Back in the 80's I was having an issue & traced it to the disconnect along the passenger side valve cover. (earlier models don't have a disconnect there).
The larger Black wire that carries the alternator charge back through the system was burned..At the time I wired around it but have since replaced the terminals.
Look there & also where the Black wire connects at the Bulkhead.. I found mine was corroded at the Bulkhead, cleaned it up & put some dielectric grease on it...So good so far but I'm gonna do the 8-guage wire bypass at some point to take the load off of the ammeter.
On another occasion my headlamps would dim & with the blinker on you could see the ammeter jump to discharge at idle.. I went on a campaign to clean every terminal & ground to body & engine under the hood, today I would use dielectric grease as well.. worked like a charm..
