09 Prostar A\C problem

Proper charge is 3.0 pounds. You should have lower low side pressure, about 25-30, (depending on hot it is out), but 50 is too high. The high side will quickly rise to 150 and keep rising to about 300 to 350 psi then the fan should kick in. The A/C system on a Prostar is all computer controlled based on the pressure reading and inlet, outlet evaporator temp sensors. If you have a proper charge and the low side is not dropping below 50 I would try an expansion valve.

 

I had this issue a few months ago when I put my glider together... now, mind you, the AC is cast aluminum and quite fragile... but I rapped on it with a plastic mallet a few times (SOLID, but not HARSH) while it was running and it cured the problem. Evidently, something inside the compressor was stuck. My old roommate is the head of maintenance for a mid-sized fleet... he said this happens with about 50% of the replacement compressors they get. I focused my "hard taps" on the bolt that holds the lines in the compressor, and the back face near the relief valve.

 

Engine fan should be coming on. The body controller is monitoring the pressure. It will tell the engine to turn on the fan at around 300. If the fan isn't coming on, the body controller is cutting off power to the compressor to protect the system. Is the fan coming on at all? As for the oil, technically to know how much to add, you need to know how much is already in the system. This is impossible to know with out removing everything and flushing in. When you recover, if your machine separates oil, I would just put back in whatever amount came out with new oil. System takes PAG oil.

 

Ok, the condenser may not be cooling the refrigerant since the pressure is not coming down once the fan kicks in. Make sure the condenser is clean, and not totally plugged with dirt or bugs. The when the system is running and pressure is building take a garden hose and run water over the condenser. If the pressure drops, keep the water on it for a bit and see if the a/c is now cold in the cab. If it is, you need a new condenser.

 

Plugged with what?

 

If the dryer/accumulator was plugged the compressor would suck the low side right down to a vacuum. Also, any point in an a/c system that is excessively restricted will behave like an expansion valve and create frost at that point.