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Old 09-05-2013, 01:57 PM
jfw432 jfw432 is offline
TT Enthusiast
Join Date: Apr 2012
Boat: other
Location: Lizella
Posts: 165
Alternator acting up

Had to call a friend to bring me a new battery on the lake over the weekend because my alternator wasn't charging the battery. My boat is an 86 with a 351w engine. Put a voltmeter on my friends battery when I got back to the dock and it read 12.8v. Then started the boat up and it settled at 12.8v at idle. Revved the engine to about 3k rpms and it still read 12.8v so I assumed the alternator was shot since the belts were tight. I also tried wiggling all the wires going to and from the alternator while watching the voltmeter but there were no jumps in the voltage.

Since I would likely have to order an alternator, I decided to crack the old one open and see if there were any problems that I could fix quickly and easily. I cleaned everything up. The brushes and commutator looked perfect, all wire connections were solid, and the bearings spun freely. After a bit of degreasing, it looked better inside than outside but I never saw anything wrong with it. Decided to bench test it with an impact gun and was able to get 14.2v from the alternator. I don't know equivalent rpms on the alternator with the impact gun but it appears to work just fine.

Reinstalled the alternator in the boat and the most voltage I get on the boat is 13.2v. There is no difference between idle and 4k rpms.

I was thinking of replacing all the wires going to the alternator but can you guys think of anything else that could be causing problems? Other than perfect pass, nothing else electrical is running on the boat.
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