1995 Ford Mustang Engine cooling fan does not come on

I have a 1995 Ford Mustang with a 3.8L engine. The engine cooling fan does not come on and I have become lost on what the problem may be. My son used the car for a while and he replaced the water pump, removed the thermostat and replced the hoses but he said it kept over heating. When I got it back I noticed the cooling fan was not coming on. The power fuse is good and the fan does work when you wire it straight to the battery. I have replaced the cooling sensor but it still does not come on with either the engine warming up or turning on the A/C. The A/C is needing to be charged as the compressor does not come on due to the system being low on freon. What is my next step?