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For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2001 🢒 Safari - 2WD 🢒 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 SFI and 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 2-phase, bi-polar, permanent magnet stepper motor that is capable of highly accurate rotation 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 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 the PCM has commanded to determine IAC pintle position. The PCM uses the IAC valve to control engine idle speed. The PCM changes 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. 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 then retracts it 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 or the following conditions:

    • A skewed (TP) sensor signal
    • A restricted air intake system
    • Objects blocking the IAC passage or throttle bore
    • The correct, properly installed, and proper operation of the (PCV) valve
    • The proper operation and installation of all air intake components
    • The proper installation and operation of the mass air flow 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 could 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 can not be overcome by the IAC valve. Refer to Symptoms - Engine Controls .

If the condition is determined to be 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 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 while the IAC counts are incrementing.

Step

Action

Yes

No

Schematic Reference:

Speed & Cruise Controls


Object Number: 655259 Size: FS
Master Electrical Component List
Powertrain Control Module Description
Transmission Shift Controls
Oxygen Sensors
OBD II Symbol Description Notice
G101, G106 and G302

1

Did you perform the Diagnostic System Check-Engine Controls?

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

Important: Ensure 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. Use the scan tool RPM control function in order to slowly increment engine speed to 1,800 RPM, down to 600 RPM, and back to 1,800 RPM.
  6. Exit the RPM control function.

Does the engine speed stabilize within 50 RPM of the commanded RPM?

Go to Diagnostic Aids

Go to Step 3

3
  1. Turn OFF the ignition.
  2. Disconnect the IAC valve.
  3. Connect anJ 37027-A IAC motor driver to the IAC valve.
  4. Start the engine.
  5. With the J 37027-A , command the IAC valve in until the engine speed is near 600 RPM.
  6. With theJ 37027-A , command the IAC valve out until the engine speed is near 1,800 RPM.
  7. Return the engine speed to the 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?

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 theJ 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 the engine speed to desired idle as indicated on the scan tool data list.
  7. Repeat the above procedure for the other IAC valve circuits.

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

Go to Step 9

Go to Step 6

6

Did the test lamp remain OFF, and never flash, while the IAC Counts were incrementing at any of the IAC valve circuits?

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 theJ 37027-A , command low RPM while observing a scan tool until the IAC Counts start to increment.
  3. With theJ 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 the engine speed to the 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?

Go to Diagnostic Aids

Go to Step 15

8
  1. Turn OFF the ignition.
  2. Disconnect the PCM harness connectors.
  3. Use a digital multimeter (DMM) in order to test for an open and for short to ground on the IAC valve circuit. Refer to Circuit Testing and to 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. Use the DMM in order to test for a short to voltage on the IAC valve circuit. Refer to Circuit Testing and 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. • Any throttle body damage or tampering
    • A skewed TP sensor
    • Any throttle lever screw tampering, if equipped
    • Any vacuum leaks
    • A faulty or incorrectly installed PCV valve and hose, if equipped
    • Any throttle shaft binding
    • Any throttle linkage or cruise control linkage binding, if equipped
  2. Remove the IAC valve and inspect for the following conditions. Refer to Idle Air Control Valve Replacement :
    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 conditions:
    2. • Any throttle body damage and tampering and any throttle lever screw tampering, if equipped
    • A 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
    • Any excessive deposits on the throttle plate
    • Any excessive deposits on the throttle bore
    • Any excessive deposits on the IAC valve pintle and in the IAC valve passage.
  2. 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 to 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 to 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 Programming .

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 any DTCs that you have not diagnosed?

Go to Diagnostic Trouble Code (DTC) List

System OK