If you have a bad diode you will get an a/c ripple. I replaced an alternator with a bad diode on a 90 vw golf. I had the auto parts store test it and they said no its good putting out 14 just like it should. I asked them to do it again and see if it had an ac ripple. Busted… Time for a new one.
Here is a good vid on how alternators work if you wanted to know.