I've been fighting a voltage drop for a few weeks now. Had a used 145amp alternator and all was well for a couple months. Now when it gets warmer I see a voltage drop on the gauges at idle and cruising. I see a steady 13.6ish volts on cold start and driving, and after a bit it drops to mid 12s and as low as 12.3. These are what is shown on my obd2 scan port device. After that, checking it under the hood with a tester shows 13.5 at the battery and alternator posts. It showed 13.0 at the drivers side post by the fuse boxes. But the wire from the battery to the post shows 0 resistance. Im a bit stumped. Same thing is happening with the alt rebuilt now too

 

When this happens, is the A/C running?

 

No AC on the car

 

So your losing .5 volts from the battery + post to the stud at the fuse box? If so replace the run. May be corrosion at the o rings or in the wire. Get s nice piece of off 4 gauge with properly crimped terminals. The only other possibility is your used alternator is on its way out and can't keep up with your electrical. Put it in park, start it and run all your accessories.... If your vom shoes less than 13 volts your alt is trash

 

I did exactly that today. Replaced the run from the battery to the post with actual battery cable and crimped/soldered the ends for good measure. Also ran 1 from the alt to the post direct. Now have 14.1 at cold idle at the ecu. I think it's fixed.
Alt was just rebuilt and didn't change the issue