For 1990-2009 cars

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
Important: A short to ground will cause an open fuse(s). Before performing this diagnostic procedure, inspect the fuse(s) for an open.
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 a fused jumper wire in order to backprobe between terminal A4 of underhood junction block connector C3 and ground.
2. Use a test lamp in order to backprobe between terminal C5 of underhood junction block connector C3 and ground.

Does the test lamp light?

--

Go to Step 4

Go to Step 23

4 |

1. Disconnect the fused jumper wire.
2. Use a test lamp in order to backprobe between terminal C5 of underhood junction block connector C3 and ground.
3. 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 left wiring 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 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 wiring 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 (connector C2 and connector C3 Oldsmobile) of the rear defogger grid.
2. Connect a test lamp between terminal A of rear defogger grid connector C1 (C2 Oldsmobile) 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. Is the repair complete? | -- | Go to Defogger System Check | --
10 |

1. Connect a test lamp between terminal A of connector C1 (C2 Olds) and terminal A of connector C2 (C3 Olds) 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 or in CKT 900 (Olds) or in CKT 850 (Pontiac) between the rear antenna/defogger grid connector and ground. Refer to Grid Line Repair in Stationary Windows and Wiring Repairs in Wiring Systems. 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 RUN 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 A4 and ground. Does the voltage measure within the specified range? | 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 RUN 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 within the specified range?

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 heater and A/C control assembly connector C2. Does the voltage measure within the specified range? | 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 within the specified range? | 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 LINK Control Assembly Replacement in HVAC Systems with A/C. 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 | --