A/C Compressor Clutch Control Diagnosis

Diagnostic Chart:

Wiring Diagram:

Circuit Description

When A/C is selected through the A/C control head a 12 volt signal is supplied to the A/C request input of the PCM. The A/C compressor clutch relay is controlled through the PCM. The PCM monitors the A/C refrigerant pressure. If the A/C refrigerant pressure, and engine operating conditions are within a specific calibrated acceptable ranges the PCM will enable the A/C compressor relay. This is accomplished by providing a ground path for the A/C relay coil within the PCM. When the A/C compressor relay is enabled battery voltage is supplied to the compressor clutch coil.

The PCM will enable the A/C compressor clutch whenever the engine is running and the A/C has been requested, unless any of the following conditions are met:

^ Throttle greater than 90%.

^ A/C head pressure greater than 414 psi (4.27 volts) or less than 34 psi (0.35 volt) (as determined by the A/C refrigerant pressure sensor).

^ Ignition voltage below 10.5 volts.

^ Engine speed greater than 4500 RPM for 5 seconds or 5400 RPM.

^ ECT greater than 125°C (257°F). IAT less than 5°C (41°F).

Test Description

The numbers below refer to numbers on the Diagnostic Table.

The A/C compressor clutch should not be engaged with the engine running if an A/C mode is not selected at the control head.

2. Stored Diagnostic Trouble Codes (That may not illuminate the Malfunction indicator Lamp) may disable the A/C compressor. This Diagnostic Table may lead to improper diagnosis and replacement of good pans if Diagnostic Trouble Codes are present.

3. The A/C compressor clutch should apply if the conditions under the Notice have been met.

6. Checks for an A/C request signal from the control head to the PCM.

12. Checks the ignition feed circuit to the A/C relay.

13. Determines whether the A/C relay or the circuitry is at fault.

21. Determines whether or not the PCM is turning the A/C relay on due to a false A/C request.

22. Isolates problem to the A/C control head or the PCM.