Jeep Repair: A/C, doors and windows, pressure switch

Question
I have a 93 cherokee 4.0 L with a not working AC. Ive read a lot of posts on the subject and have this question. In my manuel and some forums they say to test the AC with the doors and windows open on max air and blower or the AC might cycle off. Is there some kind of thermostat inside the cabin that controls when the AC kicks in and or off? If so do you know where it is located, what the part name is and how to test it? Also, Ive taken the cap off the black relay and have touched the contacts together manually and the compressor starts to turn, so I do know that it does work...Also have taken my dash apart to check the wiring on the air and heater controls and nothing looks or smells bad. Thanks

Answer
I think I answered this once but in case it did not come through: have the system charged.  It is probably a little low on refrigerant and is not allowing the low side pressure switch to turn on the compressor. Any shop that works on air conditioning systems should be able to test and charge the system for you.