GM Service Manual Online
For 1990-2009 cars only

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Action

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Schematic Reference: Defogger Schematics

1

Did you review the Rear Window Defogger system operation and perform the necessary inspections?

Go to Step 2

Go to Symptoms - Stationary Windows

2

  1. Turn ON the ignition, with the engine OFF.
  2. Depress the rear window defogger switch.
  3. Observe the rear window defogger indicator on the HVAC Control Assembly.

Does the rear window defogger indicator illuminate?

Go to Step 3

Go to Step 5

3

Connect a test lamp between the rear window defogger grid and a good ground.

Does the test lamp illuminate?

Go to Step 4

Go to Step 9

4

Connect a test lamp between the left side of the rear window defogger grid and the right side of the rear window defogger grid.

Does the test lamp illuminate?

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

Go to Step 11

5

  1. Turn OFF the ignition.
  2. Disconnect the harness connector C4 of the HVAC Control Assembly.
  3. Turn ON the ignition, with the engine OFF.
  4. Connect a test lamp between the Ignition 3 voltage circuit of the HVAC Control Assembly and a good ground.

Does the test lamp illuminate?

Go to Step 6

Go to Step 12

6

Connect a test lamp between the battery positive voltage circuit of the HVAC Control Assembly and a good ground.

Does the test lamp illuminate?

Go to Step 7

Go to Step 8

7

Connect a test lamp between the battery positive voltage circuit of the HVAC Control Assembly and the ground circuit of the HVAC Control Assembly.

Does the test lamp illuminate?

Go to Step 10

Go to Step 13

8

Test the battery positive voltage circuit of the HVAC Control Assembly for an open or short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 16

Go to Step 14

9

Test the supply voltage circuit of the rear window defogger grid for an open or high resistance. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 16

Go to Step 10

10

Inspect for a poor connection at the harness connector of the HVAC Control Assembly. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 16

Go to Step 15

11

Repair an open or high resistance in the ground circuit of the rear window defogger grid. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

Go to Step 16

--

12

Repair an open or high resistance in the Ignition 3 voltage circuit of the HVAC Control Assembly. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

Go to Step 16

--

13

Repair an open or high resistance in the ground circuit of the HVAC Control Assembly. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

Go to Step 16

--

14

Repair a short to ground in the supply voltage circuit of the rear window defogger grid. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

Go to Step 16

--

15

Replace the HVAC Control Assembly. Refer to Heater and Air Conditioning Control Replacement in HVAC Systems - Manual.

Did you complete the replacement?

Go to Step 16

--

16

Operate the system in order to verify the repair.

Did you correct the condition?

System OK

Go to Step 2