my 1990 RS 305 has always had a radiator fan problem, until i replaced the fan motor, after I did this it still wouldn't turn on by itself but it would turn on if i turned the a/c or fan on, so I am left constantly running the ac, what's the problem here? what needs to be replaced to get it to turn on at it's original programmed temperature?

 

I believe the temperature it needs to reach for it to turn on is 236* I'm not sure how I came up with that but I seem to remember it from another post. There are several stickies that show you how to diagnose your system.

 

I had this too on my manual fan switch, problem was the ground came off on the switch side so the AC still worked. If your AC turns on the fan you have a bad sensor, connector or burnt (close to exhaust) signal wire side of your circuit.

 

The AC always turns on the fan when the car is running properly, but if the sensor that taps into your passenger side head with the green wire attached to it goes bad then you either need to replace that sensor, hardwire your fan, or always run your AC.