PCM Controlled A/C Circuit Diagnosis

Diagnostic Chart:

Refer to Engine Controls Schematics A/C Compressor Control.

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 more than 96 percent.
^ A/C head pressure is more than 440 psi (4.6 volts) or less than 32 psi (0.3 volt) as determined by the A/C refrigerant pressure sensor.
^ Ignition 1 voltage is below 10 volts.
^ Engine speed is more then 4400 RPM for any amount of time.
^ ECT is less 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 the following conditions:
- Backed out terminals
- Improper mating
- Broken locks
- Improperly formed or damaged terminals
- Poor terminal to wire connection
^ Damaged harness - Inspect the wiring harness for damage.
^ If the harness appears to be OK, disconnect the PCM, turn ON the ignition and observe a Digital Multimeter 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 more than 96 percent.
^ A/C head pressure is more than 440 psi (4.6 volts) or less than 32 psi (0.3 volt) as determined by the A/C refrigerant pressure sensor.
^ Ignition 1 voltage is below 10 volts.
^ Engine speed is greater then 4400 RPM for any amount of time.
^ ECT is more than 124°C (255°F).
^ IAT is less than 5°C (41°F).