Problem: This vehicle was brought to the
shop because the A/C was blowing hot air from all vents.
Details: The system was evacuated and
recharged just to confirm the refrigerant level was at
specification (20oz). The technician attempted to command the A/C
compressor “ON” with a scan tool, but it would not engage. When
he scanned for diagnostic trouble codes (DTCs), he found DTC
P0073 – Ambient Air Temperature (AAT) sensor circuit high.
He followed the Ford diagnostic Pinpoint Test “FA” in ALLDATA and
determined that the AAT sensor was faulty (reporting -40°F). He
replaced the AAT sensor and cleared the DTC.
At that point, there were no codes, but the A/C compressor still
would not engage. If power was applied directly to the
compressor, the system would engage and operate as designed. A
call was then made to ALLDATA Tech-Assist.
The Tech-Assist consultant advised using the scan
tool to review A/C-related temperature and pressure sensors to
verify that they all reported accurately and rationally to each
other. He further suggested checking to see if the PCM was
denying any A/C requests.
The tech inspected the A/C temperature and pressure sensors as
advised. He found that the A/C pressure transducer reported only
4 psi in all conditions. Using the wiring diagrams as a guide
(see image), he verified that the circuit integrity was good and
the PCM was not at fault.
Confirmed Repair: The tech replaced the A/C
pressure transducer and cleared the DTC. After the repairs, the
A/C operated as designed and the DTC did not return. Fixed!