A/C Compressor Control Circuit Diagnosis

Circuit Description

When A/C is selected through the A/C control head a 12.0 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)

Diagnostic Aids

Check for the following conditions:

^ Poor connection at PCM or oil level switch. Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection. Refer to Diagrams.
^ Damaged harness. Inspect the wiring harness for damage. If the harness appears to be OK, disconnect the PCM and turn the ignition ON. Observe a voltmeter connected to the affected PCM output circuit while moving connectors and wiring harnesses related to the PCM output circuit. A change in voltage will indicate the location of the fault. Refer to Diagrams.

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 parts 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.