)


The act of forming and following a program to achieve a specific goal.


A flat metal plate for gauging the accuracy of a plane surface in precision metalworking, which is also known as a surface plate

Plastic Filler

A compound of resin and heavy pigments used to fill dents on car bodies.


A low molecular weight substance added to polymeric materials such as paints to improve their flexibility.


Brand name of a product that is used to determine bearing clearance.

Plate Grids

Basket like holders to contain the active material in a battery.

Plate Lug

A connection point on the battery plate.


A chamber in the air inlet system to provide even air supply to all cylinders.


Rayon cords used in tyre construction.

Plug Thread

Screwed thread that is designed for a spark plug.

Plunge Joint

A shaft joint or coupling with provision for axial or plunging movement.


Alternative term for a piston or needle in a pump, slave cylinder or injector.

Ply (Tire)

Fabric layer used in the construction of a tire.

Ply (Tyre)

Fabric layer used in the construction of a tyre.

Ply Rating

Load rating of a tyre.


Transfer of force by air or gas.

Pneumatic Suspension

Suspension system using air or gas springs.


Park/Neutral Switch


A tool in the form of a short bar, usually tapered and often incorporating a spanner/wrench at one end.


The switching contacts of an ignition contact breaker.

Polarised Light

Light that has been filtered so that its rays oscillate only in one direction.


A description of a component that must have positive and negative connections connected correctly.

Polarity (Electrical)

Electrical positive and negative charges.

Polarity (Magnetic)

Magnetic north and south poles.

Polarized Light

Light that has been filtered so that its rays oscillate only in one direction.


The ends of a magnet where the field lines are emitted.


A specially formulated blend of components designed to remove minor paint surface imperfections, such as fine scratches, light oxidation, water spots, and swirl marks left by the use of rubbing compounds.


The use of polishes, either by hand or machine, to level and provide more gloss to a finish.

Polishing Compound

A mild abrasive material applied to a vehicle's painted surface which removes minor imperfections with minimal cutting action. Buffing restores film to a high gloss appearance.

Polishing Wax

A combination of wax and polish that contains mild abrasives. The abrasives remove minor paint imperfections. The wax and other ingredients produce a durable, high-gloss finish.


Products or by-products that destroy purity or make environments foul or filthy.


A 2K resin used as the binder for plastic fillers, fibre glass laminations and high-build primers

Polyester Stopper

A finer grade 2K plastic filler material used to fill minor imperfections such as pinholes in filler prior to priming.


A naturally occurring or synthetic compound consisting of large molecules. Used for many purposes, one of which is as a polish.


Manmade plastic material used for electrical insulation, bumpers and other trim components.


A 1K or 2K plastic-based paint finish that is characterised by durability and high gloss. Also, used for manufacturing some plastic and flexible parts.

Poppet Valve

Conventional inlet and exhaust valves having a circular plate at right angles to a central stem that runs through a guide tube.


A paint defect characterized by raised domes in the surface caused by solvent vapour forming within the paint after it has begun to "skin over." Often caused by baking at too high a temperature or before adequate flash-off time has been allowed.

Population of UK

Number of people that live in the UK.


Property of a material that allows fluid to soak into or through it.


Hole or aperture shaped to facilitate the flow of gas or liquid into or from a chamber. Usually denoted by its function, as inlet port or exhaust port.

Port Fuel Injection

Petrol electronic fuel injection system with injectors in each inlet manifold spraying fuel in towards the inlet valves in the ports of each cylinder. Also known as multipoint injection.


This is usually a folder that contains the evidence chosen to illustrate competence to satisfy individual key skills requirements.

Position Switch

A switch used to feedback position of a moving component.

Positive Camber

Steering geometry where the road wheel leans outward at the top.

Positive Castor

Angle in side elevation between the steering and vertical axis. Steering axis hits the ground in front of the vertical axis.

Positive Communication

A comment or statement expressing approval.

Positive Crankcase Ventilation

A series of pipes and valves connecting the engine crankcase to the inlet manifold or air cleaner in order to reduce oil vapour emission to atmosphere.

Positive Displacement

A term applied to pumps which transfer liquid in the same quantity as the volume in the pumping chamber for each action of the pump.

Positive Offset (Wheel)

The position of the wheel nave or disc when outside the centre line of the wheel.

Positive Scrub

Positive scrub radius steering geometry where a line through the steering swivel axis meets the road surface inside of the tyre centre line.

Positive Temperature Coefficient

Type of resistor that has resistance increasing with increases in temperature. The normal response for metal conductors. These resistors are used in electrical circuits to give temperature sensing and control.


Energy stored in a stationary object or chemical combination that has the capacity to do work.

Potential Difference

Difference of electric potential between points (pd) volts.


A variable resistor with manual operation that provides an output voltage from a sliding terminal on a resistor block.

Pounds Feet

Old unit of torque. Also foot pounds. Torque is now given in the SI units of Newton metres Nm.

Pour Point

Applied to fuel oils (diesel) to indicate the temperature where it will still flow under its own weight. The pour point specified value is 3 degrees above the point where it will only just flow.

Powder Coatings

Any protective coating which is applied to the surface as a dry, finely ground powder and then carefully heated above its melting point so that the powder particles flow together to form a continuous film.


The rate of doing work. It is measured by the standard unit called the watt (W) and is equal to one joule per second. This is a very small unit and kilowatt (kW) or megawatt (MW) is more often used. Also, Horsepower which is the old term for power.

Power Assisted Steering

The use of a hydraulic or electrical system to increase the driver's effort on the steering wheel thus reducing the effort that the driver has to apply.

Power Booster

Hydraulic or pneumatic brake pedal or hydraulic pressure booster.

Power Distribution

A central point from which various components are supplied.

Power Output

Power developed by an engine.

Power split hybrid

A combination of a series and parallel hybrid that uses a mechanical splitting device.

Power Stroke

Part of the four-stroke cycle following combustion.


Term used to describe the engine, transmission and driveline components as a unit.


Personal protective equipment


Parts Per Million


Spontaneous ignition in a petrol engine that occurs before the controlled ignition point. Can be caused by an overheated engine, incandescent particles in the combustion chamber or incorrect fuel.


Spontaneous ignition in a petrol engine that occurs before the controlled ignition point. Can be caused by an overheated engine, incandescent particles in the combustion chamber or incorrect fuel.


Additional tightening of a bearing or component after the backlash or free play has been eliminated. Used to eliminate backlash after the bearings have bedded in. Usually set by a Nm or angular torque.


The way in which something is displayed.


Pressure is the product of a force multiplied by the area on which the force is applied. The standard unit of pressure is the Pascal which is equivalent to one N/m2.

Pressure Bleeder

A brake/clutch bleeder that forces fluid through the system.

Pressure Cap

Cap used to pressurise cooling system.

Pressure Chamber

An area used to hold a pressure as something expands of to supply pressure.

Pressure Charged

An engine using a pressure-charger to boost the air charging pressure above atmospheric pressure, to increase the air mass entering the cylinders of the engine.

Pressure Conscious

Variable opening valve used for sensing and adjusting the pressure in brake circuits to restrict force to the rear brakes.

Pressure Differential

The difference in pressures between two sides of a component or areas in a system that are used to operate the component or system. Atmospheric pressure is often used in conjunction with a vacuum to provide a force.

Pressure Gauge

An instrument for measuring pressure of a fluid or a gas.

Pressure Modulator

A device for controlling pressure in a system.

Pressure Plate

Component of the clutch.

Pressure Regulator

A valve in a hydraulic system that controls line pressure.

Pressure Relief Valve

The valve in an oil or hydraulic circuit that opens to release excess fluid pressure and return fluid to the feed side of the pump.

Pressure Switch

A pressure sensitive switch used to provide a signal voltage to an instrument, lamp or electronic control module.

Pressure Wave Supercharger

A supercharger using the exhaust pressure waves to boost the inlet air pressure. This supercharger uses a drum with longitudinal drillings that is rotated in time with the engine and ported to match inlet and exhaust pressure waves together.

Pressure-feed Gun

A spray gun equipped with a separate paint container that is pressurized and connected to the spray gun by means of a solvent-resistant fluid hose, in addition to the normal air hose.

Primary Shoe

A brake shoe that has a leading edge making first contact when applied. Also leading shoe.

Primary Winding

The low tension (primary circuit) coil of wire in an ignition coil.


The first coat of paint applied to a substrate. Depending on the formulation of a particular primer, its function can be to provide any combination of">adhesion, build, scratch-filling, ground coating and corrosion resistance.


An undercoat which improves adhesion of the topcoat, by sealing off old painted surfaces that have been sanded; usually does not require sanding when dry.


Lifting fuel into the system components to remove air.

Priming Pump

A manually operated pump that lifts fuel into the system.


An output device that allows data from the computer to be displayed on paper.


The central processing unit oversees all of the other components of the system. Can be thought of as the brain of the computer.


Initial acceleration of the throttle after the idle position. Going onward/upward.

Progressive Carburettor

Usually a carburettor with two chokes, the second of which only opens as the throttle is pushed further down.


Programmable Read Only Memory.

Propeller Shaft

Connects gearbox to final drive and transmits torque on rear wheel and four wheel drive vehicles.

Propellor Shaft

See Propeller Shaft.


The relation of one thing to another.

Proportioner Valve

A valve used in hydraulic brake circuits to split the force between the front and rear brake circuits.


Connects gearbox to final drive and transmits torque on rear wheel and four wheel drive vehicles.

Propshaft Joints

Universal joints used to allow a propshaft to work through an angle.


Application of a driving force.


One of a number of particles in the nucleus of an atom. The proton has a positive polarity and is electrically positive in opposition to the negative charge of the electrons.


A working version of a new design.


The origins of information.


The German word for horsepower. Pferdestärke. 1PS equals 0.986bhp.


Pound per square inch. A measure of pressure in old units. The SI unit of pressure is the Pascal (Pa).


Power Steering Pressure


Power Steering Pressure Switch.


Part Throttle.


Pending Trouble Code

PTO - Power Take Off

A supplementary drive from a vehicle transmission that is used to drive to auxiliary units.

Pull-in Winding

A winding in a starter motor solenoid that actuates the solenoid to pull the switch contacts together. It is often earthed through the armature so that the drive pinion rotates as it moves into mesh with starter ring gear.

Pull-off Spring

A simple return spring to open or close something after it is switched off.


A tool for applying a pulling force. For example, to remove a gear from a shaft.


Used to transmit or receive drive from a belt. The alternator pulley is a good example.


On/off or increase/decrease in pressure or position.

Pulsation Damper

Any device, such as a hydraulic accumulator, for damping pulsations in a fluid system, as for example the pressure pulses from a fuel pump.


A change in electrical or mechanical signal.

Pulse Air Injection

An emission control system that uses exhaust pressure pulsations to draw air into the exhaust gas flow to increase the available oxygen in the catalytic converter.

Pulse Generator

Inductive electronic device that provides information to the ignition amplifier or ECU on engine position.

Pulse Shaping

The action carried out in an ECU to a signal from a sensor. For example, a sine wave may be changed to a square wave.

Pulse Wheel

Toothed sensor wheel used to induce or affect electrical currents in magnetic inductive pulse generators, such as engine speed and position sensors.

Pulse Width

The operating phase of an actuator when it is energised by an electronic control unit.


Device that is electrically or mechanically driven to pressurize a fluid or gas.


The use of marks and signs to form words, sentences, paragraphs etc.

Purge Valve

The valve in a fuel vapour evaporative system operated from the ECM under specific conditions to draw air back through the evaporative canister. An emission control device to prevent hydrocarbon fuel vapours from the fuel tank escaping into the atmosphere.

Push 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 the same as the outer dimension of the inner component. Also known as a transition fit.

Push Rod

A rod that transfers movement from a cam follower to a rocker arm.


Power to Weight Ratio


Explosive devices!

Pyrotechnic Inflater

An explosive device used to produce a large quantity of gas to inflate an air bag.

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