why wont my ac pump shut off??

Just going off what you have told me it sounds like the fan high pressure switch, I own a KW they are a little different, mostly the colors is all. That is my opinion you are on the right track, maybe another pete owner can jump in here if we are goofed up. I did not have any gauges to check for pressure at that switch so I was pretty careful messing with it , as I was just out back in their parking lot. But from memory it did not have much pressure that wouldn't leak off when I was easing the switch out, here again keep back a bit. Shouldn't be a big deal really, I think I was back out on the interstate in an hour start to finish.

 

The system pressure on lowside has to get around 23psi for the compressor to cycle off then when it goes back up to about 40 psi it will come back on. It may be because of ambient temp being so high try turning fan speed down to first setting then see if will cycle with idel set to 1100

 

I own 98 379 Pete and my a/c compressor clutch never sycles. a/c relay works fine because I would not be able to turn my a/c off at all..May be as Gear head said a lowside pressure never gets down to the point for a low pressure switch to turn the system off. I always sought it was normal for a compressor to stay on all time.

 

If your system is working at or near capacity, the compressor will not cycle off. It sounds like that's what's going on; it's hot enough outside that your system is working at capacity, pumping all the heat that it's capable of pumping. The only times that it will cycle off are when the high side pressure exceeds the high pressure switch setpoint (will only do this if there's a problem such as an expansion valve that's stuck closed, the system is overcharged, or the condenser fan does not cycle on when needed), or when the low side pressure drops below the low pressure cutout switch setpoint. This can happen during normal operation when the air passing over the evaporator is cold enough that it doesn't boil off the liquid refrigerant quickly enough, and is completely normal. When it's NOT normal is when it cycles on the low side due to low refrigerant levels.

 

Some trucks have orfice tubes and some have expansion valves. The way to tell difference is the system that uses orfice tube will have a accumulator in low side (silver canister). The expansion valve systems will not have this. The orfice tube systems will use pressure switch to cycle the expansion valve systems use a thermostatic switch which has probe that determines temp that cycles compressor.

 

If I'm not mistaken, don't all dual systems (defrost / sleeper) have expansion valves instead of orifice tubes? And I also believe that the expansion valve systems still use the high / low pressure switches to control the compressor; the expansion valve temperature bulb only mechanically controls the amount of refrigerant that is allowed to pass into the evaporator, and there is no electrical component.

 

Some good info explaining A/C basics HERE

 

You are right all trucks use expansion valves for bunk some use expansion valves in cab but the ones that use expansion valves for cab uses thermostatic switch which electronicly cycles compressor. In these systems there is a binary pressure switch witch will shut compressor off if pressure is too low or to high. It has anouther pressure switch which turns the fan on and off its located on left side of radiator at top. But this thermostatic switch is what cycles compressor on and off based on temperature not pressure. Orfice tube systems use pressure switch for cycling compressor .

 

Some pete have orfice tube for cab and expansion valve for bunk. The new petes are using expansion valves in cab and bunk. Orfice tube or expansion valve their both called expansion divices