Idle Air Control (IAC) Valve

PRINCIPLE OF OPERATION, IDLE AIR CONTROL VALVE





Idle Air Control Valve (IAC)--When the accelerator is not activated and the throttle butterfly closed, the engine receives air only via the idle air control valve.

The control module regulates the opening of the valve to maintain idling speed constant. This means, for instance, that the valve opens slightly more on engagement of the A/C compressor, when a gear position other than P or N is selected on cars with automatic transmission, or when the control module activates the secondary air injection pump. This is done to provide idling speed compensation, as idling speed would otherwise drop.

The idle air control valve is of single-oil type and supplied with current from the main relay. It is controlled from pin 49 of the control module by a 500 Hz PWM signal. The valve opens more the longer the control module keeps pin 49 grounded.

The control module is programmed to maintain engine idling speed at 850 ~ 50 rpm. Rapid changes in idling speed cannot be compensated for by a change - in air flow. Ignition timing at idling speed is instead so designed that engine rpm is maintained constant.

If there is a break in the idle air control valve circuit, the valve opening will be set by a spring so that the idling speed of a warmed-up engine is at least 1000 rpm.