Try pulling the green wire off the alt while running with the blue and large black still attached. The black and blue wire should have battery voltage. Run a jumper wire to ground and momentarily touch it to the field terminal that you removed the green wire from. The alternator should go full charge, around 16V, if it is ok. Dont do this for long as your battery and accesories wont like the access voltage. The problem will then lie in the voltage regulator circuit. You either have a broken green wire, bad ground at the voltage regulator, no power the blue wire to the voltage regulator or a faulty regulator itself. Hope this helps.