The purpose of the Idle Air Control (IAC) system is to control engine idle speeds while preventing stalls due to changes in engine load.
The throttle blade, when closed allows a small amount of air into the intake manifold. The IAC system controls 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 opposing springs within the valve assembly (equal to a slightly elevated idle). Switched B+ is provided the drive unit within the IAC valve. The PCM controls the valve via two control circuits, one to drive the valve open and the other to drive the valve closed. The PCM pulse Width Modulates both control circuits simultaneously. The ration of the frequency between two PWM signals determines the direction and amount that the drive unit rotates the shutter within the valve. As the shutter closes, bypass airflow is reduced and idle speed decreases. To increase idle speed, the PCM commands the shutter open, allowing more air to bypass the throttle plates.
Preliminary tests for an IAC related problem include checking for the following conditions:
- If a scan tool is available, connect it to the system, and start the engine.
- Turn all accessories OFF (A/C, Heater, Rear Defroster, etc.).
- Using the scan tool, command RPM up to 1500, down to 650, and then up to 1500 while monitoring Engine Speed on the scan tool. If the IAC valve follows these commands and holds the commanded engine speed, then it is good.
- If a scan tool is unavailable, then try the following test procedure:
- Unplug the IAC connector and check resistance between the IAC terminals. Resistance between terminals A & B and terminals C & D should be 40-to 80 ohms. If resistance is not within specification, the IAC valve should be replaced.