Just to clarify for other readers,
Whether you have an early (1 field term) or later (2 field term) alternator, if you apply 12 V across the field, you should get full output from the alternator, or at least as much as it puts out at idle speed.
You can manually apply 12 V across the field by jumpering 12 V from BATT+ to one terminal and grounding the other terminal. Doesn't matter which one, but disconnect your factory field wires first. In early ones, the grounding is already done because one brush bolts to the frame without a ceramic insulator. Use a multimeter to verify that you are truly applying 12 V between the 2 brushes. If not, your "ground brush" may not be getting grounded thru the frame, due to corrosion.
With above, if your alternator does not output strongly, either it is bad or it is not getting grounded well thru the frame to engine connection (or bad engine block to BATT- connection). A bad alternator is usually either from no field current or blown output diodes. The field wiring can be checked with an ohmmeter and could be simply worn-out brushes.
Once you get it to output manually, worry about how the Vreg does the same thing, but varying the field voltage as needed to control the output it senses. Many things can go wrong there, most of which are due to your car's wiring, not the Vreg itself.
