My concern is he says he gets batt volts at all three wires one being the ground wire? I was also reading in my manual that if the diode shorts out internally the engine will run with the key off it will backfeed to the ignition. One quick test is fully charge the battery and start the motor after a bit at 2000 rpm check the voltage at the battery it should be 13.8-13.9 volts.
That is true with some systems and can be a common problem.... even with several of the V-8's.
Regarding the voltage readings that he's seeing, perhaps he's doing somethin wrong in his test procedure.
I still say have the alternator tested, and make that the starting point, and then work from there.
Use the tried and proven P of E.... (process of elimination)... one item and one item
only at a time.
The P of E won't let you down when used systematically and methodically.
justboughtaboat, if you replace the alternator, a good inexpensive replacement would be the Marine Delco 10si in either the three wire or single wire.
With the single wire, the wires within the harness can be capped off. You need only the POS charging lead and the case-to-engine Negative lead connected.
These are self exciting and require a quick bump in RPM to excite them.
Once excited, they remain excited until the engine is shut down.
It's a non-issue for most of us.
.
.
]