Charging System Problem

Depending on which battery isolator he has, there may not be any voltage to the big wire with the engine off. A solid state isolator has 2-3 large diodes for isolators.

I can think of two other possible issues.

FIRST: Measure voltage AT THE ALTERNATOR battery terminal (That's the big one) First put the ground on the alternator case/housing, then on the engine, then on the battery negative.

This has been known to be diagnostic, though I do not think you will have the same problem, If you have NO voltage on any of those paths, You have found the problem (most likely a fused fusable link).

I had a different problem.

The other problem (loose fan belts) not likely with modern serpentine belt engines.

Older GM alternator should have battery voltage on large wire connected to output stud. One wire to the two terminal connector should have battery voltage with key on for exciting the alt and the regulator. The other wire is for the dash warning light. And, if you ran it with a wire or voltage not there, don't worry,you didn't hurt a thing.

I agree, sounds like a wiring issue.

And, if you disconnect the B+ (large-gauge wire) and run it, you run a very high likelihood of blowing the diodes in the alternator. DON'T EVER RUN AN ALTERNATOR TO AN OPEN CIRCUIT.

The older external regulator style should have 2 wires plus the Batt wire . Follow the 2 wires and you should find the regulator.The older external regulator alt. were not high amp out put you might want to look at a new internal regulated one it's a easy change over.

It is GM. I didn't see an external voltage regulator. Where would it be located?

Sounds like it has an external voltage regulator that isn't working right. Is this a GM chassis?
It may have the wrong alt. (external as opposed to internal regulating)