For 1990-2009 cars

### DTC Descriptor

DTC B0285: Electric Rear Defrost Circuit Low
DTC B0286: Electric Rear Defrost Circuit High

### Diagnostic Fault Information

Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
Relay Switch B+ Circuit | 2 | 2 | -- | --
Relay Coil B+ Circuit | 2 | 2 | -- | --
HVAC B+ Circuit | 2 | 2 | -- | --
Rear Defogger Relay Control | B0285 | B0286 | B0286 | --
Relay Controlled Output Circuit | 2 | 2 | 3 | --
Rear Defogger Switch Signal | 2 | 2 | 2 | --
Defog Switch Ground | -- | 1, 2 | 1, 2 | --
Rear Window Defogger Grid Ground | -- | 2 | 2 | --
1. HVAC Defogger Indicator Inoperative 2. Rear defogger Inoperative 3. Rear Defogger Always ON

### Circuit/System Description

The body control module (BCM) monitors the voltage level on the supply voltage circuit of the REAR DEFOG relay. The voltage level should be low while the REAR DEFOG relay is de-energized. The voltage will be near system voltage when the BCM energizes the REAR DEFOG relay.

### Conditions for Running the DTC

Battery voltage must be between 9-16 volts.

### Conditions for Setting the DTC

#### DTC B0285

This DTC will set when the BCM detects a low voltage level in the supply circuit of the REAR DEFOG relay when the relay is energized.

#### DTC B0286

This DTC will set when the BCM detects an open/high resistance in the supply circuit of the REAR DEFOG relay when the relay is energized.

### Action Taken When the DTC Sets

• | The BCM stores the DTC to memory.

• | The rear window defogger will not operate.

### Conditions for Clearing the DTC

• | This DTC will change from current to history when the fault is no longer present.

• | A history DTC will clear after 100 consecutive ignition cycles if the condition for the malfunction is no longer present.

• | The technician issues a scan tool CLEAR DTCs command.

### Reference Information

#### Schematic Reference

Defogger Schematics

#### Connector End View Reference

Window Systems Connector End Views

#### Description and Operation

Rear Window Defogger Description and Operation

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### Scan Tool Reference

• | Scan Tool Data List

• | Scan Tool Data Definitions

• | Scan Tool Output Controls

### Circuit/System Verification

1. Ignition ON, observe the scan tool Rear Defog Switch parameter while pressing the Rear Defogger Switch. The reading should change between ON and OFF.

⇒ | If the reading does not change between the commanded states, refer to Rear Defogger Switch Malfunction in Circuit/System Testing

2. Command the rear window defogger ON and OFF with the scan tool. The rear window defogger and defogger indicator should turn ON and OFF when changing between the commanded states.

⇒ | If the rear window defogger does not turn ON and OFF when changing between the commanded states, refer to Rear Defogger Malfunction in Circuit/System testing.

⇒ | If the Defogger Indicator does not illuminate, replace the HVAC control module.

### Circuit/System Testing

#### Rear Defogger Switch Malfunction

1. Ignition OFF, disconnect the harness connector C2 at the HVAC control module.
2. Ignition ON, verify the scan tool Rear Defog Switch parameter is OFF.

⇒ | If not the specified value, test the signal circuit terminal 22 for a short to ground. If the circuit tests normal, replace the BCM.

3. Install a 3-A fused jumper wire between the signal circuit terminal 22 and ground. Verify the scan tool Rear Defog Switch parameter is ON.

⇒ | If not the specified value, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the BCM.

4. If all circuits test normal, replace the HVAC Control Module.

#### Rear Defogger Malfunction

1. Ignition OFF, disconnect the REAR DEFOG relay.
2. Ignition OFF, test for less than 1.0 ohm of resistance between the coil side ground circuit terminal and ground.

⇒ | If greater than the specified range, test for an open/high resistance.

3. Ignition ON, verify that a test lamp does not illuminate between the switch side control circuit terminal and ground.

⇒ | If the test lamp illuminates, test the control circuit for a short to voltage.

4. Verify that a test lamp illuminates between the switch side B+ circuit terminal and ground.

⇒ | If the test lamp does not illuminate, test the switch side B+ circuit for a short to ground or an open/high resistance. If the circuit tests normal and the switch side circuit fuse is open, test the switch side control circuit terminal for a short to ground.

5. Connect a test lamp between the coil side control circuit terminal and the coil side ground circuit terminal.
6. Command the REAR DEFOG relay ON and OFF with a scan tool. The test lamp should turn ON and OFF when changing between the commanded states.

⇒ | If the test lamp is always ON, test the control circuit for a short to voltage. If the circuit tests normal, replace the BCM.

⇒ | If the test lamp is always OFF, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the BCM.

7. If all circuits test normal, test or replace the REAR DEFOG relay.

### Component Testing

#### Relay Test

1. Ignition OFF, disconnect the REAR DEFOG relay.
2. Test for 60-180 ohms of resistance between terminals 85 and 86.

⇒ | If the resistance is not within the specified range, replace the relay.

3. Test for infinite resistance between the following terminals:

• | 30 and 86

• | 30 and 87

• | 30 and 85

• | 85 and 87

⇒ | If not the specified value, replace the relay.

4. Install a 25-amp fused jumper wire between relay terminal 85 and 12 volts. Install a jumper wire between relay terminal 86 and ground. Test for less than 2 ohms of resistance between terminals 30 and 87.

⇒ | If greater than the specified range, replace the relay.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Relay Replacement

• | Control Module References for HVAC control module and BCM replacement, setup, and programming.