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?

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Go to Step 2

Go to Diagnostic System Check

2

Verify the fault is present.

Does the system operate normally?

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Go to Testing for Electrical Intermittents

Go to Step 3

3

Does the front axle disengage when the four wheel drive (4WD) control lever is in the two wheel drive (2WD) position?

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Go to Step 4

Go to Four-Wheel Drive Does Not Disengage

4

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

Does the lamp go out?

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Go to Step 5

Go to Diagnostic System Check - Instrument Cluster in Instrument Panel, Gauges and Console

5

  1. Ensure the 4WD control lever is in the 2WD position.
  2. Using the scan tool, under engine controls, PCM/VCM, read the front axle switch status.

Does the scan tool show switch status no?

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Go to Step 10

Go to Step 6

6

  1. Disconnect the front axle actuator/switch.
  2. Using the scan tool, under engine controls, PCM/VCM, read the front axle switch status.

Does the scan tool show switch status No?

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Go to Step 9

Go to Step 7

7

  1. Turn OFF the ignition.
  2. Disconnect the VCM/PCM.
  3. Turn ON the ignition with the engine OFF.
  4. Measure the voltage of the 4WD front wheel lock telltale feed circuit at the PCM/VCM to a good ground.

Does the voltage measure near the specified value?

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Go to Step 9

Go to Step 8

8

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

Did you complete the repair?

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Go to Step 11

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9

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

Did you complete the replacement?

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Go to Step 11

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10

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

Did you complete the replacement?

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Go to Step 11

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11

Operate the system in order to verify the repair.

Did you correct the condition?

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System OK

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