Idle Speed/Throttle Actuator - Electronic: Description and Operation

Fig. 125:

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 AAC system operates under the following conditions:
1. Idle switch closed.
2. Vehicle speed lower than 5 mph (8 km/h).

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.

Air Regulator:

AIR REGULATOR
The air regulator is mounted on the intake collector. 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.