I'm just talking off the top of my head here, but I think csf is on the right track. On older alternator systems with external regulators, and some of the early internally regulated alternators, there is a wire about 18 gauge that carries "exciting" current from the ignition switch to the regulator and/or the alternator. This 12 volt source "excites" the windings in the regulator and/or alternator, allowing the alternator to develop full output voltage. If this wire is bad, eg: bad connection, broken, etc., the alternator will not put out full voltage. Get a good wiring diagram for your specific year and model, and take a good look at the system. Or, find an "OLD" guy mechanic who has seen this sort of thing in his experience; a lot of guys in the trade who are under thirty, probably don't know much about this system, because it was old tech by the time they started working on cars.