GM Service Manual Online
For 1990-2009 cars only

Step

Action

Value(s)

Yes

No

1

Did you perform the transfer case Diagnostic System Check?

--

Go to Step 2

Go to Diagnostic System Check

2

Verify the fault is present.

Does the system operate normally?

--

Go to Testing for Electrical Intermittents

Go to Step 3

3

Does the front axle engage when the four wheel drive (4WD) control lever is in the 4WD position?

--

Go to Step 4

Go to Four-Wheel Drive Does Not Engage

4

Using the scan tool, under special test in instrument cluster, activate the 4WD indicator lamp to ON.

Does the lamp come on?

--

Go to Step 5

Go to Step 13

5

  1. Ensure the 4WD control lever is in the 4WD position.
  2. Using the scan tool, under engine controls, Powertrain Control Module (PCM) and Vehicle Control Module (VCM), read the front axle switch status.

Does the scan tool show switch status yes?

--

Go to Step 12

Go to Step 6

6

  1. Ensure the 4WD control lever is in the 4WD position.
  2. Back probe the 4WD front wheel lock telltale feed circuit at the front axle actuator/switch with a test lamp that is connected to a battery positive voltage circuit.

Does the test lamp illuminate?

--

Go to Step 7

Go to Step 11

7

  1. Turn OFF the ignition.
  2. Disconnect the Powertrain Control Module (PCM) or the Vehicle Control Module (VCM).
  3. Turn ON the ignition, with the engine OFF.
  4. Probe the 4WD front wheel lock telltale feed circuit with a test lamp that is connected to a battery positive voltage circuit.

Does the test lamp illuminate?

--

Go to Step 12

Go to Step 8

8

  1. Disconnect the front axle actuator/switch.
  2. Probe the 4WD front wheel lock telltale feed circuit with a test lamp that is connected to a battery positive voltage circuit.

Does the test lamp illuminate?

--

Go to Step 9

Go to Step 10

9

Repair short to ground in the 4WD front wheel lock telltale feed circuit . Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 13

--

10

Repair open in the 4WD front wheel lock telltale feed circuit between the switch and the PCM/VCM. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 13

--

11

Replace the front axle actuator/switch. Refer to Electric Motor Actuator Replacement in Front Drive Axle.

Did you complete the replacement?

--

Go to Step 13

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12

Replace the VCM/PCM. Refer to VCM Replacement/Programming for 4.3L or Powertrain Control Module Replacement for all other engines

Did you complete the replacement?

--

Go to Step 13

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13

Operate the system in order to verify the repair.

Did you correct the condition?

--

System OK

Go to Step 3