1991 Chevy Camaro charging system problem

Alternator dosen't charge to correct voltage. I get 12.3 volts. Tried 3 alternators, all the same. I have another camaro same year & model. They work fine in that car. 2 wires plug into alternator. With car not running they both appear to have proper voltage. When I start car voltage meter dosen't move beyond where the battery voltage puts it. I drove the car about 50 miles and had to replace battery before I could restart car. At this time ( while driving ) I realized that the cruise control also wasn't working. Prior to this I had a problem with cooling fan not working & car overheated. I replaced coolant sensor thinking that was the overheating problem. I drove the car 40 miles to work. It didn't overheat, even though I removed fuse to cooling fan & forgot to put it back. Alternators are good, batteries are good and all fuses look good. Any ideas?