GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2001 🢒 Yukon XL - 4WD 🢒 Driveline/Axle 🢒 Transfer Case - NVG 236/246-NP8 (Two Speed Automatic) 🢒 Front Axle Will Not Disengage

Circuit Description

The front axle control circuit consists of an electric motor actuator which engages and disengages the front axle. The front axle actuator's 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 has four circuits connected to it, a front axle feed circuit, front axle ground circuit, front axle control circuit and a front axle switch circuit.

Whenever 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. Tests the electrical function of the front axle actuator. Helps determine between mechanical and electrical malfunctions.

    2.

  2. Tests for proper power and ground at the front axle actuator.

    3.

  3. Tests the operation of the front axle control circuit.

    4.

  4. Tests the electrical function of the front axle actuator control circuit.

    5.

  5. Tests for proper supply voltage to the front axle actuator.

    6.

  6. Tests for open fuse in the front axle feed circuit.

    7.

  7. Tests the front axle control circuit for an open or high resistance.

    8.

  8. Tests the front axle switch circuit for an open or high resistance.

    9.

  9. Helps determine if the front axle actuator has a mechanical malfunction.

    10.

  10. Tests the front axle switch circuit for a short to voltage.

    11.

  11. Inspect the front axle for a mechanical malfunction.

Step

Action

Value(s)

Yes

No

Schematic Reference: Transfer Case Control Schematics

1

Was the Transfer Case Diagnostic System Check performed?

--

Go to Step 2

Go to Diagnostic System Check - Transfer Case

2

  1. Turn the ignition ON, with the engine OFF.
  2. Ensure that the vehicle is in 2WD mode.
  3. Install a scan tool.
  4. Use the scan tool in order to read the Front Axle Switch Status.

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

--

Go to Step 10

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 front axle feed circuit and the front axle ground circuit at the front axle actuator.

Is the voltage reading within the specified values?

9-14 V

Go to Step 4

Go to Step 6

4

  1. Connect a test lamp between the front axle feed circuit and the front axle 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 light?

--

Go to Step 5

Go to Step 8

5

  1. Connect a fused jumper wire between the front axle feed circuit and the front axle switch 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 15

Go to Step 9

6

Measure the voltage between the front axle feed circuit and ground.

Is the voltage reading within the specified values?

9-14 V

Go to Step 13

Go to Step 7

7

Test for open fuse in the front axle feed circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Was the condition found and corrected?

--

Go to Step 17

Go to Step 14

8

Test the front axle control circuit for an open or high resistance. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Was the condition found and corrected?

--

Go to Step 17

Go to Step 16

9

Test the front axle switch circuit for an open or high resistance. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Was the condition found and corrected?

--

Go to Step 17

Go to Step 16

10

  1. Disconnect the front axle actuator.
  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 11

Go to Step 12

11

Test the front axle switch circuit for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Was the condition found and corrected?

--

Go to Step 17

Go to Step 16

12

Inspect the front axle for a mechanical malfunction.

Was the condition found and corrected?

--

Go to Step 17

Go to Step 16

13

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

Is the repair complete?

--

Go to Step 17

--

14

Repair an open in the front axle feed circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Is the repair complete?

--

Go to Step 17

--

15

Replace the front axle switch/actuator. Refer to Electric Motor Actuator Replacement .

Is the repair complete?

--

Go to Step 17

--

16

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

Is the repair complete?

--

Go to Step 17

--

17
  1. Use the scan tool in order to clear the DTCs .
  2. Operate the vehicle within the Conditions for allowing the symptom to re-occur.

Does the symptom re-occur?

--

Go to Step 2

System OK