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 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 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. Helps determine if the transfer case shift control module is able to monitor the front axle switch.

    5.

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

    6.

  6. Measures the voltage between the front axle feed circuit and ground.

    7.

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

    8.

  8. Inspects the front axle for a mechanical malfunction.

    9.

  9. Tests for open in the front axle feed circuit.

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. Install a scan tool.
  2. Turn the ignition ON, with the engine OFF.
  3. With 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 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 7

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 12

Go to Step 6

6

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 14

Go to Step 13

7

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 11

Go to Step 10

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 14

Go to Step 13

9

Inspect the front axle for a mechanical malfunction.

Was the condition found and corrected?

--

Go to Step 14

Go to Step 12

10

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 14

--

11

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 14

--

12

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

Is the repair complete?

--

Go to Step 14

--

13

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

Is the repair complete?

--

Go to Step 14

--

14
  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