1995 Ford Taurus

Cooling fan not cycling properly. Fan will come on when I energize AC and will run constant. Fan will not always come on when engine gets hot. Sometimes I can shut the engine off when hot and restart and fan will come on. Does this sound like a relay prob or sensor prob. I am thinking ECT sensor but I really don't know exactly how the system works or where the components are located.
September 7, 2007.

Its a 95 Ford Taurus 3.0 single cam (have had this car for four years 150K miles on it currently)

Has not overheated or boiled over yet. The temp guage has " NORMAL" in between the C and H. In normal opperation the needle hangs around the R in NORMAL. I have had problems with cooling system leaks (replaced radiator and two hose) so I am constantly watching the temp. Also the radiator cap comes loose every once and a while and I will loose coolent ( has done that on both radiators). Currently I have no leaks and radiator stays full of coolent. The needle has risen all the way up to the A in NORMAL. When this happens I turn on heat to cool coolent off, or just use AC to prevent this from happening (fan runs constant). I have been investigating and have notice that the cooling fan is not running when the engine is getting hot. I am thinking that it is not a relay prob because fan runs when AC is energized. Maybe that lower fan speeds are gone? I have not done any testing because I am not familar with the electrical sys. I am an electronics eng. So I can find my way around and test circuits if I know how they work. I will check my fan motor windings for contenuity on all speeds. How can I check the ECT and were is it located? How is the fan engaged in relation to coolent temp, what speeds when? ?

Sep 8, 2007.