Help! The last day my a/c worked the output wass so cold it made ice on the inside of the windshield - the next day it blew only warm air. I’ve checked the fuses (dashboard and under hood) and relays - all are good. This has got to be a bad switch somewhere, but where? How do it track it down?
It sounds like you have lost your refrigerant due to a leak. Have the system checked for refrigerant and dye tested if necessary. My A/C did the exact same thing, working great one day and dead the next. It turned out to be a leaky condenser. $1000 later I have ice cold A/C again. For anyone about to say “that is too expensive for a condenser” I will tell you that it was a condenser, receiver/dryer, and (most expensively) the tubing to and from the controls (this is on a Toyota Previa that is freakishly hard to work on). I had 7 hours of labor on top of parts.
There should be a pressure switch on the low pressure side of the system; usually near the accumulator, etc.
With the A/C on you should verify that power is provided to that switch.
If there is power at that switch, and no power through it, then either the switch is bad or the system pressure is low enough that it will not close the circuit.
If there is no power at that switch, then you need a wiring diagram and will have to trace the problem out.