• | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. |
• | Review Strategy Based Diagnosis for an overview of the diagnostic approach. |
• | Diagnostic Procedure Instructions provides an overview of each diagnostic category. |
Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance |
---|---|---|---|---|
HVAC Control Assembly B+ | 2 | 2 | -- | -- |
Relay Switch B+ | 2 | 2 | -- | -- |
Rear Defogger Relay Control | 2 | 2 | 1 | -- |
Rear Window Defogger Grid Control | 2 | 2 | 1 | -- |
Relay Coil Ground | -- | 2 | 2 | -- |
Rear Window Defogger Grid Ground | -- | 2 | 2 | -- |
HVAC Control Assembly Ground | -- | 2 | 2 | -- |
1. Rear defogger always On 2. Rear defogger Inoperative |
When the rear defogger button is pressed, voltage is supplied to the relay coil and the HVAC control assembly also illuminates the rear window defogger indicator. 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 through the switch contacts and out the relay switched output to the rear window defogger grids.
Rear Window Defogger Description and Operation
Ignition ON, command the rear window defogger ON and OFF by pressing the rear window defogger switch. The rear window defogger and rear window defogger indicator should turn ON or OFF each time the rear window defogger switch is pressed.
⇒ | If the indicator does not turn ON or OFF with each switch press, replace the HVAC control assembly. |
⇒ | If a rear window defogger grid does not turn ON and OFF when changing between the commanded states, refer to Single Rear Window Defog Grid Malfunction. |
⇒ | If both rear window defogger grids do not turn ON and OFF when changing between the commanded states, refer to Rear Window Defogger Malfunction. |
• | RPO A12 rear window defogger grid ground terminal A |
• | RPO A18 rear window defogger grid ground terminal B |
⇒ | If greater than the specified range, test the ground circuit for an open/high resistance. |
⇒ | 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, test or replace the rear window defogger grid. |
⇒ | If greater than the specified range, test the ground circuit for an open/high resistance |
⇒ | If the test lamp does not illuminate, test the B+ circuit for an open/high resistance |
⇒ | If greater than the specified range, test the ground circuit for an open/high resistance. |
⇒ | If the test lamp illuminates, test the control circuit for a short to voltage. |
⇒ | If the test lamp does not illuminate, test the B+ circuit for an open/high resistance. |
⇒ | If the rear window defogger grids do not activate, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal, test or replace the REAR DEFOG relay. |
⇒ | If the test lamp is always ON, test the control circuit for a short to voltage. If the circuit tests normal, replace the HVAC control assembly. |
⇒ | 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 HVAC control assembly. |
⇒ | If not within the specified range, replace the relay. |
• | 30 and 86 |
• | 30 and 87 |
• | 30 and 85 |
• | 85 and 87 |
⇒ | If not the specified value, replace the relay. |
⇒ | If greater than the specified range, replace the relay. |
⇒ | 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. |
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.
• | Control Module References for HVAC control module replacement, setup, and programming |