GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Isuzu 🢒 2002 🢒 Isuzu T-Series (Truck Line C) 🢒 Driveline/Axle 🢒 Rear Axle Controls 🢒 Two Speed Rear Axle Inoperative

Circuit Description

This symptom is usually the result of actions that were expected to take place during the transition between rear axle gears and have not occurred. When the two speed rear axle driver control switch is in the LOW position, the two speed rear axle shift relay is de-energized and ignition 3 voltage is supplied to the speedo adapter or the powertrain control module. With ignition 3 voltage supplied to the speedo adapter/powertrain control module, speedometer calibration is set for low gear.

When the two speed rear axle driver control switch is placed in the HIGH position, the switch closes to ground removing ignition 3 voltage from the speedo adapter/powertrain control module and energizes the two speed rear axle shift relay. Voltage is supplied to the two speed rear axle shift motor from the two speed rear axle shift relay and shifts to high gear. The speedometer then changes the calibration so that the speedometer reading matches road speed.

Test Description

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

  1. This step tests for proper ignition 3 voltage at the two speed rear axle shift relay.

    2.

  2. This step checks the shift motor relay control circuit and the two speed rear axle driver control switch for a short to ground.

    3.

  3. This step tests the shift motor relay control circuit and the two speed rear axle driver control switch for an open.

    4.

  4. This step tests the high gear shift motor control circuit and the shift relay for proper operation.

    5.

  5. This step tests the low gear shift motor control circuit and the shift relay for proper operation.

    6.

  6. This step tests for a proper ground circuit.

    7.

  7. This step tests the shift motor relay control circuit for a short to ground.

    8.

  8. This step tests the shift motor relay control circuit for an open.

    9.

  9. This step tests the rear axle shift relay.

    10.

  10. This step tests the rear axle shift relay.

Step

Action

Value(s)

Yes

No

Schematic Reference: Two-Speed Rear Axle Schematics

1

Did you review the Two Speed Rear Axle Shift Description and Operation and perform the necessary inspections?

--

Go to Step 2

Go to Two Speed Rear Axle Shift System Description and Operation

2

Verify the fault is present.

Does the system operate normally?

--

Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems

Go to Step 3

3

  1. Remove the two speed rear axle shift relay.
  2. Turn ON the ignition, with the engine OFF.
  3. Test the ignition 3 voltage circuit for proper voltage at the two speed rear axle shift relay connector.

Does the voltage measure within the specified value?

9-14 V

Go to Step 4

Go to Step 13

4

  1. Set the two speed rear axle driver control switch to the LOW position.
  2. Connect a test lamp from the ignition 3 voltage circuit to the shift motor relay control circuit at the two speed rear axle shift relay connector.

Does the test lamp illuminate?

--

Go to Step 9

Go to Step 5

5

Set the two speed rear axle driver control switch to the HIGH position.

Does the test lamp illuminate?

--

Go to Step 6

Go to Step 10

6

  1. Set the ignition switch to the OFF position.
  2. Set the two speed rear axle driver control 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 motor connector.
  5. Set the ignition switch to the RUN position.
  6. Test the high gear shift motor control circuit for proper voltage.

Does the voltage measure within the specified value?

9-14 V

Go to Step 7

Go to Step 11

7

  1. Set the two speed rear axle driver control switch to the LOW position.
  2. Test the low gear shift motor control circuit for proper voltage.

Does the voltage measure within the specified value?

9-14 V

Go to Step 8

Go to Step 12

8

Connect a test lamp from the low gear shift motor control circuit to the ground circuit on the shift motor connector

Does the test lamp illuminate?

--

Go to Step 14

Go to Step 15

9

  1. Do not disconnect the test lamp.
  2. Disconnect the two speed rear axle driver control switch.

Does the test lamp illuminate?

--

Go to Step 16

Go to Step 17

10

  1. Do not disconnect the test lamp.
  2. Disconnect the two speed rear axle driver control switch.
  3. Connect a fused jumper wire from cavity A at the two speed rear axle driver control switch to ground

Does the test lamp illuminate?

--

Go to Step 17

Go to Step 18

11

  1. Remove the two speed rear axle shift relay.
  2. Connect a fused jumper between terminal 30 and terminal 87 in the relay connector.
  3. Test the high gear shift motor control circuit for proper voltage.

Does the voltage measure within the specified value?

9-14 V

Go to Step 19

Go to Step 20

12

  1. Remove the two speed rear axle shift relay.
  2. Connect a jumper between terminal 30 and terminal 87A in the relay connector.
  3. Test the low gear shift motor control circuit for proper voltage.

Does the voltage measure within the specified value?

9-14 V

Go to Step 19

Go to Step 21

13

Repair the open or short to ground in the ignition 3 voltage circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 22

--

14

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

Did you complete the replacement?

--

Go to Step 22

--

15

Repair the open in the ground circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 22

--

16

Repair the short to ground in the shift motor relay control circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 22

--

17

Replace the two speed rear axle driver control switch. Refer to Two Speed Rear Axle Shift Control Switch Replacement in Rear Axle Controls

Did you complete the replacement?

--

Go to Step 22

--

18

Repair the open in the shift motor relay control circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 22

--

19

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

Did you complete the replacement?

--

Go to Step 22

--

20

Repair the open in the high gear shift motor control circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 22

--

21

Repair the open in the low gear shift motor control circuit. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 22

--

22

Operate the vehicle within the conditions for causing the symptom as specified in the supporting text.

Does the system operate normally?

--

System OK

Go to Step 3