Mike: Your diodes are fried.
Please do not run the alternator without the battery in the circuit, it will, if working (and your isn't) go way overvoltage and fry things. Not good. This usually finishes off weak diodes, by the way.
12.1V will not charge the battery, you must get at least 12.5, preferably 13 at idle, and should get 14-14.5 at 1500 rpm. anything less, the alternator isn't putting out enough voltage (let alone amperage!).
Peter
