A/C Cooland Temp. sender on '94 Tracker

A couple of years ago I had a new radiator installed on my Tracker and at the same time the mechanic replaced the fan sensor on the intake manifold/cooling casting. Now, when I turn on the ignition key, even though the engine is cold, the electric fan starts and runs until the temp gauge starts to move. Also, the A/c won't work until the car starts to warm up, Which was never the case before the mechanic replaced the sensor. I've since bought a new sensor from NAPA, who seem to be very competent, but the problem continues. I know that there are two separate Coolant Temp. senders but they appear to be the same. One controls the fan and the other the temp gauge. I've switched them back and forth to now avail. My A/C thermistor recently went out and I've taken out the evaporator to replace it. Has anyone out there had similar experiences? The Amplifier appears to be in good condition with no burn or hotspots on the solid state board. Could this brain box be causing the problems? Thanks to all!