Thanks to my radiator fan refusing to come on I can't enjoy my 90 Mustang's cool ac in this Texas heat. My car had this problem years ago and a guy at AutoZone, who also had a Mustang, hooked the fan up to the battery so it came on when the car did (not ideal I know but a quick, cheap fix). This worked for quite a while until one day I turned the car off and the fan stayed on. I detatched the fan from the battery and amazingly it still would come on when the ac did or when it started to get too hot. Now I'm back in the same boat where the fan won't come on for anything. I hooked it back up to the battery and felt the tease of cool air from that glorious ac but again, the fan stayed on when the car was turned off. Because of a brand new radiator the car thankfully isn't overheating yet.

Any suggestions on what the problem may be (why the fan won't come on when it obviously can work)?
Or how to get the fan to turn off when the car goes off if I keep it connected to the battery?

Thanks in advance.

Are you getting power to the switch that controls the fan? Is cooling system full? It is possible the temp switch that controls the fan is failing.

My girlfriends been having trouble with her Mustang overheating here latley. Her radiator fan quit, as you are discribing. Chances are that its your sensor/switch for that fan. Its located on the coolant return coming out of the block and back to the radiator. You can take it to autozone, and they can plug up a machine and tell you if any of your sensors, or switches are bad.

I'm assuming thisis a 4cyl mustang, because a 90 V8 didn't have an electric fan. On my old 89 4cyl, I just wiried the fan to a source ignition power, so the fan would run all the time when the car had power. Cheap and easy fix.