PCM Controlled A/C Circuit Diagnosis

Circuit Description

When A/C is selected through the A/C control head a signal is supplied to the PCM A/C request circuit through the Class II 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 a 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:

VIN K

^ Throttle angle is greater than 96%.
^ A/C head pressure is greater 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 9.5 volts.
^ Engine speed is greater than 4392 RPM for less than 1 second.
^ Engine speed is greater then 4400 RPM for any amount of time.
^ ECT is greater than 124°C (255°F).
^ IAT is less than 5°C (41°F).

VIN 1

^ 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 9.5 volts.
^ Engine speed is greater then 4392 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:

VIN K

^ Throttle angle is greater than 96%.
^ A/C head pressure is greater 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 9.5 volts.
^ Engine speed is greater than 4392 RPM for less than 1 second.
^ Engine speed is greater then 4400 RPM for any amount of time.
^ ECT is greater than 124°C (255°F).
^ IAT is less than 5°C (41°F).

VIN 1

^ 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 9.5 volts.
^ Engine speed is greater then 4392 RPM for any amount of time.
^ ECT is greater than 124°C (255°F).
^ IAT is less than 5°C (41°F).