Generator to alternator conversion

Well, I may aswell start the ball rolling with this one!

I recently swapped 2 engines over on a friends bug, he bought a tatty '76 bug that had the benefit of a running engine, the only difference with the engines was the car we were putting it in previously had a dynamo and the engine going in had a Bosch alternator. After swapping the engines I hooked up the Alternator without bypassing any regulators etc, using the original wiring and the engine ran fine. The only problem that I came across was the ignition light stayed on but I put that down to the amps that were being lost through the unnescassary regulator. He has since driven the car for over 2 hours back to uni and its still firing fine/ charging the battery, giving the impression there is no rush to bypass the regulator (Im sure he will do it at some stage but he is a student so cant expect that any time soon!).

If anyone could give the definitive reason for the ignition light staying on in this instance that would be awesome.