Could be the relay, could be either of 2 (depending on you model) pressure switches located on the receiver/dryer. Could be low charge, aux fan works off of the ac normally. Need a little more info. When did the ac stop working? Did it make any unusual noises? What's the state of charge in the system? Is it still R12 or has it been converted to 134a? Did the seals get replaced w/the right stuff? Has there ever been any of that nasty stop leak garbage in the system? Could be a bunch of stuff or it could be really simple.