Volt

Rear Window Demister Malfunction

When the HVAC control module receives a rear window demist request from the HVAC control faceplate, the HVAC control module will supply voltage to the relay coil. Battery positive voltage is supplied at all times to the relay switched input and the relay coil is always grounded. This allows battery positive voltage from the relay switched input to flow through the switch contacts and out the relay switched output to the rear window demister grid.

Rear window demister indicator will still function when the switch is pressed but any of the following conditions may cause the rear window demister not to function:

Schematic Reference

Demister Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Rear Window Demister Description and Operation

Electrical Information Reference

Scan Tool Reference

Control Module References for scan tool information

1. Vehicle ON, observe the scan tool Rear Demister Switch parameter while pressing the heated rear window switch, the parameter should change from Inactive to Active.

⇒	If the value does not change, replace the A20 radio/HVAC control.

2. Command the Rear Demist Indicator ON and OFF with a scan tool. The indicator should turn ON or OFF while changing between the commanded states.

⇒	If the demister indicator is always ON or always OFF, replace the A20 radio/HVAC control.

3. Command the Rear Demist ON and OFF with a scan tool, the upper and lower rear window demister grids should turn ON and OFF when changing between the commanded states.

⇒	If a single rear window demister grid does not turn ON and OFF when changing between the commanded states, refer to Single Rear Window Demist Grid Malfunction.

⇒	If both rear window demister grids do not turn ON and OFF when changing between the commanded states, refer to Rear Window Demister Malfunction.

Single Rear Window Demist Grid Malfunction

1. Vehicle OFF, disconnect the harness connector at the appropriate E18 rear window demister grid.
2. Test for less than 10 Ω between the ground circuit terminal listed below and ground:

•	Terminal A E18B rear demister grid-lower.

•	Terminal 1 E18A rear demister grid-upper

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

3. Disconnect the appropriate KR4B outside wing mirror relay or the KR5 rear demister relay.
4. Test for less than 10 Ω between the ground circuit terminal listed below and ground.

•	Terminal 40 KR5 rear demister relay.

•	Terminal 86 KR4B outside wing mirror heater relay.

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

5. Vehicle ON, connect a 40A fused jumper wire between the circuit terminals listed below, verify the rear demister grid is activated:.

•	B+ terminal 43 and control terminal 42 KR5 rear demister relay.

•	B+ terminal 30 and control terminal 87 KR4B outside wing mirror heater relay.

⇒	If the rear demister grid does not activate and the circuit fuse is open, test the control circuit for a short to ground. If the circuit tests normal, test or replace the E18 rear window demister grid.

⇒	If the rear demister grid does not activate and the circuit fuse is good, test the control circuit for an open/high resistance. If the circuit tests normal, test or replace the E18 rear window demister grid.

6. If all circuits test normal, test or replace the relay.

Rear Window Demister Malfunction

1. Vehicle OFF, disconnect the KR5 rear window demister relay.
2. Vehicle OFF, disconnect the KR4B outside wing mirror heater relay.
3. Connect a test lamp between the control circuit terminal 85 and the ground circuit terminal 86 of the KR4B outside wing mirror heater relay.
4. Vehicle ON, command the Rear Demist 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 K33 HVAC control module.

⇒	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 K33 HVAC control module.

5. Vehicle OFF, connect the KR4B outside wing mirror heater relay.
6. Vehicle ON, command the Rear Defog ON with a scan tool, the E18B rear demister grid-lower should turn ON.

⇒	If the E18B rear demister grid-lower does not turn ON, test or replace the KR4B outside wing mirror heater relay.

7. If all circuits test normal, test or replace the KR5 rear window demister relay.

Rear Window Demister Grid Test

1. Vehicle ON, turn on the E18 rear window demister grid.
2. Verify that a test lamp illuminates between each E18 rear window demister grid line and ground. The test lamp should be off when testing at the ground side of the grid and get brighter as the test lamp moves closer to the voltage supply side of the grid. The test lamp should be dim when testing each grid line in the middle.

⇒	If the test lamp remains bright at the middle of the grid line, test for an open between the test point and the ground side of the grid.

⇒	If the test lamp remains off at the middle of the grid line, test for an open between the test point and the voltage supply side of the grid

Relay Test

1. Vehicle OFF, disconnect the appropriate relay.
2. Test for 60-180 Ω between terminals 1 and 2.

⇒	If the not within the specified range, replace the relay.

3. Test for infinite resistance between the following terminals:

•

3 and 2

•

3 and 5

•

3 and 1

•

1 and 5

⇒	If not the specified value, replace the relay.

4. Install a 5 A fused jumper wire between relay terminal 2 and 12 V. Install a jumper wire between relay terminal 1 and ground. Test for less than 2 Ω between terminals 3 and 5.

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

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.