PCM Controlled Air Conditioning Diagnosis

Diagnostic Chart:

Wiring Diagram:

Circuit Description

When an A/C mode is selected, the A/C control head or the A/C programmer supplies a 12 volt signal to the PCM A/C request input. The A/C compressor clutch relay is controlled by the PCM. The PCM provides a ground path for the A/C relay coil. When the A/C compressor relay is engaged 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 one or more of the following conditions are present:

^ Throttle angle is greater than 96%.

^ A/C head pressure is greater than 440 psi (4.6 volts) or less than 35 psi (0.4 volt) (as determined by the A/C refrigerant pressure sensor).

^ Ignition 1 voltage is below 10.5 volts.

^ Engine speed is greater than 4300 RPM for longer than 1 second.

^ Engine speed is greater then 5400 RPM for any amount of time.

^ ECT is greater than 124°C (255°F).

^ IAT is less than 5°C (41°F).

Any of the conditions described above will inhibit A/C compressor relay operation.

Diagnostic Aids

Check for the following conditions:

^ Poor connection at the PCM. Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection.

^ Damaged harness. Inspect the wiring harness for damage.

^ If the harness appears to be OK, disconnect the PCM, turn the ignition ON and observe a voltmeter connected to the A/C relay control circuit at the PCM harness connector while moving connectors and wiring harnesses related to the A/C relay. A change in voltage will indicate the location of the malfunction.

Check for conditions that will disable A/C operation:

^ Throttle angle is greater than 96%.

^ A/C head pressure is greater than 440 psi (4.6 volts) or less than 35 psi (0.4 volt) (as determined by the A/C refrigerant pressure sensor).

^ Ignition 1 voltage is below 10.5 volts.

^ Engine speed is greater than 3700 RPM for longer than 5 seconds.

^ Engine speed is greater then 4400 RPM for any amount of time.

^ ECT is greater than 124°C (255°F).