Idle Speed Control System

Fig. 47 Idle Speed Control:

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.

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.

Fig. 57 Auxilary Air Control Valve:

AUXILIARY AIR CONTROL (AAC) VALVE
The AAC valve has a solenoid that is opened and closed at a rapid rate (between 100 and 200 times per second). The longer it is opened, the more air that is allowed to pass through.

Fig. 48 AAC Valve Operating Conditions:

Refer to the chart for AAC valve operating conditions.