Fault diagnosis for alternators and battery charging

I ran into an issue recently with my wifes' old car (2003 Hyundai Elantra) where the electrical system started to do some buggy stuff. First - charge light went on in the dash. Ok, probably hadn't been run for a while. Charge light goes out, but it starts running erratically. Ok, 3000 year old fuel (thought the fuel level was low as the gauge was showing empty but... :rolleyes:) So I ran a multimeter over the battery while it was running - and got 16.35v. Not good - AT ALL. So I think the voltage regulator is baked and getting one online is cheaper than buying a complete replacement alternator but as a precaution I decided to google alternator fault diagnosis and found this helpful -

https://axleaddict.com/auto-repair/A...g-System-Tests

Interesting info I pulled from this is solving the mystery "drained battery overnight" issue which turns out to be easy to test.

:rolleyes: As for my "low fuel level/crap fuel" issue, I discovered the tank was not only over half full but when I removed the in-tank fuel pump and level sender assembly I discovered the ugly truth about what ethanol fuel had been doing to this car. The sender unit has (or in this case HAD) a small copper watch spring connecting the sweep arm to the fixed end of the volt sender and was corroded away to green chalk. The fuel pressure regulator has a thick crust of gunk encasing it and the plastic pump housing was stained with a rust-like residue (the Elantra has an all plastic tank so the only sources of ferrous metals are the fuel pump and the level sender - yay)