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: Component Connector End Views .
1 | Did you perform the Diagnostic System Check - Vehicle? | Go to Step 2 | Go to Diagnostic System Check - Vehicle .
2 |

1. Start the engine.
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 8
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 . | Go to Step 20
5 |

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

Does the scan tool display On?

Go to Step 7

Go to Step 6

6 |

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

Does the test lamp illuminate?

Go to Step 19

Go to Step 15

7 |

1. Turn the ignition OFF.
2. Disconnect harness connector xx of the BCM.
3. Turn the ignition ON, with the engine OFF.
4. Connect a 3-ampere fused jumper between battery voltage and the supply voltage circuit of the rear window defogger indicator for the HVAC control assembly.

Does the indicator illuminate?

Go to Step 19

Go to Step 16

8 |

1. Install a scan tool.
2. From the miscellaneous test menu, select the rear defogger under from the BCM output controls.
3. Command the REAR DEFOG relay ON and OFF.

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

Go to Step 11

Go to Step 9

9 |

1. Turn the ignition OFF.
2. Disconnect the REAR DEFOG relay.
3. Turn the ignition ON, with the engine OFF.
4. Connect a test lamp between the supply voltage circuit and the ground circuit of the REAR DEFOG relay coil.
5. From the miscellaneous test menu, select the rear defogger under from the BCM output controls.
6. With the scan tool, command the REAR DEFOG relay ON.

Does the test lamp illuminate?

Go to Step 17

Go to Step 10

10 |

1. Install a scan tool.
2. Connect a test lamp between the supply voltage circuit of the REAR DEFOG relay coil and a good ground.
3. With the scan tool, command the REAR DEFOG relay ON.

Does the test lamp illuminate?

Go to Step 21

Go to Step 13

11 |

1. Turn the ignition OFF.
2. Disconnect the REAR DEFOG relay.
3. Turn the ignition ON, with the engine OFF.
4. Connect a test lamp between the switched battery voltage circuit input of the REAR DEFOG relay and a good ground.

Does the test lamp illuminate?

Go to Step 12

Go to Step 14

12 |

1. Connect a test lamp between the rear window defogger grid and a good ground.
2. Connect a 30-ampere fused jumper between the switched battery voltage circuit input of the REAR DEFOG relay and the switched relay output to the supply voltage circuit of the rear defogger.

Does the test lamp illuminate?

Go to Step 17

Go to Step 23

13 | Test the supply voltage circuit of the REAR DEFOG relay coil for an open or short to ground. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | Go to Step 27 | Go to Step 19
14 | Test the switched battery voltage circuit input of the REAR DEFOG relay for an open or high resistance. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | Go to Step 27 | Go to Step 22
15 | Test the rear defogger switch signal circuit of the HVAC control assembly for an open or short to voltage. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | Go to Step 27 | Go to Step 18
16 | Test the supply voltage circuit of the rear window defogger indicator for the HVAC control assembly for an open or short to ground. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | Go to Step 27 | Go to Step 18
17 | Inspect for poor connections at the REAR DEFOG relay. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | Go to Step 27 | Go to Step 24
18 | 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 . Did you find and correct the condition? | Go to Step 27 | Go to Step 25
19 | Inspect for poor connections at the harness connector of the BCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | Go to Step 27 | Go to Step 26
20 | Repair an open or high resistance in the ground circuit of the rear window defogger. Refer to Wiring Repairs . Did you complete the repair? | Go to Step 27 | --
21 | Repair an open or high resistance in the ground circuit of the REAR DEFOG relay coil. Refer to Wiring Repairs Did you complete the repair? | Go to Step 27 | --
22 | Repair a short to ground in the supply voltage circuit of the rear window defogger. Refer to Wiring Repairs . Did you complete the repair? | Go to Step 27 | --
23 | Repair an open or high resistance in the supply circuit of the rear window defogger. Refer to Wiring Repairs . Did you complete the repair? | Go to Step 27 | --
24 | Replace the REAR DEFOG relay. Did you complete the replacement? | Go to Step 27 | --
25 | Replace the HVAC control assembly. Refer to HVAC Control Module Replacement Did you complete the replacement? | Go to Step 27 | --
26 | Replace the BCM. Refer to Control Module References . Did you complete the replacement? | Go to Step 27 | --
27 | Operate the system in order to verify the repair. Did you correct the condition? | System OK | Go to Step 2