For 1990-2009 cars

### Circuit Description

The rear defogger switch is a momentary switch to ground. The intelligent switching unit (ISU) monitors the status of the rear defogger switch. The ISU energizes the internal rear defogger relay when it detects a change of status in the rear defogger switch. The ISU de-energizes the internal rear defogger relay when it detects the next change of status in the rear defogger switch or a predetermined on time interval has elapsed.

### Conditions for Running the DTC

The ignition is ON.

### Conditions for Setting the DTC

The ISU detects a high voltage level on the rear defogger switch circuit for 2 seconds.

### Action Taken When the DTC Sets

The rear window defogger will be disabled until the condition is no longer present.

### Conditions for Clearing the DTC

• | The ISU no longer detects a high voltage level on the rear defogger switch circuit.

• | A history DTC will clear after 40 consecutive ignition cycles if the condition for the malfunction is no longer present.

### Test Description

The numbers below refer to the step numbers on the diagnostic table.

1. This step tests the inactive state of the rear defogger switch.

2. This step tests the active state of the rear defogger switch.

3. After replacement of the ISU, the ISU must be reprogrammed.

Step | Action | Yes | No
--- | --- | --- | ---
Schematic Reference: Defogger Schematics Connector End View Reference: Component Connector End Views
1 | Did you perform Diagnostic System Check - Vehicle? | Go to Step 2 | Go to Diagnostic System Check - Vehicle
2 |

1. Install a scan tool.
2. Turn ON the ignition, with the engine OFF.
3. With a scan tool, observe the rear defrost switch parameter in the ISU Miscellaneous Inputs data list.

Does the scan tool display OFF?

Go to Step 3

Go to Step 6

3 |

1. Activate the defogger switch.

Important: The rear defrost switch parameter is a momentary display. Observe the scan tool carefully.

2. With a scan tool, observe the rear defrost switch parameter.

Does the rear defrost switch parameter change state?

Go to Step 13

Go to Step 4

4 |

1. Turn OFF the ignition.
2. Inspect the 30A ISU Defroster Fuse F.

Is the fuse open?

Go to Step 5

Go to Step 8

5 |

1. Repair short to ground on the ISU voltage output circuits to the rear window and heated mirrors.
2. Replace the 30A ISU Defroster Fuse F.

Is the repair complete?

Go to Step 13

--

6 |

1. Turn OFF the ignition.
2. Disconnect the defogger switch.
3. Turn ON the ignition, with the engine OFF.
4. With a scan tool, observe the rear defrost switch parameter.

Does the scan tool display OFF?

Go to Step 10

Go to Step 7

7 | Test the signal circuit of the defogger switch for a short to ground. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | Go to Step 13 | Go to Step 8
8 | Test the signal circuit of the defogger switch for a short to voltage. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition? | Go to Step 13 | Go to Step 11
9 | Inspect for poor connections at the harness connector of the intelligent switching unit (ISU). Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | Go to Step 13 | Go to Step 11
10 | Inspect for poor connections at the harness connector of the defogger switch. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | Go to Step 13 | Go to Step 12
11 | Important: Perform the program procedure for the ISU. Replace the ISU. Did you complete the replacement? | Go to Step 13 | --
12 | Replace the defogger switch. Did you complete the replacement? | Go to Step 13 | --
13 |

1. Use the scan tool in order to clear the DTCs.
2. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC reset?

Go to Step 2

System OK