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For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2009 🢒 XLR 🢒 Diagnostic Navigation 🢒 Vehicle Diagnostic Information 🢒 Air Conditioning Compressor Malfunction

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 Ignition

2

2

--

--

A/C Relay Switch B+

2

2

--

--

A/C Relay Control

P0645

P0645

P0645

--

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 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 COMP 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 less than 4,760 RPM.
    • Engine speed is more than 600 RPM.
    • A/C high side pressure is between 2929-269 kPa (425-39 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.

Reference Information

Schematic Reference

HVAC Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Automatic HVAC 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

Engine running, observe the function of the A/C compressor clutch while commanding the A/C relay ON and OFF with a scan tool. The A/C compressor clutch should engage and disengage.

Circuit/System Testing
  1. Ignition OFF, disconnect the A/C COMP relay.
  2. Ignition ON, verify that a test lamp does not illuminate between the control circuit terminal H9 and ground.
    3. ⇒If the test lamp illuminates, test the control circuit for a short to voltage.
  3. Verify that a test lamp illuminates between the ignition circuit terminal F9 and ground.
    4. ⇒If the test lamp does not illuminate, test the ignition circuit for a short to ground or an open/high resistance. If the circuit tests normal and the ignition circuit fuse is open, test or replace the A/C COMP relay.
  4. Verify that a test lamp illuminates between the B+ circuit terminal F8 and ground.
    5. ⇒If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. If the circuit tests normal and the B+ circuit fuse is open, test the control circuit terminal for a short to ground.
  5. Disconnect the harness connector at the A/C compressor clutch.
  6. Test for less than 1.0 ohm between the ground circuit terminal A and ground.
    7. ⇒If greater than the specified range, test the ground circuit for an open/high resistance.
  7. Connect the harness connector at the A/C compressor clutch.
  8. Connect a 10A fused jumper wire between the B+ circuit terminal F8 and the control circuit terminal H9. Verify the A/C compressor clutch is activated.
    9. ⇒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 clutch.
  9. Connect a test lamp between the ignition circuit terminal F9 and the control circuit terminal H8.
  10. Command the A/C relay ON and OFF with a scan tool. The test lamp should turn ON and OFF when changing between the commanded states.
    11. ⇒If the test lamp is always ON, test the control circuit for a short to ground. If the circuit tests normal, replace the ECM.
    ⇒If the test lamp is always OFF, test the control circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the ECM.
  11. If all circuits test normal, test or replace the A/C COMP relay.

Component Testing

Relay Test

  1. Ignition OFF, disconnect the A/C COMP relay.
  2. Test for 60-180 ohms between terminals 85 and 86.
    3. ⇒If the resistance is not within the specified range, replace the relay.
  3. Test for infinite resistance between the following terminals:
    4. • 30 and 86
    • 30 and 87
    • 30 and 85
    • 85 and 87
    ⇒If not the specified value, replace the relay.
  4. Install a 30-amp fused jumper wire between relay terminal 85 and 12 volts. Install a jumper wire between relay terminal 86 and ground. Test for less than 2 ohms between terminals 30 and 87.
    5. ⇒If greater than the specified range, replace the relay.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  Relay Replacement
    •  Air Conditioning Compressor Replacement
    •  Control Module References for ECM replacement, setup, and programming