GM Service Manual Online
For 1990-2009 cars only

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

Relay Switch B+

1

1

--

--

Relay Coil Ignition Voltage

1

1

--

--

A/C Compressor Clutch Relay Control

P0645

P0645

P0645

--

Relay Coil Ground

--

1

--

--

Compressor Clutch Ground

--

1

--

--

1. A/C Compressor Malfunction

Circuit/System Description

When the A/C switch is pressed, the HVAC control module grounds the A/C request signal circuit. This input will request the engine control module (ECM) to ground the A/C compressor clutch relay control circuit, which will switch the A/C CMPRSR CLUTCH relay on. 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)
    • 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

HVAC Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

    •  Air Temperature Description and Operation
    •  Air Delivery Description and Operation

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

Scan Tool Reference

Control Module References for scan tool information

Circuit/System Verification

  1. Engine running
  2. Press the A/C request switch. The A/C compressor clutch should engage.
  3. Place the mode switch in the defrost position. The A/C compressor clutch should engage.
  4. If the clutch does not engage refer to Air Conditioning (A/C) System Performance Test

Circuit/System Testing

  1. Ignition OFF, disconnect the X2 harness connector at the HVAC control module.
  2. Test for less than 1.0 ohm between the ground circuit terminal and ground.
  3. If greater than the specified range, test the ground circuit for an open/high resistance.
  4. Remove the A/C compressor clutch relay.
  5. Verify that a test lamp illuminates between the ignition circuit terminal 85 and ground.
  6. If the test lamp does not illuminate, test the ignition circuit for a short to ground or an open/high resistance.
  7. Verify that a test lamp illuminates between the B+ circuit terminal 30 and ground.
  8. If the test lamp does not illuminate, test the 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.
  9. Disconnect the harness connector at the A/C compressor clutch.
  10. Ignition OFF test for less than 1.0 ohm between the A/C compressor ground circuit terminal 2 and ground.
  11. If greater than the specified range, test the ground circuit for an open/high resistance.
  12. Connect the harness connector at the A/C compressor clutch.
  13. Connect a 10-amp fused jumper wire between the B+ circuit terminal 30 and the control circuit terminal 87. Verify the A/C compressor clutch engages.
  14. 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.
  15. Connect a test lamp between the control circuit terminal 86 and the B+ circuit terminal 85.
  16. 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.
  17. If the test lamp remains ON all the time, test the control circuit for a short to ground. If the circuit tests normal, replace the ECM.
    If the test lamp remains OFF all the time, test the control circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the ECM.
  18. If all circuits test normal, test or replace the A/C compressor.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  Air Conditioning Compressor Replacement
    •  Control Module References for HVAC control module and ECM replacement, setup, and programming