GM Service Manual Online
For 1990-2009 cars only

Step

Action

Value(s)

Yes

No

1

Did you perform the Transfer Case Diagnostic System Test?

--

Go to Step 2

Go to Diagnostic System Check in Transfer Case

2

Inspect the 4WD indicator lamp. Refer to Transfer Case Shift Control Switch Indicator Inoperative - One or More in Transfer Case.

Does the 4WD indicator lamp light?

--

System OK

Go to Step 3

3

Inspect the vacuum hoses.

Are the vacuum hoses kinked, disconnected, damaged or plugged?

--

Go to Step  4

Go to Step  5

4

Replace the vacuum hoses.

Does the 4WD engage?

--

System OK

Go to Step  5

5

Inspect for engine vacuum at the vacuum actuator.

Is there vacuum at the actuator?

--

Go to Step 7

Go to Step 6

6

Replace the vacuum actuator. Refer to Vacuum Actuator Replacement .

Does the 4WD engage?

--

System OK

Go to Step 7

7

Inspect the cable.

Is the cable damaged, disconnected or binding?

--

Go to Step 8

Go to Step 9

8

Replace the shift cable. Refer to Shift Cable Replacement .

Does the 4WD engage?

--

System OK

Go to Step 9

9

Inspect the operation of the shift fork. Use the J 33798 . Refer to Inner Axle Housing Shaft and Housing Replacement .

Does the 4WD engage?

--

System OK

Go to Step 10

10

Replace the shift fork. Refer to Shift Fork Replacement .

Does the 4WD engage?

--

System OK

Go to Step  11

11

The cause of concern may be in the front drive axle. Disassemble the front drive axle. Inspect for damaged components. Refer to Differential Carrier Assembly Disassemble .

Does the 4WD engage?

--

System OK

Go to Step 1