GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1999 🢒 Safari - 2WD 🢒 Driveline/Axle 🢒 Transfer Case 🢒 Transfer Case Diagnostic System Check

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 Transfer Case Diagnostic System Check. All ATC diagnostics must begin with the Transfer Case Diagnostic System Check. The Transfer Case Diagnostic System Check determines the following:
    2. • Proper SERVICE indicator (AWD/4WD) lamp 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 Transfer Case Diagnostic System Check. The diagnostic tables will help enable you to diagnose any ATC system malfunction. Bypassing these procedures may result in the following:
    3. • Extended Diagnostic Time
    • Incorrect Diagnosis
    • Incorrect Parts Replacement
  3. Repeat the Transfer Case Diagnostic System Check after you perform any repair or diagnostic procedures. This will verify that you correctly performed the repair. This will also ensure that no other malfunctions exist.

Circuit Description

The ignition switch supplies Ignition 3 voltage to the transfer case shift control module at connector C1 terminal E10 using the 4WD Fuse. When the ignition switch is first turned to the RUN position, the shift control module responds by flashing the SERVICE indicator (AWD/4WD) lamp for approximately 2 seconds, then turning it OFF, while performing tests on the ATC system. The shift control module will continue to perform tests on the ATC system during a normal drive cycle.

Diagnostic Aids

You must diagnose the diagnostic trouble codes in the order specified by the Transfer Case Diagnostic System Check. Failure to do so may result in the following:

    • Extended Diagnostic Time
    • Incorrect Diagnosis
    • Incorrect Parts Replacement

Test Description

The number(s) below refer to the step number(s) on the diagnostic table.

  1. This test checks for proper operation of the serial data line.

    2.

  2. This test will identify the stored diagnostic trouble codes and whether or not they are current or history.

    3.

  3. The SERVICE indicator (AWD/4WD) lamp should turn off after approximately 2 seconds.

Transfer Case Diagnostic System Check

Step

Action

Value(s)

Yes

No

1
  1. Turn the ignition switch to the RUN position.
  2. Observe the SERVICE indicator (AWD/4WD) lamp on the instrument cluster.

Did the SERVICE indicator (AWD/4WD) lamp turn ON?

--

Go to Step 2

Go to Service Indicator Inoperative

2

  1. Turn the ignition to OFF.
  2. Connect the Scan Tool to the data link connector (DLC). Follow the directions in the instruction manual of the scan tool.
  3. Turn the ignition switch to the RUN position.
  4. Attempt to communicate with the transfer case shift control module.

Does the scan tool communicate with the module?

--

Go to Step 3

Go to Diagnostic System Check - Data Link Communications

3

Request for ATC diagnostic trouble codes (DTCs) display.

Are there any Current DTCs present in the module?

--

Go to Diagnostic Trouble Codes

Go to Step 4

4

Are there any History DTCs present?

--

Go to Step 9

Go to Step 5

5

  1. Turn the ignition to OFF.
  2. Turn the ignition switch to the RUN position.
  3. Observe the SERVICE indicator (AWD/4WD) lamp on the instrument cluster.

Did the SERVICE indicator (AWD/4WD) lamp turn ON and turn OFF after approximately 2 seconds?

--

Go to Step 6

Go to Service Indicator Always On

6

Was the vehicle brought in for a performance concern?

--

Go to Powertrain On Board Diagnostic (OBD) System Check

Go to Step 7

7
  1. Take the vehicle on a road test.
  2. Attempt to duplicate the customer's complaint.
  3. After the road test check for ATC DTCs.

Were there any ATC DTCs present?

--

Go to Diagnostic Trouble Codes

Go to Step 8

8

Were there any mechanical problems identified on the road test?

--

Go to Transfer Case Mechanical System Diagnosis

System OK

9
  1. A History DTC indicates a malfunction has been repaired (but DTCs were not cleared) or that malfunction is intermittent.
  2. Record any History DTCs set.
  3. Clear ATC DTCs.
  4. Take vehicle on a road test.
  5. Attempt to engage AWD.
  6. After the road test check for ATC DTCs.

Were there any ATC DTCs present?

--

Go to Diagnostic Trouble Codes

Go to Step 10

10

Refer to Diagnostic Aids for the recorded History DTC.

Are the repairs complete?

--

Go to Step 1

--