Idle Speed/Throttle Actuator - Electronic: Description and Operation

IAC Valve:

DESCRIPTION
The purpose of the Idle Air Control (IAC) Valve is to control the engine idle speed.

OPERATION
The throttle blade, when closed, allows a small amount of air into the intake manifold. The IAC system controls the idle speed by allowing a controlled amount of air to bypass the throttle blade via a passage in the IAC valve. The IAC valve consists of a rotating shutter that is held in a neutral position by the opposing springs within the valve assembly (which is equal to a slightly elevated idle). The switched B+ is provided to the drive unit within the IAC valve. The ECM controls the valve via two control circuits, one control circuit is used in order to drive the valve open and the other control circuit is used in order to drive the valve closed. The ECM Pulse Width Modulates both of the control circuits simultaneously. The ratio of the frequency between the two PWM signals determines the direction and the amount that the drive unit rotates the shutter within the valve. As the shutter closes, the bypass air flow is reduced and the idle speed decreases. The ECM commands the shutter open, allowing more air to bypass the throttle plates in order to increase the idle speed.