GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2001 🢒 Jimmy - 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 a Front Drive Axle Clutch Solenoid which is electronically controlled by the transfer case shift control module. This 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 2 HI or neutral modes.

Test Description

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

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

    2.

  2. Visual inspection of the front axle to determine if the front axle is mechanically engaged.

    3.

  3. Helps determine if the front axle control circuit is shorted to ground.

    4.

  4. Tests the front axle control circuit for a shorted to ground.

    5.

  5. Helps determine whether the short to ground is due to faulty wiring or module.

    6.

  6. Helps determine a faulty front axle switch.

    7.

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

    8.

  8. Helps determine whether the short to voltage is due to faulty wiring or module.

    9.

  9. Helps determine a faulty front axle actuator.

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 ON the ignition, with the engine ON.
  2. Place the vehicle into 2WD mode.
  3. Connect a scan tool in order to monitor the Front Axle Switch status.
  4. Use the scan tool to disengage the front axle.

Does the Front Axle Switch status change from LOCKED to UNLOCKED?

--

Go to Step 17

Go to Step 3

3

Inspect the front axle to determine if it is mechanically engaged to the transfer case.

Is the front axle mechanically engaged to the transfer case?

--

Go to Step 4

Go to Step 7

4

  1. Turn OFF the ignition.
  2. Disconnect the connector at the front axle clutch solenoid.
  3. Turn ON the ignition, with the engine ON.
  4. Use a scan tool in order to monitor the Front Axle Switch status.

Does the Front Axle Switch status change from LOCKED to UNLOCKED?

--

Go to Step 5

Go to Step 10

5

  1. Turn ON the ignition, with the engine OFF.
  2. Use the scan tool to disengage the front axle.
  3. Connect a DMM and measure between the front axle control circuit harness connector at the front drive axle clutch solenoid and ground.

Is the resistance reading within the specified value?

OL

Go to Step 16

Go to Step 6

6

  1. Disconnect the transfer case shift control module.
  2. With the DMM still connected measure between the front axle control circuit harness connector at the front drive axle clutch solenoid and ground.

Is the resistance reading within the specified value?

OL

Go to Step 17

Go to Step 13

7

  1. Turn ON the ignition, with the engine OFF.
  2. Use a scan tool in order to monitor the Front Axle Switch status.
  3. Disconnect the front axle switch.

Does the Front Axle Switch status change from LOCKED to UNLOCKED?

--

Go to Step 15

Go to Step 8

8

Connect a DMM between the front axle switch signal circuit at the front axle switch and ground.

Is the voltage reading within the specified value?

0 V

Go to Step 17

Go to Step 9

9

Disconnect the transfer case shift control module.

Is the voltage reading within the specified value?

0 V

Go to Step 17

Go to Step 12

10

  1. Turn ON the ignition, with the engine ON.
  2. Disconnect the vacuum source from the front drive axle clutch solenoid.

Does the Front Axle Switch status change from LOCKED to UNLOCKED?

--

Go to Step 16

Go to Step 11

11

Inspect the front axle for a mechanical failure.

Are there any mechanical failures?

--

Go to Step 14

Go to Step 16

12

Repair a short to voltage in the front axle switch signal circuit. Refer to Wiring Repairs in Wiring Systems.

Is the repair complete?

--

Go to Step 18

--

13

Repair a short to ground in the front axle control circuit. Refer to Wiring Repairs in Wiring Systems.

Is the repair complete?

--

Go to Step 18

--

14

Repair a mechanical failure inside the transfer case. Refer to Transfer Case Disassemble in Transmission/Tranaxle/Transfer Case- Unit Repair Maunel.

Is the repair complete?

--

Go to Step 18

--

15

Replace the front axle switch. Refer to Four-Wheel Drive Indicator Switch Replacement .

Did you complete the replacement?

--

Go to Step 18

--

16

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

Did you complete the replacement?

--

Go to Step 18

--

17

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

Did you complete the replacement?

--

Go to Step 18

--

18
  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