GM Service Manual Online
For 1990-2009 cars only

Step

Action

Value(s)

Yes

No

1

  1. Exchange the rear defogger relay with a known good relay from the underhood junction block.
  2. Turn the ignition switch to the ON position.
  3. Press the rear defogger switch to the ON position.

Does the rear defogger grid warm?

--

Go to Step 2

Go to Step 3

2

Replace the faulty relay.

Is the repair complete?

--

Go to Defogger System Check

--

3

  1. Use J 36169-A in order to backprobe between terminal D5 of the underhood junction block connector C3 and ground.
  2. Use a test lamp in order to backprobe between terminal C5 of the underhood junction block connector C3 and ground.

Does the test lamp light?

--

Go to Step 4

Go to Step 23

4

  1. Use a test lamp in order to backprobe between terminal C5 of the underhood junction block connector C3 and ground.
  2. Press the rear defogger switch to the ON position.

Does the test lamp light?

--

Go to Step 5

Go to Step 14

5

  1. Use a test lamp in order to backprobe between terminal C1 of the left IP junction block connector C2 and ground.
  2. Press the rear defogger switch to the ON position.

Does the test lamp light?

--

Go to Step 6

Go to Step 7

6

  1. Use a test lamp in order to backprobe between terminal C12 of the left IP junction block connector C3 and ground.
  2. Press the rear defogger switch to the ON position.

Does the test lamp light?

--

Go to Step 8

Go to Step 9

7

Repair the open in CKT 293 between the underhood junction block and the left IP junction block. Refer to Wiring Repairs in Wiring Systems.

Is the repair complete?

--

Go to Defogger System Check

--

8

  1. Disconnect connector C1 and connector C2 of the rear defogger grid.
  2. Connect a test lamp between terminal A of the rear defogger grid connector C1 and ground.
  3. Press the rear defogger switch to the ON position.

Does the test lamp light?

--

Go to Step 10

Go to Step 11

9

Replace the left IP junction block. Refer to Relay/Fuse Center Replacement in Wiring Systems.

Is the repair complete?

--

Go to Defogger System Check

--

10

  1. Connect a test lamp between terminal A of connector C1 and terminal A of connector C2 of the rear defogger grid connector.
  2. Press the rear defogger switch to the ON position.

Does the test lamp light?

--

Go to Step 12

Go to Step 13

11

Repair the open in CKT 293 between the rear defogger grid and the left IP junction block. Refer to Wiring Repairs in Wiring Systems.

Is the repair complete?

--

Go to Defogger System Check

--

12

Repair the open in the rear defogger grid. Refer to Grid Line Repair .

Is the repair complete?

--

Go to Defogger System Check

--

13

Repair the open in CKT 850 between the rear defogger grid and ground splice pack G402. Refer to Wiring Repairs in Wiring Systems.

Is the repair complete?

--

Go to Defogger System Check

--

14

  1. Turn the ignition switch to the ON position.
  2. Use a test lamp in order to backprobe between terminal A11 of underhood junction block connector C3 and ground.

Does the test lamp light?

--

Go to Step 15

Go to Step 23

15

Use the J 39200 in order to backprobe underhood junction block connector C3 and measure the voltage between terminal D5 and ground.

Does the voltage measure to the specified value?

B+

Go to Step 16

Go to Step 23

16

  1. Disconnect connector C2 of the heater and A/C control assembly.
  2. Turn the ignition switch to the ON position.
  3. Use the J 39200 in order to measure the voltage between terminal C of heater and A/C control assembly connector C2 and ground.

Does the voltage measure to the specified value?

B+

Go to Step 17

Go to Step 18

17

Use the J 39200 in order to measure the voltage between terminal C and terminal K of the heater and A/C control assembly connector C2.

Does the voltage measure to the specified value?

B+

Go to Step 19

Go to Step 20

18

Repair the open in CKT 41 between the heater and A/C control assembly and the underhood junction block. Refer to Wiring Repairs in Wiring Systems.

Is the repair complete?

--

Go to Defogger System Check

--

19

Use the J 39200 in order to measure the voltage between terminal D and terminal K of heater and A/C control assembly connector C2.

Does the voltage measure to the specified value?

B+

Go to Step 21

Go to Step 22

20

Repair the open in CKT 251 between the heater and A/C control assembly and ground splice pack G204. Refer to Wiring Repairs in Wiring Systems.

Is the repair complete?

--

Go to Defogger System Check

--

21

Replace the heater and A/C control assembly. Refer to Control Assembly Replacement in HVAC Systems - A/C Manual.

Is the repair complete?

--

Go to Defogger System Check

--

22

Repair the open in CKT 193 between the heater and A/C control assembly and the underhood junction block. Refer to Wiring Repairs in Wiring Systems.

Is the repair complete?

--

Go to Defogger System Check

--

23

Replace the underhood junction block. Refer to Underhood Fuse Block Replacement in Engine Electrical.

Is the repair complete?

--

Go to Defogger System Check

--