It is just a in and out switch......power goes through it.....when the evaporator core reaches a certain temp....I think it is close to 30 degrees fahrenheit...it will open and cycle the clutch to keep the evaporator from freezing up....A good rule of thumb when retrofitting to 134a from R-12 is to use about 10% less refrigerant....but be sure to flush the condenser and all lines with chemical flush and compressed nitrogen....if you do not have compressed nitrogen, you may use compressed air if run through a sufficient water separator....one for automotive painting will work well.....make sure you replace the filter drier, and evacuate well with a vacuum pump.......remember the longer the evacuation....the better......1 hour should be sufficient.....replace all the "o" rings and use mineral oil on them and on the threads of the connections(this will help reduce corrosion in case you need to service in a few years or so.