C 3500 Truck 2WD V8-6.0L VIN U (2001)
Idle Air Control Motor (IAC): Testing and Inspection
IDLE AIR CONTROL (IAC) SYSTEM DIAGNOSIS
CIRCUIT DESCRIPTION
The Idle Air Control (IAC) valve is located in the throttle body of both Sequential Multi-port Fuel Injection (SFI) and 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. This change in polarity can be seen when observing 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 to determine IAC pintle position. The PCM uses a predicted number of counts, 1 count represents 1 change
in polarity which equals 1 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 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:
^
The engine RPM
^
The battery voltage
^
The air temperature
^
The Engine Coolant Temperature (ECT)
^
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. 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-up. This procedure is known as an IAC Reset.
DIAGNOSTIC AIDS
Inspect for the following conditions:
^
A skewed high TP sensor
^
Restricted air intake system
^
Objects blocking the IAC passage or throttle bore
^
The correct Positive Crankcase Ventilation (PCV) valve
^
A properly installed PCV valve
^
Proper operation of the PCV valve
^
Proper operation and installation of all air intake components
^
Proper installation and operation of the Mass Air Flow (MAF) sensor, if equipped
^
Evidence of damage or tampering of the following components:
-
The throttle stop screw
-
The throttle plate
-
The throttle shaft
-
The throttle linkage
-
The cruise control linkage, if equipped.
^
Excess deposits in the IAC passage or on the IAC pintle
^
Excess deposits in the throttle bore or on the throttle plate
^
Vacuum leaks
^
Excess load on engine, such as energy-draining conditions of the transmission, power steering, or alternator
^
A high or unstable idle condition-This condition could be caused by a non-IAC system problem that can not be overcome by the IAC valve. refer
to Symptoms - Computers and Control Systems. See: Computers and Control Systems/Testing and Inspection/Symptom Related Diagnostic
Procedures
^
A low or unstable idle condition-This condition could be caused by a non-IAC system problem that can not be overcome by the IAC valve. refer to
Symptoms - Computers and Control Systems. See: Computers and Control Systems/Testing and Inspection/Symptom Related Diagnostic
Procedures
^
An intermittent condition-If the condition is intermittent, Refer to Intermittent Conditions. See: Computers and Control Systems/Testing and
Inspection/Initial Inspection and Diagnostic Overview/Intermittent Conditions
TEST DESCRIPTION
