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Automotive Service Glossary - Houston's Premier Automotive Service Center

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Idle air control:  Controls the amount of air that bypasses the throttle plate in order to regulate idle engine speed.

Idler arma conventional steering system component consisting of an arm that swivels in a bushing on a shaft, which is attached to the frame. The idler arm is mounted on the right side of the vehicle and is the same length and set at the same angle as the pitman arm. It's function is to hold the right end of the center link level with the left end, which is moved by the pitman arm, and transfer the steering motion to the right side tie-rod.

Ignitability: characteristic of a solid that enables it to spontaneously ignite. Any liquid with a flashpoint below 140F is said to possess ignitability.

Ignition coil: transforms the low 12-volt battery ignition primary current into the high voltage secondary current that fires the spark in the plugs. The current through the primary coil windings builds up an electromagnetic field around the ferrous core of the coil. When the current is suddenly shut off, the electromagnetic field collapses and generates the high voltage in the secondary windings.

Ignition control module (ICM): the computer does not directly operate the ignition coil, because of the comparatively high voltages and currents involved. Instead, it signals the ignition control module (ICM) when to fire the spark. The ICM has a power transistor which turns on the ignition primary circuit to charge the coil by building an electromagnetic field around the ferrous core, and fires the spark by shutting off the current to the primary circuit, allowing the field to collapse and generate a high voltage spark current in the coil secondary circuit. The ignition control module typically includes additional internal circuits to perform other functions such as calculating dwell. 

Ignition switch: a key operated switch located on the steering column, that connects and disconnects power to the ignition and electrical systems.

Ignition system: the components that produce the spark to ignite the air/fuel mixture in the combustion chamber.

Ignition timing: refers, in crankshaft degrees, to the position of the piston in the cylinder when the sparks occur.

Included angle: the sum of the angle of camber and steering axis inclination; the sum of two intersecting angles.

Independent suspension: a suspension in which each wheel can travel up and down without directly affecting the position of the opposite wheel.

Induction: the process by which an electric or magnetic effect is produced in an electrical conductor or magnetic body when it is exposed to variation of a field of force. Induction is the principle used in an ignition coil to increase voltage.

Inertia switch: a device used to shut off a system when disturbed, such as jarring, tipping or inversion; a switch that automatically shuts off the fuel pump if the vehicle rolls over or is involved in a collision.

Injector: a device which receives metered fuel under relatively low pressure and is activated either electrically or mechanically to spray the fuel under relatively high pressure into the engine.

Inner bearing race: inner part of a ball or roller bearing that provides a surface for the balls or rollers to rotate.

Inner tie rod end: transfer motion from steering linkage to the wheel hub assembly.

Insert spring: exerts pressure on the inserts in a synchronizer assembly.

Insert guides: valve guides that are pressed fit in the cylinder head.

Insulator: a non-conductive material used to insulate wires in an electrical circuit.

Intake air temperature (IAT) sensor: works by a negative coefficient thermistor that loses resistance as its temperature goes up (like the engine coolant temperature sensor). When the computer applies its 5-volt reference signal sensor, this voltage is reduced through a ground circuit by an amount corresponding to the temperature of the intake air. 

Intake manifold: a part with runners that connect the fuel system to the intake valve ports.

Intake port: the passage or opening in a cylinder head that is closed by the intake valve.

Intake stroke: the first stroke of a 4-stroke cycle engine in which the intake valve is open and the exhaust valve is closed, during which the downward motion of the piston draws the fuel/air mixture into the cylinder.

Intake valve: also called inlet valve, it closes off the intake port and opens it at the correct time in response to movement from the cam lobe.

Integral ABS: an anti-lock braking system that substitutes the traditional master cylinder and power booster with a self-contained hydraulic modulator and high-pressure accumulator.

Integral guides: valve guides that are part of the cylinder head.

Integral power steering: a power steering system in which the power cylinder and control valve are contained in one housing.

Integral steering gear: Uses a recirculating ball gearbox along with a pitman arm to steer the vehicle to the left or right.

Intercooler: a component on some turbocharged engines used to cool the compressed intake air.