If you disconnected your voltmeter and the problem still exists, that's not your problem.
If the Ammeter wires are cleanly connected to each other and insulated, that's not you problem either.
You said that you took the belt off................. @Dana67Dart suggested removing and insulating the field terminals. This would electrically disconnect the alt from the system. The alt would still be fed with the belt off (sorry, just spitballing)