Achieva L4-144 2.4L DOHC VIN T SFI (1996)
Idle Air Control (IAC) Valve: Description and Operation
Description
Notice: Numbers in text correspond to numbered components in image.
Engine idle speed is controlled by the PCM through the Idle Air Control (IAC) valve (1) mounted on the throttle body.
Operation
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 (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 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.
Programmed Engine Speeds
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 coolant temperature, park/neutral switch status, vehicle speed, battery voltage, and A/C refrigerant pressure (if equipped).
PCM Learned Idle Speeds
The PCM learns the proper IAC valve positions 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 (information is retained after 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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Engine idle speed is a function of total airflow into the engine based on IAC valve pintle position plus throttle valve opening plus 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 (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 adjustment.
Notice: Do Not try to remove the stop screw filler and readjust the stop screw. Misadjustment may result in damage to the IAC valve or the
throttle body and may set a DTC P0506 or a DTC P0507.
Establishing Zero Count Position
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The PCM 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 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.
^
The IAC valve also can be reset without driving the vehicle by using this service procedure:
1. Turn ignition switch to the ON position (engine OFF).
2. Turn ignition OFF for ten seconds.
3. Start engine and check for proper idle operation.
Disconnecting IAC Valve
