GM Service Manual Online
For 1990-2009 cars only

Diagnostic Instructions

    â€¢ Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    â€¢ Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    â€¢  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:

    â€¢ Battery voltage is between 9-18 volts
    â€¢ Engine coolant temperature (ECT) is less than 123°C (253°F)
    â€¢ Engine speed is greater than 600 RPM
    â€¢ Engine speed is less than 4,760 RPM
    â€¢ A/C high side pressure is between 2929-269 kPa (39-425 psi)
    â€¢ Throttle position is less than 100 percent
    â€¢ Evaporator temperature is greater than 0°C (32°F)
    â€¢ ECM does not detect excessive torque load
    â€¢ ECM does not detect insufficient idle quality
    â€¢  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

    â€¢  Air Temperature Description and Operation
    â€¢  Air Delivery Description and Operation

Electrical Information Reference

    â€¢  Circuit Testing
    â€¢  Connector Repairs
    â€¢  Testing for Intermittent Conditions and Poor Connections
    â€¢  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.

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