II-20 GLOSSARY
SECONDARY CIRCUIT: The high voltage side of the ignition system, usu-
ally above 20,000 volts. The secondary includes the ignition coil, coil wire,
distributor cap and rotor, spark plug wires and spark plugs.
SENDING UNIT: A mechanical, electrical, hydraulic or electromagnetic de-
vice which transmits information to a gauge.
SENSOR: Any device designed to measure engine operating conditions or
ambient pressures and temperatures. Usually electronic in nature and designed
to send a voltage signal to an on-board computer, some sensors may oper-
ate as a simple on/off switch or they may provide a variable voltage signal
(like a potentiometer) as conditions or measured parameters change.
SHIM: Spacers of precise, predetermined thickness used between parts to es-
tablish a proper working relationship.
SLAVE CYLINDER: In automotive use, a device in the hydraulic clutch sys-
tem which is activated by hydraulic force, disengaging the clutch.
SOLENOID: An electrically operated, magnetic switching device.
SPARK PLUG: A device screwed into the combustion chamber of a spark ig-
nition engine. The basic construction is a conductive core inside of a ceramic
insulator, mounted in an outer conductive base. An electrical charge from the
spark plug wire travels along the conductive core and jumps a preset air gap
to a grounding point or points at the end of the conductive base. The resul-
tant spark ignites the fuel/air mixture in the combustion chamber.
SPLINES: Ridges machined or cast onto the outer diameter of a shaft or in-
ner diameter of a bore to enable parts to mate without rotation.
TACHOMETER: A device used to measure the rotary speed of an engine,
shaft, gear, etc., usually in rotations per minute.
THERMOSTAT: A valve, located in the cooling system of an engine, which
is closed when cold and opens gradually in response to engine heating, con-
trolling the temperature of the coolant and rate of coolant flow.
TOP DEAD CENTER (TDC): The point at which the piston reaches the top
of its travel on the compression stroke.
TORQUE: Measurement of turning or twisting force, expressed as foot-pounds
or inch-pounds.
TORQUE CONVERTER: A turbine used to transmit power from a driving
member to a driven member via hydraulic action, providing changes in drive
ratio and torque. In automotive use, it links the driveplate at the rear of the en-
gine to the automatic transmission. TRANSDUCER: A device that changes energy from one form to another. For
example, a transducer in a microphone changes sound energy to electrical en-
ergy. In automotive air-conditioning controls used in automatic temperature
systems, a transducer changes an electrical signal to a vacuum signal, which
operates mechanical doors.
TRANSISTOR: A semi-conductor component which can be actuated by a
small voltage to perform an electrical switching function.
TUNE-UP: A regular maintenance function, usually associated with the re-
placement and adjustment of parts and components in the electrical and fuel
systems of a vehicle for the purpose of attaining optimum performance.
TURBOCHARGER: An exhaust driven pump which compresses intake air
and forces it into the combustion chambers at higher than atmospheric pres-
sures The increased air pressure allows more fuel to be burned and results
in increased horsepower being produced.
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VACUUM ADVANCE: A device which advances the ignition timing in re-
sponse to increased engine vacuum.
VACUUM GAUGE: An instrument used for measuring the existing vacuum
in a vacuum circuit or chamber. The unit of measure is inches (of mercury in
a barometer).
VALVE: Devices that can open or close fluid passages in a hydraulic system
and are used for directing fluid flow and controlling pressure.
VALVE CLEARANCE: The measured gap between the end of the valve stem
and the rocker arm, cam lobe or follower that activates the valve.
VISCOSITY: The ability of a fluid to flow. The lower the viscosity rating, the
easier the fluid will flow. 10 weight motor oil will flow much easier than 40
weight motor oil.
VOLTMETER: An instrument used for measuring electrical force in units
called volts. Voltmeters are always connected parallel with the circuit being
tested.
WHEEL CYLINDER: Found in the automotive drum brake assembly, it is a
device, actuated by hydraulic pressure, which, through internal pistons, pushes
the brake shoes outward against the drums.
