[greywurm]

I just learned about this yesterday after doing a lot more digging. There is a module a few inches from the battery on the negative post that is called a battery generator regulator. It senses battery flow and adjusts the alternator regulator to system demands. It's part of a regulated voltage control system. GM put this on 05 - 10 full size trucks and SUVs (I think they stopped in 2010)

When the engine is in fuel economy mode, and electrical demand isn't too high, this low voltage mode kicks in and drops the alternator output to between 12.5 and 13.1. My readings at the alternator were right in this range.

To confirm, you can turn on tow mode (if equipped) or turn on your headlights and it will trigger the alternator to slowly step up the voltage to the usual 14.x range. Mine did just this, but it is slow.

I didn't connect the dots that by the time I was done with work and got to changing the alternator, it was dark enough for the headlights to come on on their own, so it explains why it was "working fine for the better part of the day" after each change.

Never knew this existed until yesterday. I can't tell if the confusion is worth the fuel economy for day time driving, but I guess it lowers the stress put on the alternator.

Now I feel bad for going through 4 alternators that probably were just fine.