Alternator voltage jumping

P

pocket change

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Every now and then the voltage jumps up to 16 volts, then goes back down. Batteries are 4 years old and both read 12.6 volts without the motor running. The alternator puts out 14 volts with the motor running. With the motor running I have jiggled wires looking for a short but it won't do it at the dock, only once in a while when going somewhere. An auto mechanic friend said it is the voltage regulator in the alternator. It's a one wire marine unit. It's less than 2 years old but a mounting bolt came loose and it bounced around for 3 months until I saw it and fixed it. Thanks.
 
Define "bounced around". It couldn't have in the front or else your belt would have slipped off. It must have been in the rear, correct? Depending on the unit (you don't mention the brand of the alternator or even the engine), a bolt could have been in close proximity of the voltage regulator and broke one of the insulators causing an intermittent short. With the battery(ies) switched off, you should check that the insulator around the wire is not cracked and that everything is secure and tight. If the voltage continues to jump, you should probably get it bench tested.
 
The engine is a 3.7 merc 4 cylinder with a Wagner electric alternator conversion kit installed. With the slotted adjustment bolt tight I could twist it about 1/2 inch. The mounting hole was worn oblong. I didn't hear it rattling over engine noise. It's a 1 wire MES alternator with an internal regulater.
 
I would have it bench tested and the housing inspected. It's cheap, if not free (less your time to remove it).
 
just had the same issue, you could see it on the voltmeter gauge and hear the pulsing whine from the alt. Turned out to be the internal regulator. Replaced with the newer style regulator that eliminated the resistor and diode trio .If using a standard battery isolator it does need to be triggered from the starters other small post.
the one that used to supply coil power during cranking
 
I replaced the alternator and the problem went away. Probably was the internal regulator. When I get a chance I'll take the old one to an auto electric shop and have it fixed. I'll keep it for a spare.
 
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