A/C Request

WARNING: This page is about a different car, the 2018 Ford Focus. 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 PCM engages the A/C clutch relay when all of the following conditions are met:

The PCM does not detect excessively high or low refrigerant pressure from the A/C pressure transducer.
The PCM does not detect excessively high engine coolant temperature.
The PCM does not detect an ambient air temperature below approximately 0°C (32°F).
The PCM does not detect a WOT condition.
The PCM does not receive a message from the BCM detecting an evaporator temperature below approximately 1°C (33.8°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).

The PCM monitors multiple temperature sensors correlation (AAT, CHT, ECT IAT, MAF...). The PCM runs this logic after an engine off and a calibrated soak period, typically 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 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.