Savana 1/2 Ton Van V8-5.7L VIN R (2001)
Idle Air Control Motor (IAC): Testing and Inspection
STANDARD MODELS (NON-ALTERNATIVE FUELS)
IDLE AIR CONTROL SYSTEM DIAGNOSIS
CIRCUIT DESCRIPTION
The Idle Air Control (IAC) valve is located in the throttle body of both Sequential Multiport Fuel Injection (SFI) and Multiport Fuel Injection (MFI)
systems. The IAC valve 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 B+ 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 Powertrain Control Module (PCM)
does not use a physical sensor 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 the PCM has 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:
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The engine RPM
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The battery voltage
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The air temperature
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The Engine Coolant Temperature (ECT)
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The Throttle Position (TP) 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 and then retract the IAC
pintle a predetermined amount of counts to allow the proper amount of air to bypass the throttle plate for engine start-up. This procedure is known as
an IAC Reset.
DIAGNOSTIC AIDS
Inspect for the following conditions:
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A skewed high Throttle Position (TP) sensor
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A restricted air intake system
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Objects blocking the IAC passage or throttle bore
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The correct Positive Crankcase Ventilation (PCV) valve, properly installed and proper operation of the PCV valve
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Proper operation and installation of all air intake components
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Proper installation and operation of the Mass Air Flow Sensor (MAF), if equipped
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A tampered with or damaged throttle stop screw
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A tampered with or damaged throttle plate, throttle shaft, throttle linkage, or cruise control linkage, if equipped.
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Excessive deposits in the IAC passage or on the IAC pintle
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Excessive deposits in the throttle bore or on the throttle plate
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Vacuum leaks.
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Excessive load on engine i.e. transmission, power steering, alternator, etc.
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A high or unstable idle condition could be caused by a non-IAC system problem that cannot be overcome by the IAC valve. refer to Symptoms -
Computers and Control Systems. See: Testing and Inspection/Symptom Related Diagnostic Procedures/Standard Model (Non-Alternative Fuels)
^
A low or unstable idle condition could be caused by a non-IAC system problem that cannot be overcome by the IAC valve. refer to Symptoms -
Computers and Control Systems. See: Testing and Inspection/Symptom Related Diagnostic Procedures/Standard Model (Non-Alternative Fuels)
^
If the problem is determined to be intermittent, refer to Intermittent Conditions in symptoms. See: Testing and Inspection/Symptom Related
Diagnostic Procedures/Standard Model (Non-Alternative Fuels)
TEST DESCRIPTION
