P0506 - P0507
The idle air control (IAC) valve is located in the throttle body of the SFI systems. The IAC consists of a movable pintle, driven by a gear attached to an electric motor called a stepper motor. The IAC valve motor is a two phase bi-polar permanent magnet stepper motor that is capable of highly accurate rotation, or movement, every time the polarity of a winding is changed.
This change in polarity can be seen when observing a test lamp connected between ground or battery positive voltage and an IAC valve circuit while the PCM is attempting to change engine RPM. The test lamp will flash on or off each time the polarity is changed. The PCM does not use a physical sensor in order to determine IAC pintle position, but uses a predicted number of counts. One count represents one change in polarity which equals one step of the stepper motor. The PCM counts the steps that have been commanded to determine IAC pintle position. The PCM uses the IAC valve to control engine idle speed. The PCM does this by changing the pintle position In the idle air passage of the throttle body. This varies the air flow around the throttle plate when the throttle is closed. To determine the desired position of the IAC pintle at idle or during deceleration, the PCM refers to the following inputs: Engine RPM, Battery voltage, Air temperature, engine coolant temperature, throttle position sensor angle, engine load, and vehicle speed. When the ignition key is turned OFF after an ignition cycle, the PCM will first seat the IAC pintle in the air bypass bore. The PCM will then retract the IAC pintle a predetermined amount of counts in order to allow the proper amount of air to bypass the throttle plate for engine start-up. This procedure is known as an IAC Reset.
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