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For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2001 🢒 Jimmy - 4WD 🢒 Engine 🢒 Engine Controls - 4.3L 🢒 Idle Air Control (IAC) System Diagnosis

Circuit Description

The idle air control (IAC) valve is located in the throttle body of both the throttle body fuel injection (TBI) and the multiport fuel injection (MFI) systems. The 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 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 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 the 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 in order to determine the IAC pintle position. The PCM uses the IAC valve in order to control the engine idle speed. The IAC does this by changing the pintle position in the idle air passage of the throttle body. This process varies the air flow around the throttle plate when the throttle is closed. In order 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

After an ignition cycle, when the ignition key is turned OFF, the PCM will first seat the IAC pintle in the air bypass bore, and will then retract the pintle a predetermined number 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.

Diagnostic Aids

Inspect for the following conditions:

    • A skewed TP sensor signal
    • A restricted air intake system
    • Objects blocking the IAC passage or the throttle bore
    • The correct, properly installed, and proper operation of the positive crankcase ventilation (PCV) valve
    • Proper operation and installation of all air intake components
    • Proper installation and operation of the mass air flow (MAF) sensor, if equipped
    • A tampered with or damaged throttle stop screw
    • A tampered with or damaged throttle plate, throttle shaft, throttle linkage, or cruise control linkage, if equipped
    • Excessive deposits in the IAC passage or on the IAC pintle
    • Excessive deposits in the throttle bore or on the throttle plate
    • Vacuum leaks
    • A high or unstable idle condition can be caused by a non-IAC system condition that can not be overcome by the IAC valve. Refer to Symptoms - Engine Controls .
    • A low or unstable idle condition could be caused by a non-IAC system condition that cannot be overcome by the IAC valve. Refer to Symptoms - Engine Controls .

If the condition is intermittent, refer to Intermittent Conditions .

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. This test will determine the ability of the PCM and the IAC valve circuits to control the IAC valve.

    2.

  2. This test will determine the ability of the PCM to provide the IAC valve circuits with a ground. On a normally-operating system, the test lamp should not flash when the IAC counts are incrementing.

Step

Action

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Diagnostic System Check-Engine Controls?

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

Important: Ensure that the engine speed stabilizes with each commanded RPM change in order to determine if the engine speed stays within 50 RPM of the commanded RPM.

  1. Set the parking brake, and block the drive wheels.
  2. Install a scan tool.
  3. Start the engine.
  4. Turn OFF all accessories.
  5. With the scan tool RPM control function, slowly increment engine speed to 1,800 RPM, then to 600 RPM, then to 1,800 RPM.
  6. Exit the RPM control function.

Did the engine speed stabilize within 50 RPM of the commanded RPM during the above test?

Go to Diagnostic Aids

Go to Step 3

3
  1. Turn OFF the ignition.
  2. Disconnect the IAC valve.
  3. Connect an J 37027-A IAC Motor Driver to the IAC valve.
  4. Start the engine.
  5. With theJ 37027-A , command the IAC valve in until near 600 RPM is reached.
  6. With theJ 37027-A , command the IAC valve out until near 1,800 RPM is reached.
  7. Return engine speed to desired idle as indicated on the scan tool data list.

Did the engine speed steadily decrease to near 600 RPM and steadily increase to near 1,800 RPM when the IAC valve was commanded in and out?

Go to Step 5

Go to Step 4

4

Did you observe an excessively high idle condition that could not be controlled with the IAC motor driver during the last test?

Go to Step 10

Go to Step 11

5

  1. With a test lamp connected to ground, probe on of the IAC valve circuits at the IAC valve harness connector using the J 35616 Connector Test Adaptor Kit.
  2. Start the engine.
  3. With the J 37027-A IAC Motor Driver, command low RPM while observing a scan tool until the IAC counts start to increment.
  4. With the J 37027-A , command high RPM while observing a scan tool until the IAC counts start to increment.
  5. While the IAC counts are incrementing observe the test lamp.
  6. Return engine speed to the desired idle, as indicated on the scan tool data list.
  7. Repeat the above procedure for the other three IAC valve circuits.

Did The test lamp remain ON and never flashing when the IAC Counts were incrementing at any of the IAC valve circuits during the above test?

Go to Step 9

Go to Step 6

6

Did the test lamp remain OFF and never flashing when the IAC Counts were incrementing at any of the IAC valve circuits during the above test?

Go to Step 8

Go to Step 7

7

  1. Connect a test lamp between the IAC coil A low circuit and the IAC coil A high circuit at the IAC valve harness connector using the J 35616 .
  2. With the J 37027-A , command low RPM while observing a scan tool, until the IAC counts start to increment.
  3. With the J 37027-A , command high RPM while observing a scan tool until the IAC counts start to increment.
  4. While the IAC counts are incrementing observe the test lamp.
  5. Return engine speed to desired idle as indicated on the scan tool data list.
  6. Repeat the above procedure with the test lamp connected between the IAC coil B low circuit and the IAC coil B high circuit at the IAC valve harness connector.

Did the test lamp stay illuminated and never flashing while the IAC counts were incrementing during the above test?

Go to Diagnostic Aids

Go to Step 15

8
  1. Turn OFF the ignition.
  2. Disconnect the PCM harness connectors.
  3. With a DMM, test for an open or short to ground on the IAC valve circuit where the test lamp remained OFF. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 16

Go to Step 12

9
  1. Turn ON the ignition.
  2. With a DMM, test for a short to voltage on the IAC valve circuit where the test lamp remained illuminated. Refer to Circuit Testing , and refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 16

Go to Step 12

10
  1. Inspect for the following conditions:
    2. • Throttle body damage and tampering
    • Skewed TP sensor signal
    • Throttle lever screw tampering, if equipped
    • Vacuum leaks
    • A faulty or incorrectly installed PCV valve and hose, if equipped
    • Throttle shaft binding
    • Throttle linkage or cruise control linkage binding, if equipped
  2. Remove the IAC valve. Refer to Idle Air Control Valve Replacement . Inspect for the following:
    3. • Debris in the IAC passage
    • Excessive deposits on the throttle plate
    • Excessive deposits on the throttle bore
    • Excessive deposits on the IAC valve pintle
  3. Repair any of the above conditions as necessary. Refer to the appropriate repair procedure.

Did you find and correct the condition?

Go to Step 16

Go to Step 13

11
  1. Inspect for the following:
    2. • Throttle body damage and tampering, throttle lever screw tampering if equipped
    • Restricted air intake system--Inspect for a possible collapsed or clogged air intake duct before and after the air filter element, a restricted air filter element, a restriction at the throttle body intake screen, if equipped.
    • Excessive deposits on the throttle plate
    • Excessive deposits on the throttle bore
  2. Remove the IAC valve and inspect for excessive deposits on the IAC valve pintle and in the IAC valve passage.
  3. Repair any of the above conditions as necessary. Refer to the appropriate repair procedure.

Did you find and correct the condition?

Go to Step 16

Go to Step 13

12

Inspect for a poor connection at the PCM harness connectors. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 16

Go to Step 15

13

Inspect for a poor connection at the IAC valve harness connectors. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 16

Go to Step 14

14

Replace the IAC valve. Refer to Idle Air Control Valve Replacement .

Did you complete the replacement?

Go to Step 16

--

15

Replace the PCM. Refer to Powertrain Control Module Replacement .

Did you complete the replacement?

Go to Step 16

--

16
  1. Use the scan tool in order to clear any DTCs that may have set.
  2. Turn OFF the ignition for 30 seconds.
  3. Operate the vehicle within conditions to verify the repair.

Did you verify the repair?

Go to Step 17

Go to Step 2

17

With a scan tool observe the stored information in Capture Info.

Does the scan tool display and DTCs that you have not diagnosed?

Go to Diagnostic Trouble Code (DTC) List

System OK