Ford

Even though it is not prime AC season I have recently had a slew of Ford vehicles with no compressor engagement. First you check the basics, AC charge/pressures, fuses, make sure there is a belt on the compressor, etc. Then we move to checking dtc's and making sure there is nothing related to misfires, evaporator temperature, ac compressor relay circuits or any other typical related AC faults. We check modules for command inputs most importantly the AC request pid. The AC request pid changes with the HVAC input, but still no compressor engagement. Next we check active commands and the PCM can command the AC compressor clutch to engage. Retry the HVAC controls and it will not command the clutch on. I have seen this issue with Evap Temp sensors, but this read normally.

I have found other temperature sensors out of range can cause this issue as well. The AAT or Ambient Air Temperature was inaccurate (more than 30° off) on one car, and missing on another (showed -40°F) causing this fault. I believe these were both Ford Focuses (Foci?), but the most recent was a Transit connect with a Skewed IAT or Intake Air Temperature sensor. After a few circuit checks and a new Mass Air Flow sensor (integral IAT) the AC operation was restored.