Okay I figured out why my car is getting so hot the fan isn't kicking on. When I first got it it was and sometimes it wouldn't shut off. Where should I start? Oh its an 88 turbo coupe by the way.
There is a fan controller under the dash (on my 86 it was behind the glove box). These are pr0ne to melting. There are threads here and on mustang forums about this. I bypassed mine with relays, since when it melted, it melted pretty good and distorted the connector. I figured it wouldn't make good contact with a new one (plus I heard the part-store replacements are unreliable (surprise!)).
You could also have a bad motor. My fan motor died, but not without a lot of tell-tale bearing squeal first. Easy enough to test, just disconnect it at the fan, and apply power and see if it runs.
I would think if it just suddenly doesn't turn on, it's the controller though.
The fans are controlled by relays inside IRCM, if they run continually someone's been jerking with the wiring or relay(s) is stuck...
You'll need to do a code scan to see if they turn on during test(will be very brief)... If you can't tell, ground pin #17(pink wire) on IRCM and lower fan should run continually...
If above checks out, the engine temp sensor between #2 & #3 injectors is probably bad(no they won't always set a code)...