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.

Idle Air Adjusting Unit:

IACV-Auxiliary Air Control Valve:

AUXILIARY AIR CONTROL VALVE
The idle air adjusting (IAA) unit is mounted on the rear of the left intake collector. The unit consists of the auxiliary air control (AAC) valve and the fast idle control device (FICD). Both units control idle speed by modifying the amount of air allowed to bypass the throttle plates. The AAC is constantly pulsed by the ECU at a frequency of approximately 160 hz. The ECU varies the duty cycle ("On" time versus "OFF" time) to raise or lower the idle as needed. The FICD is a solenoid valve that opens/closes an air passage and is used by the ECU to adjust the idle speed to instantly compensate for engine load.

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.

Auxiliary Air Regulator Valve:

AIR REGULATOR
The intake collector mounted air regulator is used to provide a cold fast idle. The unit consists of a shutter wheel and an electrically heated bimetallic strip. When air temperature is cold, the bimetallic strip causes the shutter wheel to open. When open, the shutter wheel allows air to bypass the throttle plate, raising idle speed. Once the engine is started, voltage is applied to the heater coil wrapped around the bimetallic strip. When heated the strip begins to close the shutter wheel which in turn closes the bypass air passage.