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Alternator

Put in 3 alternators none seem to charge the battery. Last one was tested and reported a bad diode. I have traced all the wiring and cannot find a short. Alt seems to work for about a day then takes a dump. New battery also (week old). Nissan 2002 4 cyl truck.

You need to hand this job off to a Pro. as something is terribly wrong and you are missing it.

There may be a problem with the main output wire of the alternator to the battery. There should be very little voltage drop across those points with a good load on the system. The trouble could also be with one of the other wires to the alternator. Be sure warning light turns on when you turn the key to ON. Most systems need that light to work since it passes current to the field windings of the alternator.

I would look at the cable that connects the alternator to the battery. If it is heating up when in use, it might need to be replaced.