Toyota Repair: Toyota AC, ac repair, intermittent problem

Question
QUESTION: I have a 94 Toyota pickup with the same problem as the guy with the Avalon:  The AC will stop working for no apparent reason. This has happened many time over the years, and when you live in Florida, it gets quite uncomfortable with no AC. Typical scenario: You drive to the grocery store and the AC works fine. An hour later for the ride home, you can't get any cold air. The AC will also stop working while you are sitting at a light. My mechanic, who advertises AC repair, has replaced every part of the system at least once over the years, always for freon leaks. He once mentioned that there was a solenoid that might have something to do with this problem, but he has never specifically looked to solve this problem. I got tired of going to him for AC repair because the freon always leaked out after a few months. I finally serviced the system myself with a can of AC Pro freon with leak sealer and that seems to have fixed the leaking, but still have the intermittent problem. The little control board that sits on top of the blower I have replaced twice myself as well. This thing  reads information from a number of sensors, and has an adjustment knob on it which I believe has something to do with rpm but I don't know how to adjust it.

ANSWER: That control knob is for setting at what idle speed the compressor engages and turns off, it's basically trial and error, turn it all the way one way and then the other and watch the result,
as soon as the clutch engages the idle will go up as long as the compressor is running.

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QUESTION: Okay, that answers my question about the knob.  Any idea which sensor may tell the compressor not to run when I am not getting any cold air?  If I knew where the bad signal was coming from I could fix a sensor or maybe bypass it.

Answer
There is a pressure switch on the high pressure line it has a connector with two wires on it bypassing it will cause the compressor to run all the time but doing this doesn't take care of the real problem, other than that the compressor is controlled by engine rpm and line pressures so if the line pressures are incorrect due to a restriction or low pressure the compressor will shut off to protect the system, this can be caused by a bad expansion valve, receiver drier, condenser and evaporator, another reason for the compressor to turn off is engine overheating.