I recently had an alternator problem, but all the guages were glowing slightly and the voltmeter was much lower than normal, but was still charging enough to drive it.

I first tried replacing the regulator/brush assembly, since my alternator had at some point been upgraded to the later internal regulator type, but the brushes had worn such deep grooves in the commutator that the new brushes wouldn't make adequate contact and I had the same problem.

So I took the regulator back to Autozone and got a new alternator for $72 and change. It has a lifetime warranty, which is a good thing, because cheap rebuilt items are not subject to great quality control. I ended up taking the new alternator back a month later when it started failing for a free replacement.

Fortunately, it's an easy swap. If you haven't upgraded to a newer alternator with internal regulator, I would recommend it. I just asked for an alternator for an '84 240 (mine's an '81.)