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Idle speed control

Idle speed control



When the accelerator is in the idle position (not depressed) and the throttle closed, the engine obtains air mainly via the idle air control valve.

The control valve regulates the degree of opening of the IAC valve in order to maintain idling speed constant. This means, for instance, that the valve will open slightly more to compensate for the drop in idling speed that would otherwise occur when the A/C compressor starts, when a gear other than P or N is selected on cars with automatic transmission, or when the control module activates the secondary air injection pump.

The idle air control valve, which has double windings, is supplied with power from the main relay. It is controlled by a 100 Hz PWM signal via pins 2 and 29 of the control module. The longer the control module keeps pin 29 grounded, the more the valve opens. The longer the control module keeps pin 2 grounded, the more the valve closes.

The control module is programmed to maintain idling speed at 900 ±50 rpm when the engine is hot. Compensation for rapid changes in idling speed cannot be achieved by changing the air flow. Instead, ignition timing control at idling speed is so designed as to maintain engine rpm constant.

With a hot engine, the increase in the air flow through the valve is as follows on engagement of:


• DRIVE position. . . . . . . . . . . . . . . . . . . about 1.1 g/s

• A/C compressor . . . . . . . . . . . . . . . . about 1.4 g/s

• Secondary air injection pump. . . . . . . . . . . . . about 0.8 g/s

• Radiator fan. . . . . . . . . . . . . . . . . . . . . . . about 0.8 g/s


In the event of a break (no continuity) in the idle air control valve circuit, the opening of the valve is set by a spring so that idling speed will be at least 1000 rpm with a hot engine.