Battery positive voltage is supplied through the BATT 1 fuse in the underhood fuse block to the rear defog relay switched input. When the rear defog switch button is pressed, the body control module (BCM) energizes the rear defog relay by providing a B+ to the control circuit of the rear defog relay. This allows battery positive voltage from the relay switched input through the switch contacts and out the relay switched output to the rear window defogger grid. The HVAC control module houses the rear defog switch and also illuminates the rear defog indicator on the switch button, upon this request. Ground for the rear window defogger grid is provided by G301 for the Sedan or G302 for the Extended Sedan.

When you start the engine and press the rear defog switch button, the defog cycle lasts for 10 minutes. Further operation will result in 5-minute defog cycles.

The rear defogger feature will not time out if vehicle speed is above 48 km/h (30 mph).

If battery voltage is low, the rear defogger feature will be disabled.

The system voltage must be between 9-16 volts.

B0283 02

This DTC will set when the BCM detects a short to ground in the rear defog relay control circuit or with the C60 Manual A/C system, the defogger indicator voltage supply circuit for 1 second or greater.

B0283 05

This DTC will set when the BCM detects a short to voltage or an open/high resistance in the rear defog relay control circuit or with the C60 Manual A/C system, rear defog indicator voltage supply circuit for 1 second or greater.

On vehicles equipped with either a C60 or C68 A/C system, a short to voltage or a short to ground in the rear defog relay control circuit will set DTC B0283. However, an open in this circuit will cause DTC B0283 to set on vehicles with C68, but not on vehicles with C60.

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

Scan Tool Reference

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.

Rear Defogger Malfunction

1. Ignition OFF, disconnect the R/WDO DEFOG relay.
2. Test for less than 1 ohm of resistance between the relay coil ground circuit and ground.

⇒	If greater than the specified range, test the relay coil ground circuit for an open/high resistance.

3. Connect a test lamp between the rear defogger relay control circuit and the relay coil ground circuit.
4. Ignition ON, 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 rear defogger relay control circuit for a short to voltage. If the circuit tests normal, replace the BCM.

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

5. If all circuits test normal, test or replace the R/WDO DEFOG relay.

Rear Defog Indicator Malfunction (C60 Manual A/C only)

1. Ignition OFF, connect a test lamp between the control circuit terminal B8 and ground.
2. Ignition ON, 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.

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

Defog Indicator (C60 Manual A/C only)

1. Ignition OFF, disconnect the harness at the HVAC Control Module.
2. Install a 5-amp fused jumper wire between circuit terminal B8 and 12 volts. Install a jumper wire between the ground circuit terminal B5 and ground. Verify the Defog Indicator illuminates.

⇒	If the Defog Indicator does not illuminate, replace the HVAC Control Module.

Relay Test

1. Ignition OFF, disconnect the R/WDO 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.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.