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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