Idle Speed Control

Idling speed is controlled by means of a single winding Idle Air Control (IAC) valve with the aid of PWM signals from the electronic control module.

The automatic idling control function is adaptive, i.e. it adapts continuously to the changes taking place throughout the life of the engine. Such changes might be wear in the engine, dirt or leakage in the intake manifold or throttle body, etc.

The control module is programmed to maintain idling speed at a constant 750 ± 50 rpm when the engine is at normal operating temperature and the throttle position sensor indicates that the throttle butterfly is in the idling position.

Engine speed compensation occurs on engagement of the A/C, secondary air injection and a gear other than P or N on cars with automatic transmission.





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 module regulates the degree of opening of the valve in order to maintain idling speed constant. This means that the valve will open slightly more to compensate for the drop in idling speed that would otherwise occur on engagement of the A/C compressor, 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 a single winding, receives its power supply from the main relay. The valve is controlled by a 100 Hz PWM signal via pin 4 of the control module. The longer the control module keeps pin 4 grounded, the more the valve opens.

The control module is programmed to maintain idling speed at 750±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:
- The DRIVE position___ about 1.7 grammes per second
- The A/C compressor___ about 1.3 grammes per second
- The secondary air injection pump___about 0.7 grammes per second

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.