Idle Speed/Throttle Actuator - Electronic: Description and Operation

Idle Air Control (IAC) Valve Assembly:

IAC Valve Air Flow Diagram:

DESCRIPTION:

^ Engine idle speed is controlled by the ECM through the idle air control (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" or "OUT" 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, displayed on a "Scan" tool. The pintle movement controls the air flow around the throttle valve, which in turn, controls engine idle speed.

^ The "controlled" or desired speed for all engine operating conditions is programmed into the Mem-Cal of the ECM. The 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 positions to achieve warm, stabilized idle speeds desired for the various operating conditions. This information is stored in "Keep Alive" memories (information is retained after ignition is turned "OFF"). All other IAC valve positioning is calculated based on these memory values. As a result, engine variations due to wear, and variations in minimum 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 plus throttle valve opening plus calibrated vacuum loss through accessories.

^ The minimum throttle valve position is set at the factory with a stop screw. This setting allows enough air to flow by the throttle valve to cause the IAC valve pintle to be positioned a calibrated number of steps (counts), from the seat, during "controlled" idle operation. The minimum throttle valve position setting on this engine should not be considered the "minimum idle speed," as on other fuel injected engines. Low internal friction, provisions for slight production variations, and the various altitudes production engines may operate at, the calibrated air rate at the minimum throttle position is too low to allow most engines to idle. The throttle stop screw is covered with a plug at the factory following adjustment.

NOTE: Do Not attempt to remove the plug and readjust the stop screw, see ADJUSTMENT PROCEDURES/THROTTLE VALVE MINIMUM POSITION (THROTTLE STOP SCREW) Misadjustment may result in damage to the IAC valve or throttle body, and may cause a Code 35 to set.

^ 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 next 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 the desired position. If, after the reset, the ECM is able to control the idle speed within the calibrated count range, the IAC valve will not be reset again until after the ignition has been turned "OFF," the engine restarted and then 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 is "lost," and enables another reset to occur. This will continue until the pintle position is learned.

^ 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 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 the IAC valve position resulting in starting or idle control problems until the IAC valve is reset by the ECM, and pintle position is relearned.

^ For diagnosis and testing procedures, see COMPUTERIZED ENGINE CONTROLS.