An alternator with a leaky diode bridge will charge normally, or at least have partial output. Try disconnecting the charging lead (the big one "with the nut"). That's the wire that's connected to the diodes. An alternator is a 3-phase AC generator with a rectifier to turn the AC into DC to keep the battery charged. A shorted leg of the stator will cause the alternator to charge at less than 1/3 of its rated output- - - -like 10 or 15 amps from a 65 amp stator. This will keep a battery charged during the day, but it won't support headlights, heater blower, etc. If you can't test the maximum amperage output with an accurate ammeter, you can't test an alternator properly.
Jerry
