Do you have one or both old alternators? How about checking out the
diodes in them?
I try to fix everything that breaks myself. I'll let something stay
broken for years while I try to figure out how to fix it, rather than
pay someone. Of course I can't let the car stay broken for more than a
day.
When my brother gave me his car, he had a problem with a battery that
repeatedly went dead, for more than two years, ever since he bought the
car new. It came with a 2-year warranty and the dealer had replaced the
alternator, the regulator, and the battery, each of them twice, and
still the problem. Then the dealer said the warranty had expired and
wouldn't do anymore.
Annoyed, he gave me the car. It had a another bad battery by this time
and I took it to Sears. They said do you want our free 235,000 point
multi-check? I said, No, just a battery. He said, It's free. I said
okay. And in less than 5 minutes he found the problem that the dealer
couldn't find in 2 years. In my case it was a bad connection at the
starter motor with the + battery cable. He took it apart and cleaned
it**. I don't know what the problem is with yours but sometimes it
pays to pay someone. Also many people tend to spend all their time on
the big parts and not enough on the wires between them.
**I also bought a new battery, but it's cleaning it that made it work
fine, until months later, I left the headlights on all night. Jumped the
battery and ran the car an hour but it still wouldn't start the next
day. I had to crawl underneath, take the cable off, clean everything
and put it back. Until the next time I left the lights on.
Eventually I learned, even in good clothes, to just stick my arm
underneath, grab the battery cable and pull it back and forth, rotating
on the stater motor stud, once or twice, and then the car would run fine
until the next time I left the lights on. I've run down the battery by
leaving the lights on on other cars without having any endruing
problem. I don't know what made this car different, but my point is that
Sears found the problem in less than 5 minutes.
He gave me the car when he went to Viet Nam, in 1967, but the story
still applies.