Hi Bolens, you really need to give to give some of the clever dudes here a little more information. I am guessing that you have an alternator/generator built by Brush. These are interesting in that they built a generator, a dynamo and a starter motor all into the same unit.
My initial thought is that if it is only pumping out 12 volts, when it should be doing 110/240 volts, you have a wiring problem or are measuring the output from the battery charging circuit.
There are a great many possible issues please provide more information.
Bob