Originally Posted by Wattapunk
Another crude way of checking it would be disconnect the battery negative cable with engine running. Bad alternator would kill the engine.
That isn't true with the TJ's alternator which requires 12 volts from the battery for it to work. Disconnecting the battery will always kill the engine on most modern engines, not many vehicles still use the type of alternator that can run without a battery. GM is one except and there are others but...
In fact, if you have a completely dead battery and jump start any modern Jeep's engine, the engine will die again after you disconnect the jump start battery. Same thing is true with my daily-driver Toyota whose battery gives me fits.