PCM Controlled A/C Circuit Diagnosis

Refer to HVAC Schematic HVAC Compressor Controls CJ3 and HVAC Compressor Controls CJ2 for wiring diagrams.

Diagnostic Chart:

CIRCUIT DESCRIPTION
When A/C is selected through the A/C control head a signal is supplied to the Powertrain Control Module (PCM) A/C request circuit through the Class 2 Serial Data circuit on vehicle with CJ2 systems, on vehicles with C60 and CJ3 systems the A/C control head supplies the signal to the PCM A/C request circuit. The A/C 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 specific calibrated acceptable ranges the PCM will enable the A/C 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 positive 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 angle is more than 96 percent.
^ 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.
^ Ignition 1 voltage is below 9.5 volts.
^ Engine speed is more then 4392 RPM for any amount of time.
^ ECT is more 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
- 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 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 more than 96 percent.
^ 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.
^ Ignition 1 voltage is below 9.5 volts.
^ Engine speed is more than 4392 RPM for any amount of time.
^ ECT is more than 124°C (255°F).
^ IAT is less than 5°C (41°F).