Refer to the gazillion "alternator - Vreg" threads, w/ responses by 67Dart273. You can "full-field" the alternator by disconnecting both spade terminals and applying 12 V to one and ground the other (doesn't matter which). See if it charges full then. If so, get a multimeter and read what field voltage drop your Vreg is applying to those terminals.
I assume you have 2 field terminals (1971+), but only because your name Scamp suggest a mid-70's car. Always tell us when asking a question. It sure matters for this question.
