A/C Request

When an A/C request is received by the PCM, the PCM engages the A/C clutch relay when all of the following conditions are met:

the refrigerant pressure is not excessively high or low.
the engine coolant temperature is not excessively high.
the ambient air temperature is above approximately 0deg.C (32deg.F).
a WOT condition is not present.
the evaporator discharge air temperature is above approximately 0deg.C (32deg.F).
The PCM does not detect temperatures from the Mass Airflow/Intake Air Temperature sensor that does not correlate with other temperature sensor readings at ignition ON. (Most Mass Airflow (MAF) sensors have an integrated Intake Air Temperature (IAT) sensor).

Compressor control and the evaporator temperature are a function of many parameters, not just a straight on/off, to avoid freezing the evaporator. The PCM monitors multiple temperature sensors for correlation including, but not limited to, AAT, CACT, CHT, ECT, IAT, IAT2, MAF, MAPT, TCB and TCIPT Parameter Identifications (PIDs) (as applicable). The PCM runs this logic after an engine off and a calibrated soak period of 6 to 8 hours. This soak period allows the Ambient Air Temperature (AAT) sensor and the other temperature sensors to stabilize and not differ by greater than a calibrated value, typically 16.7 deg.C (30 deg.F). If a temperature sensor input is found to be reporting a temperature imbalance the PCM does not allow the A/C compressor operation to engage. For more information on PCM sensors, refer to the appropriate service information.

The PCM monitors the discharge pressure measured by the A/C pressure transducer. The PCM interrupts A/C compressor operation in the event the A/C pressure transducer indicates high system discharge pressures. It is also used to sense low charge conditions. If the pressure is below a predetermined value for a given ambient temperature, the PCM does not allow the A/C clutch to engage.