Automotive Glossary of Terms - P -
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Definitions for automotive terms and illustrations from park neutral safety switch to pushrod.
PARK NEUTRAL SAFETY SWITCH: On vehicles with automatic transmissions, a neutral safety switch (often referred to by various names by the different manufacturers, such as: transmission range sensor, neutral safety switch, park/neutral switch, etc.) on the side of the transmission is wired to the relay or solenoid. Its function is to prevent activation of the starter (by creating an open circuit) when the transmission is in any gear other than P (park) or N (neutral). The vehicle can only be started in P or N. Most manual transmission vehicles have a clutch switch to prevent starting the vehicle unless the clutch is depressed.
PARTS WASHER: A basin or tub, usually with a built-in pump mechanism and hose used for circulating chemical solvent for the purpose of cleaning greasy, oily and dirty components.
PART-TIME FOUR-WHEEL DRIVE: A system that is normally in the two-wheel drive mode and only runs in four-wheel drive when the system is manually engaged because more traction is desired. Two or four-wheel drive is normally selected by a lever to engage the front axle, but if locking hubs are used, these must also be manually engaged in the Lock position. Otherwise, the front axle will not drive the front wheels.
PASSIVE RESTRAINT: Safety systems such as air bags or automatic seat belts which operate with no action required on the part of the driver or passenger. Mandated by Federal regulations on all cars sold in the U.S. after 1990.
PAYLOAD: The weight the car is capable of carrying in addition to its own weight. Payload includes weight of the driver, passengers and cargo, but not coolant, fuel, lubricant, spare tire, etc.
PCM: Powertrain control module. See Electronic Control Unit (ECU).
PCV VALVE: A valve usually located in the rocker cover that vents crankcase vapors back into the engine to be reburned.
PERCOLATION: A condition in which the fuel actually "boils" due to excessive heat. Percolation prevents proper atomization of the fuel causing rough running.
PICK-UP COIL: The coil in which voltage is induced in an electronic ignition.
PING: A metallic rattling sound produced by the engine during acceleration. It is usually due to incorrect ignition timing or a poor grade of gasoline.
PINION: The smaller of two gears. The rear axle pinion drives the ring gear which transmits motion to the axle shafts.
PISTON RING: An open-ended ring which fits into a groove on the outer diameter of the piston. Its chief function is to form a seal between the piston and cylinder wall. Most automotive pistons have three rings: two for compression sealing; one for oil sealing.
PITMAN ARM: A lever which transmits steering force from the steering gear to the steering linkage.
PLY RATING: A rating given a tire which indicates strength (but not necessarily actual plies). A two-ply/four-ply rating has only two plies, but the strength of a four-ply tire.
POLARITY: Indication (positive or negative) of the two poles of a battery.
POWER-TO-WEIGHT RATIO: Ratio of horsepower to weight of car.
POWERTRAIN: See Drivetrain.
Ppm: Parts per million; unit used to measure exhaust emissions.
PREIGNITION: Early ignition of fuel in the cylinder, sometimes due to glowing carbon deposits in the combustion chamber. Preignition can be damaging since combustion takes place prematurely.
PRELOAD: A predetermined load placed on a bearing during assembly or by adjustment.
PRESS FIT: The mating of two parts under pressure, due to the inner diameter of one being smaller than the outer diameter of the other, or vice versa; an interference fit.
PRESSURE PLATE: A spring-loaded plate (part of the clutch) that transmits power to the driven (friction) plate when the clutch is engaged.
PRIMARY CIRCUIT: The low voltage side of the ignition system which consists of the ignition switch, ballast resistor or resistance wire, bypass, coil, electronic control unit and pick-up coil as well as the connecting wires and harnesses.
PROFESSIONAL TECHNICIAN: A repair technician that has been properly trained in a vehicle’s systems. Usually affiliated with ASE or other certification system. This technician will also have the proper tools to diagnose and repair your vehicle.
PROFILE: Term used for tire measurement (tire series), which is the ratio of tire height to tread width.
Psi: Pounds per square inch; a measurement of pressure.
PUSHROD: A steel rod between the hydraulic valve lifter and the valve rocker arm in overhead valve (OHV) engines.