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Ac wiring question

F250 on Sun April 26, 2009 12:14 AM User is offline

Year: 99
Make: Ford
Model: F250
Engine Size: 7.3
Refrigerant Type: 134a

Another newbie question here. According to the Ford sevice manual wiring diagrams for a manual ac system (like mine), pin 41 from the PCM is connected to the "High Pressure Switch". My question: Does the PCM tell the switch what to do or does the PCM just receive information about the ac system via this wire from the HP switch? According to the wiring diagram, the wire comes from the PCM to the ac clutch diode then splits off to the HP switch and ac compressor clutch solenoid. Just trying to figure out how the PCM is involved in the ac system. Thanks again.

99 Ford F250 Superduty with oem AC

NickD on Sun April 26, 2009 7:39 AM User is offline

One reason is to kill the compressor if the PCM detects an engine overheat condition, another, mostly on foreign vehicles, is to kill the compressor when you slam on the gas to ease on engine loading.

GM Tech on Sun April 26, 2009 9:30 AM User is offline

High pressure is an input to the PCM, not an output....The PCM will use that to decide when to disable compressor for high and low pressures. (or turn on cooling fans)

The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

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