Idle Speed/Throttle Actuator - Electronic: Description and Operation

Fig. 81 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.

Fig. 82 Auxillary Air Control Valve:

AUXILIARY AIR CONTROL VALVE
The auxiliary air control (AAC) valve 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.

When the engine is cold, the air cut valve is fully open allowing the AAC to function normally. The ECU opens the AAC causing the engine to fast idle. Once the engine temperature begins to rise coolant flowing through the air cut valve causes the valve to close. When closed the AAC is limited to the amount of air that it can allow into the plenum. The idle then drops to curb speed where the limited function of the AAC is sufficient to properly maintain base idle speeds.