All I have for automotive so far is a small booklet and this it what is says:
The Evaporator pressure and temperature in most late model vehicles are controlled by the compressor turning on and off, a process called compressor cycling. The vehicles computer controls the compressor cycling based on information the computer gets from its engine sensors, vehicle speed sensors, and a pressure or temperature switch located at the evaporator.----- Sorry I can't be more helpful, maybe the AC guy will show up soon! lol.

Sounds like there should be some kind of thermistor at the end of the evaporator.. or maybe it's like matt was saying and it's controlled by the pressure switches.

 

I would think a thermostat would only be for a system that has automatic climate control....

 

actuall that helps my jeep is 89 ac is not ecm controlled so its either my pressure switch or thermostat according to you book i have a low pressure switch and my pressure are good so that leaves that out only one thing left thermostat thanks

 

in the fsm it shows there is a pre set thermostat

 

same here but the fsm show it weird

 

now on the expansion valve is that not a thermostat on top??????????

 

what does that thermostat connect to? I also read many systems contain an additional valve aside from the expansion valve which controls the refrigerant pressure in the evaporator, but it doesn't go into any more detail.

 

Does something connect to the top of that expansion valve? If so then what?

 

to me that looks like a diaphragm. Do you have a picture of an installed expansion valve? I'm working out of town other wise I would go out and look at my jeep.

 

thats the expansion valve on a jeep

 

Do two hoses connect on each side of the expansion valve? What I'm wondering is if the sensing element is self contained in the expansion valve. One line would be the liquid line into the evaporator, the other would be the suction line with super heated vapor to the compressor. That would make sense. Who knows

 

Yeah I have a brand new one at home because I never got to finish my system, but I can't remember what hooks to it.

 

That is a picture of the thermostatic expansion valve (TXV). The ones on vehicles usually operate on refrigerant pressure, not temperature. There is a sensing "bulb" that controls the valve position which in turn controls how much refrigerant enters the evaporator coil. The symptoms from the original post point to low charge in the system. This is the most common cause of the compressor cycling. If you want a quick test and/or fix, go to a dollar general store and look for a can of R134A. It's about 8 dollars compared to 15-20 at the autoparts store. Buy one of the cheap (about $30) handheld gauge/dispenser that screws onto the can (the cans are all the same fitting). Screw the dispenser onto the can, start your jeep and turn the a/c on full blast, connect the fitting on the dispenser tube to the INLET or LOW side of the compressor (fitting will only go on the correct side), ALWAYS hold can upright with hose coming out the top, pull trigger and watch the pressure on the gauge. When the compressor clutch is on or "engaged" is when you want to read the gauge. Hold the trigger until your in the green zone on the gauge. If I remember right, it's around 40 to 50 psi. Unhook dispenser from low side fitting and put cap back on. Enjoy and remember - never have flames or sparks around refrigerant.

 

If it still cycles after you add refrigerant, chances are your low pressure switch is bad. You can jump it out to test if you feel comfortable doing that, just don't leave it jumped out. PS - I'm a liscensed class 2 and 3 refrigeration tech.

 

im using freeze 12 pressures a re still good never heard of low pressure switch going bad to wharte it cuts on and off when systems full might be possible its like six bucks had that planned all ready just stumped