Cooling fan comes on but won't shut off

Well, another problem reared its head. Apparently the radiator cooling fan comes on but won't shut down after the coolant temperature drops. 

Any suggestions where to start looking ?  Also, it seems that the temp gauge reads on the low side.

If you have a bad engine coolant temp sensor, the one that is mounted in between the injectors on an 88TC, that could cause it to act strange. Go to a local junkyard and grab one form an 80's Ford and swap it in to see if it corrects the problem. Mine was bad and replaced with a parts store replacement, but it never worked well, replaced with a genuine Ford one from an 80's Crown Victoria and works great now.

In addition to what Kuch said, it is possible the fan relay(s) are sticking shut. Next time the fan wont turn off, give the IRCM a good rap or 2 with a small hammer, wrench, etc. If fan turns off, replace IRCM.

Ok, I'm going to try changing the sending unit first. See if I can get one from a ford of similar vintage.  I have a new RCM which I can install if that doesn't work.