GM Service Manual Online
For 1990-2009 cars only

Circuit Description

The vehicle control module (VCM)/powertrain control module (PCM) monitors the class 2 serial data circuit for communications from the transfer case shift control module. If the transfer case shift control module is unable to communicate with the VCM this DTC will set.

Conditions for Setting the DTC

No class 2 serial data from the transfer case shift control module.

Action Taken When the DTC Sets

This DTC will not illuminate the MIL.

Conditions for Clearing the MIL/DTC

    • A history DTC will clear after 40 consecutive warm up cycles with no failures of any non-emissions related diagnostic test.
    • VCM/PCM battery voltage is interrupted.
    • Use a scan tool.

Diagnostic Aids

This DTC will not set unless the VCM/PCM and the transfer case shift control module have already established communications. This DTC will only diagnosis an open or intermittent connection from the transfer case shift control module to the splice.

Test Description

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

  1. This step verifies that communication may be established between the scan tool and the transfer case shift control module.

  2. This step tests for any opens in the class 2 serial data circuit between the VCM and the transfer case shift control module. This step also tests for any intermittent malfunctions associated with the serial data circuit.

  3. Use a scan tool in order to clear and check for any DTCs in all the modules connected to the class 2 serial data circuit (the VCM/PCM, the EBCM and the transfer case shift control module).

Step

Action

Value(s)

Yes

No

Important: Before clearing the DTCs, use the scan tool in order to record the Freeze Frame and the Failure Records for reference. This data will be lost when the Clear DTC Information function is used.

1

Did you perform the Powertrain On-Board Diagnostic (OBD) System Check?

--

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check in Engine Controls - 4.3L

2

  1. Install the scan tool.
  2. Turn the ignition switch to the ON position. Do not start the engine.
  3. Attempt to establish communications with the transfer case shift control module.

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

--

Go to Step 3

Go to Diagnostic System Check - Data Link Communications

3

Inspect the class 2 serial data circuit for the following intermittent conditions:

    • An open between the VCM and the transfer case shift control module
    • A short to ground
    • A short to voltage
    • Loose or damaged terminals at the DLC, the VCM, or the transfer case shift control module

Did you find an intermittent condition?

--

Go to Step 6

Go to Step 4

4

  1. Turn the ignition switch to the OFF position.
  2. Connect or install any connectors or components that were disconnected or removed.
  3. Clear all DTCs using a scan tool.
  4. Turn the ignition switch to the OFF position for 15 seconds.
  5. Operate the vehicle within the conditions listed in the Failure Records.

Does DTC U1026 reset?

--

Go to Diagnostic System Check - Data Link Communications

Go to Step 5

5

This DTC is intermittent.

Are any additional DTCs stored?

--

Go to the Applicable DTC

Go to Diagnostic System Check - Data Link Communications

6

Repair the circuit as necessary. Refer to Wiring Repairs in Wiring Systems.

Is the repair complete?

--

Go to Step 7

--

7

  1. Turn the ignition switch to the OFF position.
  2. Connect or install any connectors or components that were disconnected or removed.
  3. Clear all DTCs using a scan tool.

Are any DTCs displayed?

--

Go to the Applicable DTC

Go to Diagnostic System Check - Data Link Communications