Alternator failures

In a recent Car Talk program, a caller wanted to know what caused her alternator to go bad. The Boys gave a lot of good reasons, but left out what, in my 20 years of automotive experience, was the most common cause of diode failure in alternators. Back in the 70’s, when I was in training, one of my diagnostic and electrics instructors warned us to never jump another car with the engine running. This, he said, would strain the charging system beyond its limits, and diodes would blow. I didn’t really believe him because it didn’t make engineering sense. But I remembered his stern warning. On day, when a customer came to me with a bad alternator, I asked him if he had jumped another car recently. “Just last week.” was his reply. I asked if his engine was running when he attempted this service. Again, his answer was affirmative. This got me intrigued and for the next 7 years I asked every customer who came in with a bad alternator the same two questions. Slightly over 60% said yes to both. My informal polling was not in any sense scientific, but does make me think that the instructor was on to something.


Yes, I agree with you and your trainer… I have no “scientific evidence” but have experienced alt. failures on ranch pickups and tractors that were used as “jump start mules.” In fact, the two biggest failure points were: dead battery replacement, followed by alt. death- since alt. was straining to charge a dead battery. And, then, jumpstarting.

Funny, the AC Delco SI alternators could handle abuse like Leece Neville types. Newer Delphi alternators don’t seem as robust…

And Lucas alternators on Ford tractors— heh, don’t ask!