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Message: <blockquote data-quote="UnexpectedBill" data-source="post: 123000" data-attributes="member: 21648">Back in the 80's I worked at a shop rebuilding alternators and starters to support myself. Do not automatically believe that just because the alt makes charging current that it can't have a bad diode. I saw plenty of times an alt will charge the battery when engine is running, but leak current in the reverse direction when engine is off.  Most places just check output, not for leaky diodes, so it will test good, but still drain the battery. A diode can be bad w/out being completely shorted to ground. If you don't have a VOM, a simple test is to just disconnect the red output wire from the alternator. t's hot from the battery so be careful and tape it w/electric tape to prevent it touching anything. If the battery quits going dead, that's it.</blockquote>