Idle Speed Control System

Idle Speed Control System:

CIRCUIT OPERATION
The idle speed is controlled by the amount of air that is allowed to bypass the throttle plate through the auxiliary air control (AAC) valve. The AAC valve is turned ON and OFF at a rate between 100 and 200 Hz. The ECU calculates the duty cycle (frequency of ON/OFF signals) necessary to maintain the proper idle speed, compensating for transmission gear position, electrical and accessory loads.

IACV-Auxiliary Air Control Valve:

AUXILIARY AIR CONTROL VALVE
The auxiliary air control (AAC) valve, located on the idle air adjusting (I.A.A) unit, controls the idle speed. It receives a pulse signal from the ECU which opens and closes an air passage allowing a precise amount of air to bypass the throttle plate.

Auxiliary Air Regulator Valve:

AIR REGULATOR
The air regulator is mounted on the air intake plenum. The air regulator is used to provide a cold fast idle to the engine. It consist of a rotary shutter wheel that is controlled by an electrically heated bimetallic strip. When engine temperature is low, the bimetal strip allows the rotary shutter to open. With the shutter open, a relatively large amount of air is allowed to bypass the throttle plate, causing a fast idle. Once the engine is started, voltage is applied to the heater which slowly causes the bimetallic strip to close the shutter. With the shutter closed, the air bypass channel is closed lowering the idle speed to curb speed. The IAA then takes over controlling the idle speed.

Idle Air Adjusting (I.A.A.) Unit:

FAST IDLE CONTROL SOLENOID
There is also a fast idle control solenoid (FICD) that opens a separate passage, allowing additional air to bypass the throttle plate. It compensates in the event the AAC valve cannot allow a sufficient amount of air through to keep the idle speed within the preset limits.