The fans are turned on by the thermostat sending 12v to the RElAY. the relay then sends current to the fans and the compressor. At least that is the way it is supposed to work. I have seen somereal creative wiring here. If yours has the shiny, origional relay it has been replaced by a relay kit. 71335290 is th n complete with proper wiring diagram. like American Express - don't leave home without the fans working when the compressor is on.