[Mike C]

Alternators designed/rated for a 12v system put out 14 volts. Usually 14.1 or 14.2 by design to charge the battery need higher voltage output than the battery. It sounds like the internal regulator is shorted in the alternator you have. Just replace it with an alternator of similar ouput.

You can also rebuild the regulator portion of the alternator and see if that doesn’t solve your problems.

I like 12si alternators from an mid 80’s Suburban. Should be 95 amps and can be picked up brand new on Rock Auto for less than $100. Fits in the same spot as the 10si just the fan looks different.