GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2004 🢒 Jimmy - 4WD 🢒 Driveline/Axle 🢒 Transfer Case - NVG 236-NP8 🢒 Front Axle Will Not Engage

Circuit Description

The front axle control circuit consists of a front drive axle clutch solenoid, which is electronically controlled by the transfer case shift control module. The front drive axle clutch solenoid opens and closes a vacuum control valve, allowing vacuum to apply and release a clutch pack inside the front axle housing. The transfer case shift control module determines whether or not the clutch pack has been applied or released, based on a signal from the front axle switch mounted on the front axle housing assembly. The front axle is normally locked, or engaged, during AUTO 4WD, 4WD HI, and 4WD LO modes. The front axle is unlocked, or disengaged, only during 2HI or NEUTRAL modes.

Test Description

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

  1. This step helps determine whether the transfer case shift control module has electronic control of the front axle actuator.

    2.

  2. This step tests the operation of the front axle switch by determining whether the state of the front axle switch is correct.

    3.

  3. This step determines whether correct vacuum is supplied to the front axle actuator.

    4.

  4. This step tests for proper voltage supplied to the front axle solenoid.

    5.

  5. This step helps determine continuity through the front axle control circuit, testing the wiring and module driver.

    6.

  6. This step tests the front axle circuit for an open.

    7.

  7. This step tests the front axle switch for an internal open.

    8.

  8. This step tests the front axle switch feed circuit for proper voltage.

    9.

  9. This step tests the front axle switch circuit for an open.

Step

Action

Value(s)

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 - 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 ON.
  3. With the scan tool, command the front axle to Locked and Unlocked.

Does the Front Axle Switch status change with each command?

--

Go to Step 5

Go to Step 3

3

Drive the vehicle in four wheel drive mode in order to determine if torque is being applied to the front wheels. The vehicle will display crow-hop or jerking during tight turns.

Is torque being applied to the front wheels?

--

Go to Step 8

Go to Step 4

4

While the engine is running, inspect the front drive axle clutch solenoid for proper vacuum supply.

Is there proper vacuum supplied to the front drive axle clutch solenoid?

--

Go to Step 5

Go to Step 12

5

  1. Turn ON the ignition, with the engine OFF.
  2. Disconnect the connector at the front drive axle clutch solenoid.
  3. Test the ignition 3 voltage circuit at the front axle vacuum solenoid for proper voltage.

Does the voltage reading indicate battery voltage?

--

Go to Step 6

Go to Step 13

6

  1. Connect a DMM between the front axle control circuit harness connector at the front drive axle clutch solenoid and ground.
  2. Use a scan tool in order to lock the front axle while monitoring the resistance value on the DMM.

Is the resistance reading within the specified values?

0-2 ohms

Go to Step 15

Go to Step 7

7

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

Was the condition found and corrected?

--

Go to Step 17

Go to Step 16

8

  1. Turn ON the ignition, with the engine OFF.
  2. Disconnect the connector at the front axle switch.
  3. Use a fused jumper wire and jump across the front axle switch harness connector.
  4. Observe the status of the Front Axle Switch with the scan tool.

Does the Front Axle Switch status change from Unlocked to Locked?

--

Go to Step 14

Go to Step 9

9

  1. Turn ON the ignition, with the engine OFF.
  2. Using a DMM, check the ignition 3 voltage circuit for proper voltage.

Does the voltage reading indicate battery voltage?

--

Go to Step 10

Go to Step 11

10

  1. Disconnect the transfer case shift control module.
  2. Test the front axle switch signal circuit for an open. Refer to Testing for Continuity and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 17

Go to Step 16

11

Repair an open or short to ground in the front axle switch feed circuit. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 17

--

12

Repair a vacuum leak between the engine and the front drive axle clutch solenoid.

Did you complete the repair?

--

Go to Step 17

--

13

Repair an open or short to ground in the front axle clutch solenoid feed circuit. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 17

--

14

Replace the four-wheel drive indicator switch. Refer to Four-Wheel Drive Indicator Switch Replacement .

Did you complete the replacement?

--

Go to Step 17

--

15

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

Did you complete the replacement?

--

Go to Step 17

--

16

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

Did you complete the replacement?

--

Go to Step 17

--

17
  1. Use the scan tool in order to clear the DTCs .
  2. Operate the vehicle in order to determine if the symptom has been corrected.

Is the symptom still present?

--

Go to Step 2

System OK