Alero L4-144 2.4L DOHC VIN T SFI (2000)
Idle Air Control Motor (IAC): Description and Operation
The engine idle speed is controlled by the PCM through the IAC valve (1) mounted on the throttle body. The PCM sends voltage pulses to the IAC valve
motor windings causing the IAC valve pintle (3) to move in toward the seat, or out away from the seat a given distance, called a step or a count, for each
pulse. The commanded location can be observed as a number of counts displayed on a scan tool. The pintle movement controls the airflow around the
throttle valve (2), which in turn, controls engine idle speed: Pintle Extended=Decrease RPM=Lower Counts, Pintle Retracted=Increase RPM=Higher
Counts.
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The controlled or desired idle speed for all engine operating conditions is programmed into the EEPROM of the PCM. The programmed engine
speeds are based on the coolant temperature, the park/neutral switch status, the vehicle speed, the battery voltage, and the A/C refrigerant pressure
(if equipped).
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The PCM learns the proper IAC valve positions in order to achieve warm, stabilized idle speeds (RPM), desired for the various conditions (P/N or
Drive, A/C on or off, if equipped). This information is stored in PCM keep alive memories and retained after the ignition is 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 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 PCM can result in incorrect idle control or the necessity to partially depress the accelerator when starting, until the
PCM relearns idle control.
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The engine idle speed is a function of the total airflow into the engine based on the IAC valve pintle position, the throttle valve opening, and a
calibrated vacuum loss through accessories.
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The minimum throttle valve position is set at the factory with a stop screw. This setting allows enough air flow by the closed throttle valve to cause
the IAC valve pintle to be positioned a calibrated number of steps or counts from the seat during controlled idle operation. The minimum throttle
valve position for this engine is not the same as the minimum idle speed associated with other fuel injected engines. The throttle stop screw is
filled at the factory following an adjustment.
IMPORTANT: DO NOT try to remove the filler and readjust the stop screw. Misadjustment may set a DTC P0506 or a DTC P0507.
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The PCM normally resets the IAC valve pintle position once during each ignition cycle when vehicle speed increases above 32 km/h (20 mph) on
moderate acceleration. During the reset, the PCM 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 desired position. The IAC is relearned only once per ignition cycle.
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The IAC valve also can be reset without driving the vehicle by using following procedure:
1. Turn on the ignition, leaving the engine off.
2. Turn off the ignition for 10 seconds.
3. Start the engine and verify the proper idle operation.
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Whenever 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, only disconnect or connect the valve after the ignition has been off for at least 10
seconds. This allows time for the PCM to move the IAC valve to the 150 count position where the valve is parked while the ignition is off.
Whenever this procedure is not followed, the PCM 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.
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AC system problems may cause improper idle speeds, resulting in a DTC P0506 or a DTC P0507. The DTC P0506 or the DTC P0507 tables
should be used to diagnose these problems.
