GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2002 🢒 Envoy - 4WD 🢒 Driveline/Axle 🢒 Transfer Case - NVG 226-NP8 🢒 DTC C0379

Circuit Description

The front axle control circuit consists of an electric motor actuator, which engages and disengages the front axle. The front axle actuator motor consists of a permanent magnetic (PM) motor, a worm gear controlled plunger, a front axle switch and an electronic control circuit, all within the actuator assembly.

The front axle actuator consists of the following circuits:

    • The front axle control circuit, which is also connected to the transfer case shift control module
    • The front axle switch circuit, which is also connected to the transfer case shift control module
    • An ignition 3 voltage circuit
    • A ground circuit

When a shift to AUTO 4WD, 4HI, or 4LO is requested, the transfer case shift control module engages the front axle by grounding the front axle control circuit through a current limiting driver.

This DTC detects a short to ground, short to voltage or an open circuit.

Conditions for Running the DTC
    • The ignition is ON.
    • System voltage is 9-18 volts.

Conditions for Setting the DTC
    • The transfer case shift control module monitors the state of the front axle switch. The transfer case shift control module reads back the front axle switch circuit for a high and low voltage. The DTC sets if this circuit is high when a low voltage is present or when the circuit is low when a high voltage is present for 10 seconds.
    • The DTC sets if this circuit is high when a low voltage is present or when the circuit is low when a high voltage is present for 10 seconds.

Action Taken When the DTC Sets

The SERVICE 4WD indicator remains illuminated for the remainder of the current ignition cycle.

Conditions for Clearing the DTC
    • The transfer case shift control module clears the DTC if the condition for setting the DTC is not currently present.
    • A history DTC clears after 100 consecutive ignition cycle without a fault present.
    • History DTCs can be cleared using a scan tool.

Test Description

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

  1. This step tests the front axle circuit for a current malfunction.

    2.

  2. This step helps determine if the failure is the result of a short to voltage or ground.

    3.

  3. This step helps determine if the front axle control circuit is shorted to ground.

    4.

  4. This step tests the integrity of the front axle switch and the ignition 3 voltage circuits.

    5.

  5. This step tests the front axle control circuit for short to ground.

    6.

  6. This step tests the front axle control circuit for short to voltage or ground.

    7.

  7. This step tests the front axle switch signal circuit for a short to voltage.

    8.

  8. This step tests the front axle ignition 3 voltage circuit and the front axle switch circuit for an open or short to ground.

    9.

  9. This step tests the front axle control circuit and the front axle ground circuit for an open or high resistance.

Step

Action

Yes

No

Schematic Reference: Transfer Case Control Schematics

Connector End View Reference: Transfer Case Control Connector End Views

1

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 2

Go to Diagnostic System Check - Transfer Case

2

  1. Install a scan tool.
  2. Turn the ignition ON, with the engine OFF.
  3. Place the transfer case into 2WD mode and change to 4WD.
  4. With a scan tool, observe the Front Axle Switch parameter.

Does the Front Axle Switch parameter change from Unlocked/Disengaged to Locked/Engaged?

Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems

Go to Step 3

3

Place the transfer case into 2WD mode.

Does the scan tool parameter continue to display Locked/Engaged?

Go to Step 4

Go to Step 5

4

Disconnect the front axle switch/actuator.

Does the parameter display Unlocked/Disengaged?

Go to Step 6

Go to Step 8

5

  1. Turn the ignition OFF.
  2. Disconnect the front axle switch/actuator.
  3. Turn the ignition ON, with the engine OFF.
  4. Connect a fused jumper across the ignition 3 voltage and the axle switch signal circuits.
  5. With a scan tool, observe the Front Axle Switch parameter.

Does the Front Axle Switch parameter change from Unlocked/Disengaged to Locked/Engaged?

Go to Step 11

Go to Step 10

6

  1. Turn the ignition OFF and disconnect the transfer case shift control module connectors C1 and C2.
  2. Test the front axle actuator control circuit for short to ground. Refer to Testing for Short to Ground and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 13

Go to Step 7

7

  1. With the transfer case shift control module connected, turn the ignition ON, with the engine OFF.
  2. Place the transfer case into 2WD.
  3. Use a DMM to probe the front axle control circuit and check the module for an internal short to ground or voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did the DMM indicate continuity to ground?

Go to Step 12

Go to Step 9

8

  1. Turn the ignition OFF.
  2. Disconnect the transfer case shift control module.
  3. Test the front axle switch signal circuit at the front axle actuator connector for a short to voltage.

    4. Refer to Testing for a Short to Voltage in Wiring Systems.

Did you find and correct the condition?

Go to Step 13

Go to Step 12

9

  1. Connect the transfer case shift control module connectors C1 and C2.
  2. Test the front axle switch signal circuit for short to voltage. Refer to Testing for a Short to Voltage and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 12

Go to Step 10

10

Test the front axle ignition 3 circuit, the front axle switch circuit, and the front axle control actuator ground circuit for an open or high resistance. Refer to Testing for Continuity and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 13

Go to Step 12

11

Replace the front axle actuator. Refer to Front Drive Axle Actuator Replacement in Front Drive Axle.

Did you complete the repair?

Go to Step 13

--

12

Replace the transfer case shift control module. Refer to Transfer Case Shift Control Module Replacement and Transfer Case Shift Control Module Reprogramming .

Did you complete the repair?

Go to Step 13

--

13
  1. Use the scan tool in order to clear the DTCs.
  2. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC set?

Go to Step 2

System OK