Low Voltage Problem
 

Okay, to start I just replaced my battery, ran all new 2 gauge wires to the alt, battery and ground along with the original wires, found a alternator off a 2000 GMC Sierra V8 pretty sure it was the 5.3 in a junkyard, brought it to Autozone tested it and it turned out to have a 130 max load. Brackets matched and all I had to do is tap off and butt one wire that didn't match up my old wiring and the belt fit fine, goes to 13.8 - 14 right when I start it, but after I drive around the block the voltage starts to drop towards 12 - 11 area which affects the performance of the blazer and just ticks me off, I'm not sure if its the same alternator I've seen some people use on the 2nd Gen trucks but I'm wondering what on earth the problem is, minus maybe it having a load in drive and idling too low to push that alternator. It has 3 ground wires, new battery and the alt passed the autozone test so I'm stumped.

did you test the alt output while its running at the alt itself? test the input to the battery also, the alt charge voltage should be the same at the battery

Yeah I had them both tested after I did the wires, and they both tested fine, it doesn't always make the problem in park or just starting it, it starts dropping when I drive around for awhile

Just because its a new battery doesnt mean its good..sounds like it may have a weak cell in it..its also possible when the alt gets hot it malfunctions

I'm not sure, I had the alt and battery tested and both turned out fine and it only drops voltage when I'm in drive or reverse it'll go back up when I put it in park