GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2002 🢒 Envoy - 4WD 🢒 Driveline/Axle 🢒 Transfer Case - NVG 126-NP4 🢒 Diagnostic System Check - Transfer Case

These diagnostic procedures will help you to find and repair Automatic Transfer Case (ATC) system malfunctions. This service category also contains information for repairing ATC system malfunctions. For best results, use the diagnostic tables, and follow the sequence listed below:

  1. Perform the Diagnostic System Check - Transfer Case. All ATC diagnostics must begin with the Diagnostic System Check - Transfer Case.

    2. The Diagnostic System Check - Transfer Case determines the following:

    • Proper SERVICE AWD indicator operation
    • Ability of the transfer case shift control module to communicate through the Data Link Connector (DLC)
    • Existence of ATC diagnostic trouble codes (DTC)
  2. Refer to the diagnostic table as directed by the Diagnostic System Check - Transfer Case. The diagnostic tables help you to diagnose any ATC system malfunction.

    3. Bypassing these procedures may result in the following:

    • Extended diagnostic time
    • Incorrect diagnosis
    • Incorrect parts replacement
  3. Repeat the Diagnostic System Check - Transfer Case after you perform any repair or diagnostic procedures. This will verify that you correctly performed the repair and also ensure that no other malfunctions exist.

Test Description

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

  1. Lack of communication may be due to a partial malfunction of the class 2 serial data circuit or due to a total malfunction of the class 2 serial data circuit. The specified procedure will determine the particular condition.

    2.

  2. A fuse failure will cause no communication this step checks for this condition.

    3.

  3. This step determines if the fuse blows when the ignition is turned on.

    4.

  4. This step determines if there is a short to ground in the battery positive circuit causing the fuse to blow.

    5.

  5. The presence of DTCs indicates that the transfer case shift control module has detected a fault. Go to symptoms for other concerns.

    6.

  6. The presence of DTCs which begin with "U" indicate some other module is not communicating. The specified procedure will compile all the available information before tests are performed.

    7.

  7. Vehicle speed is provided to the transfer case shift control module via Class 2 and can effect the setting of DTC C0300. Perform the diagnostics for powertrain DTCs P0502 and P0503 first.

    8.

  8. Perform diagnostics for DTC C0300 and C0305 first before addressing other DTCs. DTC C0300 and C0305 can be responsible for setting other DTCs.

Step

Action

Yes

No

1

Install a scan tool.

Does the scan tool power up?

Go to Step 2

Go to Scan Tool Does Not Power Up

2

  1. Turn ON the ignition, with the engine OFF.
  2. Attempt to establish communication with the transfer case shift control module.

Does the scan tool communicate with the transfer case shift control module?

Go to Step 6

Go to Step 3

3

  1. Turn off the ignition.
  2. Check the underhood ATC fuse.

Did the ATC fuse blow?

Go to Step 4

Go to Scan Tool Does Not Communicate with Class 2 Device

4

  1. Replace the ATC fuse.
  2. Turn on the ignition.

Does the ATC fuse blow?

Go to Step 5

Go to DTC C0306

5

Check the battery positive circuit for a short to ground.

Did you find and correct the condition?

Go to Step 2

Go to Transfer Case Shift Control Module Replacement and Transfer Case Shift Control Module Reprogramming

6

Select the transfer case shift control module display DTCs function on the scan tool.

Does the scan tool display any DTCs?

Go to Step 7

Go to Symptoms - Transfer Case

7

Does the scan tool display any DTCs which begin with a "U"?

Go to Scan Tool Does Not Communicate with Class 2 Device

Go to Step 8

8

Does the scan tool display DTC P0502 or P0503?

Go to DTC P0502 or DTC P0503

Go to Step 9

9

Does the scan tool display DTC C0300 or C0305?

Go to DTC C0300 or DTC C0305

Go to Diagnostic Trouble Code (DTC) List