A/C pressure

Are the heater valves shut? There’s a chart somewhere for ambient, if it was low on Freon the compressor would short cycle most likely,

Could also check the blend door being stuck

 

Compressor and fan engage/cut out pressure depends on the pressure switch for each. I’d guess you’re a bit low on refrigerant. That’s probably why the compressors constantly running. Although you may have a bad high pressure switch. I would add a bit, very slowly, in gas form ( can right side up) till the compressor shuts off. May take a while. Always worked for me when in doubt. If you add too much with a bad pressure switch you risk over charging and blowing a hose or even splitting the compressor wide open. I’ve done both. On my Freightliner the fan always comes on just before the compressor cycles off. There’s so many different rated pressure switches for different Trucks. I’d try to get the info from the Dealer. The numbers are stamped on the switches. Example Normally open NO325, meaning it closes and shuts off the compressor at 325 Another example is NC325, it’s normally closed and opens at 325. You might have only 1 switch that is a combo low and high switch. Called a bianary switch. Fan switch is usually separate. Though there is such a thing as a trianary switch for all 3. It’s rare, probably no longer used. Maybe the low side of the switch is working and the high side isn’t. I’d throw a new switch on it anyways, keep old one as a spare, or for future diagnosis. You might be able to read the stamp on your existing switch. I doubt it though. It’s hard to see even when new. Call the Dealer, find out what switch or switches your Truck has, then add refrigerant at least up to that pressure. Compressor should shut off. If not I’d suspect a bad switch. Good luck.

 

The specification is the same no matter what the vehicle is.

The fan kicks on at 325 psi.
A good pressure is around 140 with fan running on semis. Cars is 225.
The low should read 30-40. Shuts the compressor off if it hits 20ish.

Semis read lower because they have different fans with more air draw then cars.

Readings are with engine running. Not idle. So kick the idle up.

Equalized pressure after motor sits overnight or whatever. 125 psi @ 120* outside air temperature. Freon expands and contracts with hot amd cold. So at 75* sunshine. Equalized pressure is probably between 80-90.