single field to dual isolated field alt

What specifically are you using for a regulator?

Have you checked charging voltage when it "seems" to be working?

It might be something like (example) a shorted turn in the alternator field, which would cause the field to draw excessive current, not charge efficiently, and of course the regulator would have to supply MORE field current to get the output, so =--------hard on regulators

There is a figure for field current in the shop manuals. If you cannot find it, I'll have to look it up