This varies by engine, year and model, but here's how it works on the earlier cars. The fan switch on the top hose fitting serves two functions:


1) It turns on the fan at HIGH speed only, usually around 105C.
2) It turns off the A/C compressor at very high temps, usually around 110C


There should be two switches on the reciever/dryer. The first is a high/low pressure switch, which allows the compressor to engage only if refrigerant pressure is above some limit (to prevent running with low freon), and also prevents it from engaging above some high limit (around 300 psi), to prevent damage to system components.


The other switch is a pressure switch for the LOW speed fan function. It will engage the fans at low speed only if refrigerant pressure exceeds some limit. The fans DO NOT come on automatically with the compressor.