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I need some serious help. I did a 3g alternator conversion on my '91 F-150 5.0 following these instructions. Now, 2 weeks and around 400 miles later, the thing is shot (and this is not the first time). The problem is, the shaft moves forward and back, causing internals to hit the case. I do not understand why this keeps happening. Is it the alternators, or is it me? I really would love to know. Any help will be appreciated, wether it's good or bad.
Sounds like you have a mechanical problem. So I would double check belt tension, and pulley alignment. If the alternator is not sitting perfectly square with the drive belt, it could put a side load on the alternator shaft.
I figured it out. Took the alternator to an alternator and starter shop only to find out there was absolutley nothing wrong with it. I came home, put it back in and drove it until the noise started again. Used a metal pipe to listen for sound, and sure enough it was not the alternator, but the tensioner. They are bolted directly together, so the sound seemed to be coming from the alt. Anyways, I also noticed that my belt was a bit long and the noise was from the tensioner bottoming out. Got a 1" shorter belt and haven't heard the sound since. Man do I feel dumb.