A/C Request

WARNING: This page is about a different car, the 2016 Ford F-150. However, it is still accessible from the selected car via links, so may be relevant.

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

Excessively high or low refrigerant pressure from the A/C pressure transducer is not detected.
Ambient air temperature is above approximately 32.0 °F (°C).
Evaporator temperature is above approximately 33.8 °F (1 °C).
Engine coolant temperature conditions are within normal parameters.
Wide Open Throttle (WOT) condition is not present.
Engine torque conditions are within normal parameters.
Battery state of charge conditions are within normal parameters.

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, MAF, MAP, and MAPT (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 18 °C (32.4 °F). If a temperature sensor input is found to be reporting a temperature imbalance the PCM does not allow the A/C clutch to engage. For more information on PCM sensors, refer to Engine Controls - Introduction (Except Diesel & Hybrid) .

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.