Firebird V8-5.7L VIN G (2001)
Idle Speed: Description and Operation
CIRCUIT DESCRIPTION
The Idle Air Control (IAC) valve is located in the throttle body of both the Sequential Multi-port Fuel Injection (SFI) and the Multi-port Fuel
Injection (MFI) systems. The IAC valve consists of a movable pintle, driven by a gear attached to an electric motor, which is called a stepper motor.
The IAC valve motor is a 2-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. You can see this change in polarity when you observe a test lamp connected between ground or B+ and an IAC valve
circuit while the Powertrain Control Module (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. The PCM uses a predicted number of counts. One count represents
1 change in polarity, which equals 1 step of the stepper motor. The PCM counts the steps that have been commanded in order to determine IAC pintle
position. The PCM uses the IAC valve to control engine idle speed 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.
In order to determine the desired position of the IAC pintle during idle or during deceleration, the PCM refers to the following inputs:
^
The engine RPM
^
The battery voltage
^
The air temperature
^
The Engine Coolant Temperature (ECT)
^
The throttle position sensor angle
^
The engine load
^
The 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. Next, the PCM will retract
the pintle a predetermined amount of counts. This retraction allows the proper amount of air to bypass the throttle plate in order for the engine to start.
This procedure is known as an IAC Reset.
