With AC on, only one fan (of two) comes on; why?
Car overheats if AC is not on, but with AC on, only one fan comes on; is that normal or
both fans should come on?
It is normal for just one fan to turn on with the A/C but as the engine temps rise the other fan should also turn on and run until the radiator has cooled down enough to turn it off but the other fan should continue to run as long as the A/C is on. I would suggest that you have you radiator cooling fans circuits and fan motor tested, you may either a bad relay, wiring or a fan motor.