Alternator stops working around 2800 rpm?? Replaced alt and still occurs! HELP!

[bjrjr16]

Hi, I have a 99 s10 zr2, for the past couple months I noticed when It downshifts my lights will go dime and my blower fan will not turn as fast and my voltage meter shows 10-12 volts, it's getting very bad now to the point where it's affecting my fuel pumps output, if I try to pass a car it will acts
like I'm getting spurts of water going through my fuel system. I replaced the alt with a known working one from a 99 zr2 blazer, I recede it up before installing it to be sure it was good, all my connections are free of corrosion and are tight, my belt
tensioner is good aswell. I'm
not sure if there is some sort of external voltage regulator that could be failing? I thought most alternators have them built into then, any help would
be greatly appretiated, thanks alot, Brian

[MagicRat]

My guess is that you have an electrical short somewhere in the system. The voltage should not drop that low unless there is a tremendous electrical draw on the battery.

Possibly, as you downshift, the engine is shifting a bit, allowing a positive wire to make contact with something, such as the frame or engine block. I suggest taking a look at the positive wire going to the starter, and the starter terminals themselves, and look for bare wires or some kind of interference.