For 1990-2009 cars

### Test Description

The number below refers to the step number on the diagnostic table.

1. Listen for an audible click when the relay operates. Command both the ON and OFF states of the REAR DEFOG relay. Repeat the commands as necessary.

Step | Action | Yes | No
--- | --- | --- | ---
Schematic Reference: Defogger Schematics Connector End View Reference: Master Electrical Component List in Wiring Systems
1 | Did you perform the Defogger Diagnostic System Check? | Go to Step 2 | Go to Diagnostic System Check - Defogger
2 |

1. Start the engine.
2. Depress the rear defogger switch.
3. Observe the rear defogger indicator on the HVAC control assembly.

Does the rear defogger indicator illuminate?

Go to Step 3

Go to Step 5

3 | Connect a test lamp between the rear defogger grid and a good ground. Does the test lamp illuminate? | Go to Step 4 | Go to Step 8
4 | Connect a test lamp between the left side of the rear defogger grid and the right side of the rear defogger grid. Does the test lamp illuminate? | Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems | Go to Step 15
5 |

1. With a scan tool, observe the Rear Defog Switch parameter in the body control module (BCM) Switch Input data list.
2. Depress the rear defogger switch.

Does the scan tool display On?

Go to Step 7

Go to Step 6

6 |

1. Turn OFF the ignition.
2. Disconnect the harness C2 connector of the BCM.
3. Turn ON the ignition, with the engine OFF.
4. Connect a test lamp between battery voltage and the rear defogger switch signal circuit of the HVAC control assembly at the BCM.
5. Depress the rear defogger switch.

Does the test lamp illuminate?

Go to Step 14

Go to Step 10

7 | Connect a test lamp between the rear defog switch indicator control circuit and the rear defog switch indicator supply voltage circuit. Does the test lamp illuminate? | Go to Step 13 | Go to Step 11
8 |

1. With a scan tool select from the Special Functions, the Rear Defogger Relay under the BCM output controls.
2. Command the REAR DEFOG relay ON and OFF.

Do you hear a click when you command the relay ON and OFF?

Go to Step 9

Go to Step 12

9 | Connect a test lamp between the supply voltage circuit of the REAR DEFOG relay switched input, at the left instrument panel fuse block, and a good ground. Does the test lamp illuminate? | Go to Step 16 | Go to Step 19
10 | Test the rear defogger switch signal circuit and the rear defogger switch ground circuit of the HVAC control assembly for an open or short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 21 | Go to Step 13
11 | Test the supply voltage circuit of the rear defogger indicator of the HVAC control assembly for an open or short to ground and test the rear defog relay circuit of the rear defogger indicator of the HVAC control assembly for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 21

Go to Step 13

12 | Inspect for poor connections at the REAR DEFOG relay. 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 21 | Go to Step 17
13 | Inspect for poor connections 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 21 | Go to Step 18
14 | Inspect for poor connections at the harness connector of the BCM. 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 21 | Go to Step 20
15 | Repair an open or high resistance in the ground circuit of the rear defogger. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | Go to Step 21 | --
16 | Repair an open, a high resistance, or a short to ground in the supply circuit of the rear defogger. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | Go to Step 21 | --
17 | Replace the REAR DEFOG relay. Refer to Relay Replacement in Wiring Systems. Did you complete the replacement? | Go to Step 21 | --
18 | 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 21 | --
19 | Replace the left instrument panel fuse block (LIPFB). Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems. Did you complete the replacement? | Go to Step 21 | --
20 | Important:: Perform the programming or set up procedure for the replaced control module if required. Replace the BCM. Refer to Body Control Module Replacement in Body Control System for applicable replacement procedure. Did you complete the replacement? | Go to Step 21 | --
21 | Operate the system in order to verify the repair. Did you correct the condition? | System OK | Go to Step 2