i have a problem with my trucks alternator, a 2000 5.3l silverado, i have tried 2 different alternators a excessive amperege 200amp and a 300amp, and they all do the same, ecxept the stock alternator, ok this is whats happening, when ever i shift to drive or reverse my voltage drops to like 11 volts, i have done the big 3, but still doesnt work , whats going on?????