Terms Starting with P


Parallel circuit-An electrical circuit that has more than one path for the current to flow.

Park/neutral switch-A safety switch that is connected to the mechanical motion of the shift lever so the engine cannot be started in any position but park and neutral.

Passive restraint system-A supplemental safety system using air bags that inflate immediately upon impact.

Passive seal-A seal that has no extra springs or tension devices to help make the seal.  O-ring seals on cylinder head valves are called passive valve seals.

Performance chart-A chart that has been produced from a dynamometer.  It shows the horsepower, torque, and fuel consumption of an engine at various rpm.

Photodiode-A type of light-sensitive semiconductor.  In some cruise control systems, a photodiode uses light to create an electrical signal based upon the speed of a rotating shaft.

Piezoelectric-A crystalline material that generates a small voltage when it is vibrated, often used in knock or pre-ignition sensors.

Pinging-A sound heard inside the combustion chamber that is caused by two combustion fronts hitting each other.

Pinion gear-The small gear attached to the armature shaft used to crank the flywheel ring gear.  The pinion gear is also the smaller of two gears.

Pintle-The center pin used to control a fluid passing through a hole, or a small pin or pointed shaft used to open or close a passageway.

Piston-A cylindrical object that slides in the cylinder.

Piston slap-The movement of the piston back and forth in the cylinder in a slapping motion.

Pitch-The angle of the valve spring twist.  A variable pitch valve spring has unevenly spaced coils.

Pitman arm-A steering linkage component that connects the steering gear to the linkage at the left end of the center link.

Pivot point-The point on a rocker arm that is the center of its rotation.

Planet carrier-The part of a planetary gear system that connects the axis of the planet gears together.

Planetary gear system-A gear assembly that includes a sun gear, planet gears, and a ring gear.  By locking up one gear, various gear ratios and speeds can be produced.

Plastigage-A small, thin, plastic strip that is used to determine the clearance between the main and/or connecting rod bearings and the crankshaft journals.

Platinum-A slivery-white material that does not oxidize with air, but does act as a catalyst to change or affect other chemicals, often used in catalytic converters.

Plies-Layers of material that wrap around the outer area of a tire.

Pole shoes-Soft iron pieces that wire is wrapped around inside the starter motor.

Pollution-Addition of harmful products to the environment.  Types of pollution may include air, water, noise, chemical, thermal, and nuclear.

Poppet-type valve-A type of valve, such as an engine intake or exhaust valve that opens and closes a port.

Ported vacuum-Vacuum taken from the carburetor slightly above the throttle plate.

Positive displacement-In reference to a hydraulic or pneumatic pump, one that pumps an exact amount of fluid volume with each revolution of the pump, such as an oil pump.

Post-ignition-Ignition that occurs after the engine ignition system is shut off due to carbon buildup in the combustion chamber.

Potentiometer-An electrical device that has a variable resistance.  As the resistance changes, the voltage drop across the device also changes.  This can then be used as an input signal for a vehicle computer.

Power-A measure of work being done.

Power control module-A module or computer used in an electronic transmission to aid in control of the shift solenoids.

Pre-ignition-The process of a glowing spark or carbon deposit igniting the air-fuel mixture before the spark plug does.

Pressure gauge-A gauge used to measure pressures such as fuel pump, transmission oil, and fuel injection pressures.

Pressure plate-The part in a clutch system used to squeeze or clamp the clutch disc between it and the spinning flywheel.

Pressure regulator valve-A valve used in an automatic transmission to regulate the pressure of the oil inside of the valve body.  Also, a valve used in a fuel system to regulate the pressure of the fuel.

Prevailing torque nuts-Nuts designed to develop an interference fit between the nut and bolt threads.

Primary battery-A type of battery that cannot be recharged after use.

Primary circuit-A circuit in an old ignition system that uses 12 volts to operate.  It includes the ignition switch, a ballast resistor or resistive wire, primary coil wires, condenser, and contact points.

Primary shoe-The forward shoe on a two-shoe drum brake system having shorter linings than the other.

PROM-Programmable read only memory, or permanent storage in a computer that can be easily accessed, removed, and/or replaced by installing a new chip.

Property class-A number stamped on the end of a metric bolt to indicate the hardness of the bolt.

Proportioning valve-A valve in the brake hydraulic system that can be adjusted to reduce pressure to the rear wheels to achieve better braking balance.

Proton-The positive (+) part of the atom.

Psig-A type of pressure scale read as pounds per square inch on a gauge.

Pulse width-A term used to describe the length of time that a fuel injector solenoid is in the ON position.

Pulse wiper system-A wiper system using electronic circuits that cause the wipers to pulse or turn on, then off for a certain number of seconds.

Purge-To separate or clean by carrying off gasoline fumes.  The carbon canister has a purge line to remove impurities.

Pushrod-A connector between the lifter and the rocker arm.


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