Anything under 13 volts is a bad alternator charge output (Notice I did not say "a bad alternator"). First thing you should do is look for any disconnected/loose wires on the alternator. Second may be replace the voltage regulater (brushes) with This . If that does not fix your problem post back here.

(I just noticed the Bosch regulator is now $45! I'm glad I got mine when it was $18 )