Idle Speed/Throttle Actuator - Electronic: Description and Operation

Idle Air Control (IAC) Valve Assembly:

IAC Valve Air Flow Diagram:

Engine idle speed is controlled by the ECM through the IAC valve, mounted on the throttle body. The ECM sends voltage pulses to the IAC valve motor windings causing the IAC valve pintle to move "IN" (toward the seat) or "OUT" (away from the seat) a given distance (a step or count) for each pulse. The commanded location (steps away from the seated position) can be observed as a number of counts, as displayed on a SCAN tool. The pintle movement controls the airflow around the throttle valve, which in turn, controls engine idle speed.

^ Pintle Extended = Decrease RPM = Lower Counts
^ Pintle Retracted = Increase RPM = Higher Counts

^ The "Controlled" or desired idle speed for all engine operating conditions is programmed into the Mem-Cal of the ECM. The programmed engine speeds are based on coolant temperature, park/neutral switch status, vehicle speed, battery voltage, and A/C pressure (if equipped).

^ The ECM learns the proper IAC valve positions to achieve warm, Stabilized idle speeds, desired for the various conditions (P/N or Drive, A/C ON or OFF, if equipped). This information is stored in the ECM Keep Alive memories (the information is retained after the ignition is turned OFF). All other IAC valve positioning is calculated based on memory values. As a result, engine variations due to wear, and variations in minimum throttle valve position (within limits) do not affect engine idle speeds. This system provides correct idle control under all conditions. This also means that disconnecting power to the ECM can result in incorrect idle control or necessity to partially depress the accelerator when starting, until the ECM relearns idle control.

^ Engine idle speed is a function of total air flow into the engine based on IAC valve pintle position + calibrated vacuum loss through accessories.

^ The ECM normally resets the IAC valve pintle position once during each ignition cycle, when vehicle speed increases above 20 mph on moderate acceleration. During the reset, the ECM commands the IAC valve pintle to retract completely, then move IN to the seated position (to establish the zero count position), and then back out to the calibrated position. If, after the reset, the ECM is able to control the idle speed within the calibrated range, the IAC valve will not be reset again until the ignition has been turned OFF, the engine restarted and the car driven above 20 mph. If the ECM is not able to control idle speed within the calibrated count range after the first reset, the ECM assumes the IAC valve pintle position is "lost" and enables another reset to occur. This will continue until the pintle position is relearned.

^ The IAC valve also can be reset without driving the vehicle by using this procedure:

1. Turn the ignition switch to the ON position (engine OFF).
2. Ground the diagnostic test terminal for five seconds. This causes the IAC valve pintle to extend to its seated position.
3. Remove the ground. This causes the pintle to retract a predetermined number of steps based on coolant temperature.
4. Turn the ignition OFF for ten seconds.
5. START the engine and check for proper idle operation.

^ If the IAC valve is disconnected and reconnected while the engine is running, the resulting IAC valve counts may not correspond with the actual IAC valve pintle position. When servicing the IAC valve, it should only be disconnected or connected after the ignition has been OFF for at least 10 seconds. This allows time for the ECM to move the IAC valve to the 150 count position where it is parked while the ignition is OFF. If this procedure is not followed, The ECM will lose track of IAC valve position resulting in starting or idle control problems until the IAC valve is reset and pintle position is relearned.

^ IAC problems may cause improper idle speeds, resulting in a Code 35. The Code 35 chart should be used to diagnose these problems. Refer to COMPUTERIZED ENGINE CONTROLS/DIAGNOSIS AND TESTING PROCEDURES/DIAGNOSTIC CHARTS/TROUBLE CODES.