Alternator output fluctuations
 

OK, new alternator, all grounds cleaned and good continuity, good battery showing 12.6V at rest. Start the car and we see 14.2 volts as measured at the battery. Load with headlights and blower and still 13.8 volts, sweet. Turn off everything and leave the car running and volts measured at the battery steadily falls to 13.3V over a minute or two. Turn off and restart immediately volts are 13.6 or 13.8 so take it for a drive and the volts still vacillate. Swapped in another known good voltage reg and the same thing. Any ideas?

Follow up.
Not the battery, swapped in another virtually new same phenomena. *Good voltage 14.25 , go 5 miles down the hwy, no issues no change. *Stop get gas and restart and I am at 13.25V! *What's going on? I think I get DC, but maybe someone can chime in. *As voltage fell as measured by the battery, I figured I would hook the DVM direct to the alternator and sure enough about a 3/4s volt drop from the alt, 14+ to the battery 13.25. **I then moved the DVM downstream to the starter. *Starter had the same 14.25V as the alt while the battery was 13.25. *So somewhere between the starter and battery I am losing voltage and seemingly only as the car warms up and stays warm. *When the car is "cold", both the battery and the alt read the same voltage. *Any BTDT?

perhaps the cable between the alternator and battery is old/corroded and increasing in resistance as it gets warmer?

On this car the positive from the alternator runs to the starter, then another runs to the battery from there. I think I eliminated the alt to starter lead as OK, but maybe the starter to battery is the culprit. That seems to be the problem. No idea if it is the pos or the neg lead but will replace both and see.