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Can anyone tell me from the picture below of why the connection between the Alternator and the battery gets very hot and melts the insulation? Also I see small sparks come out from the connection and get larger as I rev the engine.

 

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The stud appears to be off center as if the insulator, the red item, is broken.
 

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Looks like the insulator is broken which could make the stud to touch metal on the alternator which would cause the heat. It might be slightly touching the metal. Also bad crimp on the terminal will cause excessive heating.

Take the alternator apart and see how it looks around the insulator.
 

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Could have been a loose connection where it was arcing and the additional resistance from that caused heating of the connection.. Maybe?

If it were a short curcuit, the entire loom would have melted, not just the connection.
 

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i see lots of corrosion on the wire, which causes resistance, which causes heat, which causes the plastic insulator bushing (which was probably already weak from being torqued on too much and several thousand heat cycles over the years) to give way and allow the stud to make contact with the alternator case.
the fix is a new plastic bushing in the alternator and a new heavy gauge wire from the alt to the + terminal on the battery, skipping the oem wiring from the alt to the horn relay altogether.
 

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Like the guys say above..bad crimps corroded terminals causing excess resistance /heat
Also a bad diode, warn bushes or stuffed bearings in the alternator will cause the same thing.
 

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It probably started as a bad connection in the crimp or between the nuts. The bad connection caused it to get hot. After prolonged heating, the insulating washer melted through and let the stud contact the case. As it arced over the stud got hot enough to melt everything and a scorch the metal stud.

Disconnect the negative ground cable immediately to prevent a fire. You may be able to rebuild the alternator. When you hook everything back up, make sure the metal is bright and clean. Put a dob of dielectric grease on it to keep the oxidation at bay. Those rubber boots they originally came with are also a good idea since they not only protect a high current power source from accidental shorts, they also keep the weather out.
 
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