GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1999 🢒 GMC C-Series Medium Duty Truck B7 🢒 Driveline/Axle 🢒 Rear Axle Controls 🢒 Two Speed Rear Axle Inoperative

Two Speed Rear Axle Inoperative Air-Operated

Step

Action

Value(s)

Yes

No

1

Does the symptom relate to the any of the following components?

    • The two-speed rear axle shift control valve
    • The interconnecting air lines
    • The two-speed rear axle shift actuator
    • Speedometer readings

--

Go to Step 2

Go to Step 3

2

Perform the necessary repairs. Refer to Two Spd R/Axle Shft Dash Panel Switch Replacement or Two Speed Rear Axle Shift Motor Replacement .

Does the two-speed rear axle operate properly?

--

System OK

Go to Step 3

3
  1. Disconnect the two-speed rear axle air shift switch.
  2. Connect a test lamp between terminal A in the two-speed rear axle air shift switch harness connector and the chassis ground.
  3. Set the ignition switch to the RUN position.

Did the test lamp light?

--

Go to Step 5

Go to Step 4

4

Locate and repair open in CKT 441 (BRN) between terminal A in the two-speed rear axle air shift switch harness connector and the AUX 2 circuit breaker.

Does the two-speed rear axle operate properly?

--

System OK

Go to Step 5

5
  1. Disconnect the Speedo Adapter.
  2. Connect a jumper between terminal A and terminal B of the two-speed rear axle air shift switch connector.
  3. Connect a test lamp between terminal A1 at the speedo adapter connector in the IP harness and the chassis ground.

Did the test lamp light?

--

Go to Step 7

Go to Step 6

6

Locate and repair the open in CKT 1467 (GRY/BLK) between terminal B in the two-speed rear axle air shift switch harness connector and terminal A1 in the Speedo Adapter connector in the IP harness.

Does the two-speed rear axle operate properly?

--

System OK

Go to Step 7

7
  1. Use a J 39200 between terminal A and terminal B in the two-speed rear axle air shift switch.
  2. Start the engine.
  3. Set the two-speed rear axle shift control valve to the HIGH position.

Does the DMM indicate continuity?

--

Go to Step 8

Go to Step 9

8

Replace the speedo adapter.

Does the two-speed rear axle operate properly?

--

System OK

--

9

Replace the two-speed rear axle shift switch. Refer to Two Spd R/Axle Shft Dash Panel Switch Replacement .

Does the two-speed rear axle operate properly?

--

System OK

--

Two Speed Rear Axle Inoperative Electric-Operated

Step

Action

Value(s)

Yes

No

1
  1. Remove the two-speed rear axle shift relay.
  2. Connect a test lamp between terminal 85 and the chassis ground.

Did the test lamp light?

--

Go to Step 3

Go to Step 2

2

Locate and repair the open in CKT 441 (BRN) between the axle circuit breaker and terminal 85 in the two-speed rear axle shift relay connector.

Does the two-speed rear axle operate properly?

--

System OK

Go to Step 3

3

Connect a test lamp between terminal 30 in the two-speed rear axle shift relay connector and the chassis ground.

Did the test lamp light?

--

Go to Step 5

Go to Step 4

4

Locate and repair the open in CKT 441 (BRN) between S418 and terminal 30 in the two-speed rear axle shift relay connector.

Does the two-speed rear axle operate properly?

--

System OK

Go to Step 5

5
  1. Set the two-speed rear axle shift switch to the HIGH position.
  2. Connect a test lamp between terminal 30 and terminal 86 in the two-speed rear axle shift relay connector.

Did the test lamp light?

--

Go to Step 6

Go to Step 11

6
  1. Set the ignition switch to the OFF position.
  2. Set the two-speed rear axle shift switch to the HIGH position.
  3. Put the two-speed rear axle shift relay back into the relay connector.
  4. Disconnect the two-speed rear axle shift actuator (motor) connector.
  5. Connect a test lamp between terminal A and the chassis ground in the two-speed rear axle shift actuator (motor) connector.
  6. Set the ignition switch to the RUN position.

Did the test lamp light?

 

Go to Step 7

Go to Step 18

7
  1. Set the two-speed rear axle shift switch to the LOW position.
  2. Connect a test lamp between terminal C and the chassis ground in the two-speed rear axle shift actuator (motor) connector.

Did the test lamp light?

--

Go to Step 8

Go to Step 21

8

Connect a test lamp between terminal C and terminal B in the two-speed rear axle shift actuator (motor) connector.

Did the test lamp light?

--

Go to Step 9

Go to Step 10

9

Replace the two-speed rear axle shift actuator (motor). Refer to Two Speed Rear Axle Shift Motor Replacement .

Does the two-speed rear axle operate properly?

--

System OK

Go to Step 11

10

Locate and repair open in CKT 150 (BLK) between G101 (Cat© Diesel) or G102 (Gasoline) and terminal B in the two-speed rear axle shift actuator (motor) connector.

Does the two-speed rear axle operate properly?

--

System OK

Go to Step 11

11
  1. Do not disconnect the test lamp.

    2. Disconnect the two-speed rear axle shift switch connector.

  2. Connect a jumper between terminal A in the two-speed rear axle shift switch connector and the chassis ground.

Did the test lamp light?

--

Go to Step 13

Go to Step 12

12

Locate and repair open in CKT 1467 (GRY/BLK) between terminal A in the two-speed rear axle shift switch connector and terminal 86 in the two-speed rear axle shift relay connector.

Does the two-speed rear axle operate properly?

--

System OK

Go to Step 13

13

Use a J 39200 between terminal B in the two-speed rear axle shift switch connector and the chassis ground.

Does the DMM indicate continuity?

--

Go to Step 15

Go to Step 14

14

Locate and repair open in CKT 150 (BLK) between terminal B in the two-speed rear axle shift switch connector and G101 (Cat© Diesel) or G102 (Gasoline).

Does the two-speed rear axle operate properly?

--

System OK

Go to Step 15

15
  1. Set the two-speed rear axle shift switch to the HIGH position.
  2. Use a J 39200 between terminal A and terminal B in the two-speed rear axle shift switch.

Does the DMM indicate continuity?

--

Go to Step 16

Go to Step 17

16

Replace the two-speed rear axle shift relay. Refer to Two Speed Rear Axle Shift Motor Relay Replacement .

Does the two-speed rear axle operate properly?

--

System OK

Go to Step 18

17

Replace the two-speed rear axle shift switch. Refer to Two Spd R/Axle Shft Dash Panel Switch Replacement

Does the two-speed rear axle operate properly?

--

System OK

Go to Step 18

18
  1. Do not disconnect the test lamp.

    2. Remove the two-speed rear axle shift relay.

  2. Connect a jumper between terminal 30 and terminal 87 in the relay connector.

Did the test lamp light?

--

Go to Step 19

Go to Step 20

19

Replace the two-speed rear axle shift relay. Refer to Two Speed Rear Axle Shift Motor Relay Replacement .

Does the two-speed rear axle operate properly?

--

System OK

Go to Step 21

20

Locate and repair open in CKT 1469 (LT BLU) between terminal 87 in the two-speed rear axle shift relay connector and terminal A in the two-speed rear axle shift actuator (motor) connector.

Does the two-speed rear axle operate properly?

--

System OK

Go to Step 21

21
  1. Do not disconnect the test lamp.

    2. Remove the two-speed rear axle shift relay.

  2. Connect a jumper between terminal 30 and terminal 87A in the relay connector.

Did the test lamp light?

--

Go to Step 22

Go to Step 23

22

Replace the two-speed rear axle shift relay. Refer to Two Speed Rear Axle Shift Motor Relay Replacement .

Does the two-speed rear axle operate properly?

--

System OK

--

23

Locate and repair open in CKT 1468 (LT BLU) circuit between terminal 87A in the two-speed rear axle shift relay connector and terminal C in the two-speed rear axle shift actuator (motor) connector.

Does the two-speed rear axle operate properly?

--

System OK

--