Hi I'm having alternator issues, intermittently.

I have a voltmeter at all times in the car so I automatically see all drops, what I'm seeing is it's charging right after starting the car at 14.4v like normal operation. However when I start driving, especially when coming to a stop my voltage drops to 12.8V like a healthy battery voltage should be...

I've heard your electrical is only as good as the worst ground, so I tried removing the worst ground I have which is the wimpy 4 gauge tranny ground but same deal. Yes my car still starts because all other grounds are 2/0 AWG including the valve cover, and everything is also grounded to the block...

I'll be honest, I have a ridiculous sized stereo for this weak 90 amp alternator but I don't play it for extended times ever, and hence why I have my voltmeter to make sure it's not dropping like crazy. Usually when it's working I have no problem maintaining 14V at the volumes I play it at. I'm thinking it's bad and tried checking at autozone with their free test, but of course it passed because it was intermittently charging at 14.4V like normal.

Any suggestions? I realize it very well may be time for a rebuild, this alternator has 15 years on it now. Thanks for any help guys. Fairly new to these forums hope this in the correct place.

 

Also I'll mention that the 12.8V drop is with all accessories off, and not even the head unit on... May be a stupid question and obviously the alternator but I don't like to change the wrong part.

 

they typically last for about 160k, usually the brushes are worn out by then.
mine were almost done, replaced and good to go.

http://www.ebay.com/itm/ALTERNATOR-BRUSH-TOYOTA-LEXUS-ACURA-MINI-COOPER-/271929190396?fits=Model%3AIntegra&hash=item3f503e27fc&vxp=mtr