Help!!!!!!!!!!!!!!

I have a 71 Duster, 360 and a 904 auto. The electical system is a MSD box, coil and distribator. When I bought the car the seller told me that it had a special made 100 Amp alternator because of the stereo he had in it. Here is my problem. I know the amp meter in these cars are crap. It stated to show a negative charge and I took the black wire from behind the gauge off. Then I noticed that when I would hook up the black wire again it would spark. I know that the black wire comes from the alternator (ground) and the red one for the other side of the gauge comes from the starter relay (Hot). With the gauge hooked up, I get voltage from both sides of the post on the gauge. I replaced the gauge and all was well for one short drive. Then it started to discharge again like before. When I checked the gauge again it was hot on both sides. I have gone through all of the other wiring and even replaced the harness in the front lights and engine. I am wondering if this alternator that is putting out 100 amps is killing the ampmeter? Any thoughts?