PCM Controlled A/C Circuit Diagnosis

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 powertrain control module (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 enables the A/C compressor clutch whenever the engine is running and the A/C has been requested unless one or more oil the following conditions are present:
^ The Throttle angle is more than 96%.
^ The A/C head pressure is more than 440 psi (4.6 volts) or less than 35 psi (0.4 volt) as determined by the A/C refrigerant pressure sensor.
^ The ignition 1 voltage is below 10.5 volts.
^ The engine speed is more than 4300 RPM for longer than 1 second.
^ The engine speed is more than 5400 RPM for any amount of time.
^ The engine coolant temperature (ECT) is more than 124°C (255°F).
^ The intake air temperature (IAT) is less than 5°C (41°F).

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

Diagnostic Aids
Inspect for the following conditions:
^ A faulty connection at the PCM. Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connections.
^ 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 the connectors and wiring harnesses related to the A/C relay. A change in voltage indicates the location of the malfunction.

Test for the conditions that disable the A/C operation:
^ The Throttle angle is more than 96%.
^ The A/C head pressure is more than 440 psi (4.6 volts) or less than 35 psi (0.4 volt) as determined by the A/C refrigerant pressure sensor.
^ The ignition 1 voltage is below 10.5 volts.
^ The engine speed is more than 4300 RPM for longer than 1 second.
^ The engine speed is more than 5400 RPM for any amount of time.
^ The engine coolant temperature (ECT) is more than 124°C (255°F).
^ The intake air temperature (IAT) is less than 5°C (41°F).