Diagnostic Instructions
| • | 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. |
Diagnostic Fault Information
Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance |
|---|---|---|---|---|
A/C Relay Coil B+ | 2 | 2 | -- | -- |
A/C Relay Switch B+ | 2 | 2 | -- | -- |
A/C Relay Control | P0645, P0646 | P0645, P0647 | P0645, P0647 | -- |
A/C Clutch Control | 2 | 2 | 1 | -- |
A/C Compressor Clutch Ground | -- | 2 | 2 | -- |
1. A/C Clutch Always Engaged 2. A/C Clutch Inoperative | ||||
Circuit/System Description
When the A/C switch is pressed, the HVAC control assembly sends a GMLAN message to the Engine control module (ECM) for an A/C request. This input will request the ECM to ground the A/C compressor clutch relay control circuit, which will switch the A/C CMPR relay. With the relay contacts closed, battery voltage is supplied to the A/C compressor clutch assembly.
Diagnostic Aids
The following conditions must be met in order for the ECM to turn on the compressor clutch:
| • | Battery voltage is between 9-18 volts. |
| • | Engine coolant temperature (ECT) is less than 123°C (253°F). |
| • | Engine speed is greater than 600 RPM. |
| • | Engine speed is less than 4,760 RPM. |
| • | A/C high side pressure is between 2929-269 kPa (39-425 psi) Air Conditioning (A/C) System Performance Test |
| . |
| • | Throttle position is less than 100 percent. |
| • | Evaporator temperature is greater than 0°C (32°F). |
| • | ECM does not detect excessive torque load. |
| • | ECM does not detect insufficient idle quality. |
| • | The ambient temperature is above 1°C (34°F). |
Reference Information
Schematic Reference
Connector End View Reference
Description and Operation
Automatic HVAC Description and Operation
Electrical Information Reference
Scan Tool Reference
Control Module References for scan tool information
Circuit/System Verification
Engine running, press the A/C request switch. The A/C compressor clutch should engage.
Circuit/System Testing
- Ignition ON, with a scan tool, verify the A/C Switch parameter cycles between On and Off with each press of the A/C switch
- Remove the A/C CLTCH relay.
- Verify that a test lamp illuminates between the B+ circuit terminal 85 and ground.
- Verify that a test lamp illuminates between the B+ circuit terminal 30 and ground.
- Disconnect the harness connector at the A/C compressor clutch.
- Ignition OFF test for less than 1.0 ohm between the A/C compressor ground circuit terminal 2 and ground.
- Connect the harness connector at the A/C compressor clutch.
- Connect a 10A fused jumper wire between the B+ circuit terminal 30 and the relay switch control circuit terminal 87. Verify the A/C compressor clutch engages.
- Connect a test lamp between the relay control circuit terminal 86 and the B+ circuit terminal 85.
- Using a scan tool, command the A/C relay output ON and OFF. The test lamp should turn ON and OFF when changing between the commanded states.
- If all circuits test normal, test or replace the A/C compressor.
| ⇒ | If the parameter does not cycle, replace the HVAC control module. |
| ⇒ | If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. |
| ⇒ | If the test lamp does not illuminate, test the relay switch B+ circuit for an open/high resistance. If the A/C clutch fuse is open, test the relay switch control circuit for a short to ground. If all circuits test normal, test or replace the A/C compressor clutch. |
| ⇒ | If greater than the specified range, test the ground circuit for an open/high resistance. |
| ⇒ | If the A/C compressor clutch does not activate, test the control circuit for an open/high resistance. If the circuit tests normal, test or replace the A/C compressor. |
| ⇒ | If the test lamp remains ON all the time, test for a short to ground on the control circuit connector X1 terminal 53. If the circuit tests normal, replace the ECM. |
| ⇒ | If the test lamp remains OFF all the time, test for a short to voltage or an open/high resistance on the control circuit. If the circuit tests normal, replace the ECM. |
Repair Instructions
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.
| • | Control Module References for HVAC control module and ECM replacement, setup, and programming |
