GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2002 🢒 Envoy - 4WD 🢒 Driveline/Axle 🢒 Transfer Case - NVG 226-NP8 🢒 Front Axle Will Not Engage

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
    • A battery feed 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.

Test Description

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

  1. This step tests the electrical function of the front axle actuator and helps determine between mechanical and electrical malfunctions.

    2.

  2. This step tests for proper power and ground at the front axle actuator.

    3.

  3. This step tests the operation of the front axle control circuit.

    4.

  4. This step helps determine if the transfer case shift control module is able to monitor the front axle switch.

    5.

  5. This step tests the front axle switch circuit for an open or high resistance.

    6.

  6. This step measures the voltage between the front axle feed circuit and ground.

    7.

  7. This step tests the front axle control circuit for an open or high resistance.

    8.

  8. This step inspects the front axle for a mechanical malfunction.

    9.

  9. This step tests for open fuse in the front axle feed circuit.

Step

Action

Values

Yes

No

1

Did you perform the Diagnostic System Check - Transfer Case?

--

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. Using a scan tool, command the front axle Unlocked and Locked.

Does the front axle turn ON and OFF with each command?

--

Go to Step 9

Go to Step 3

3
  1. Turn the ignition OFF.
  2. Disconnect the front axle actuator.
  3. Turn the ignition ON, with the engine OFF.
  4. Measure the voltage between the ignition 3 voltage circuit and the front axle ground circuit at the front axle actuator.

Is the voltage reading within the specified value?

11-13 V

Go to Step 4

Go to Step 7

4
  1. Connect a test lamp between the ignition 3 voltage circuit and the front axle actuator control circuit at the front axle actuator.
  2. Observe the test lamp, while using the scan tool, in order to engage the front axle.

Does the test lamp illuminate?

--

Go to Step 5

Go to Step 8

5
  1. Connect a fused jumper wire between the ignition 3 voltage circuit and the front axle actuator signal circuit.
  2. Use the scan tool in order to read the Front Axle Switch status.

Does the scan tool display the Front Axle Switch status Locked?

--

Go to Step 13

Go to Step 6

6

Test the front axle actuator signal 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 15

Go to Step 14

7

Measure the voltage between the ignition 3 voltage circuit and ground.

Is the voltage reading within the specified range?

9-14 V

Go to Step 12

Go to Step 10

8

Test the front axle control 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 15

Go to Step 14

9

Inspect the front axle for a mechanical malfunction.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 13

10

Test for an open fuse in the Ignition 3 voltage circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 15

Go to Step 11

11

Repair an open in the Ignition 3 voltage circuit. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 15

--

12

Repair an open in the front axle ground circuit. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 15

--

13

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

Did you complete the repair?

--

Go to Step 15

--

14

Important: After transfer case shift control module replacement and reprogramming, ensure all doors are closed and the key is out of the ignition, then open and close the doors again and wait 2 minutes to avoid incomplete programming or false communication DTCs.

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 15

--

15
  1. Use the scan tool in order to clear the DTCs.
  2. Operate the vehicle within the conditions for the symptom to occur.

Does the symptom occur?

--

Go to Step 2

System OK