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. The air flow is also changed by the temperature of the engine coolant flowing through the wax valve. As the coolant temperature rises, the wax in the valve softens and expands. This pushes a plunger out and restricts the air flow through the bypass circuit.

Fig. 65 Idle Air Adjusting Unit:

Auxiliary Air Control Valve:

AUXILIARY AIR CONTROL VALVE
The auxiliary air control (AAC) valve, located on the idle air adjusting (I.A.A.) unit at the back of the intake plenum, 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. It also closes as the coolant temperature rises.