I'm afraid to say, but I'd guess you are on the wrong track. I'd say, offhand, that someone has done some sort of ammeter bypass.
What you need to do is get the engine running at a good "fast idle" and check battery voltage at the alternator output stud and at the battery. if it's the same at both points, and low, below 12V you are not charging.
What do you have for an alternator / regulator?
A.........Old style, 69/ earlier with one green field wire?
B.........70/ later style with two field wire connections at the alternator?
C........Some swap or aftermarket alternator?
