Calais L4-138 2.3L VIN D FI (1990)
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.
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Pintle Extended = Decrease RPM = Lower Counts
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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 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).
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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.
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Engine idle speed is a function of total air flow into the engine based on IAC valve pintle position + calibrated vacuum loss through accessories.
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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.
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The IAC valve also can be reset without driving the vehicle by using this procedure:
