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Old 19-03-2014, 17:28   #1
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12 volt Blues

Marine electrician says my 1 month old alternator is good if the tach is working.
It is working, but at 5 knots where it should be reading 2000 RPMS, it goes all over the place (200, 500, 1000 RPM's and not necessarily in that order).
It will remain at one for 15 minutes and the next one for an hour, for example.
Also, I can't get the batteries to charge unless the solar panels are doing it.
(With the engine OFF at anchor, the batteries voltage will go up quite a bit.)
(Motoring, or charging at idle for 1/2 hour, they won't move enough to notice.
"Sounds like a personal problem", I know.
The batteries are all new and their voltage checks out with the house meter when compared to my digital tester.
Anyone have an idea.

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Old 19-03-2014, 18:22   #2
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Re: 12 volt Blues

Output side alternator fittings- are they properly crimped on proper wire? Is there a corroded nick in the wire from the alternator forward? Have you looked at every inch of the wire casing that's carrying current from the alt to batteries?
Seems the tach is screwy, but unrelated. Unless an alt wire is contacting the engine and messing with the tachometer magnets.
Either way, my shot is a bare wire, wire broken in a casing, or poor contact of a wire at a terminal.

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Old 28-03-2014, 23:06   #3
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Re: 12 volt Blues

If you bought your 30 day old alternator because the old one wasn't charging, and if the new one isn't charging, the problem is probably somewhere else. Probably wiring or a ground. Or if the alternator has a outside voltage regulator, that could be it. Most better auto parts stores will check an alternator or voltage regulator for free. If they test ok, it's somewhere else.

Also, the voltage at the battery when running should be about 14.5 volts. You might hook up a meter and go thru your tack experiences again and see what voltage readings change.

Knowing nothing about your tack or engine type, variable readings could be caused by low voltage. Gas engine tacks usually count distributor spark and most newer diesels have a small generator like device that sends signals to the tack. Both need reliable voltage.

Corrosion on any cables or wire ends are common on boats. I would check both ends of battery cables by removing them at the posts and where they attach to the engine and starter. Clean the ends and their contact point.

A problem I had when a alternator tested ok, was corrosion traveling down the main cables. Somehow the starter could blow thru the corrosion, but the resistance was enough to stop the charging current. When I cut open the cables there was the green corrosion you usually see at the terminals.

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Old 29-03-2014, 04:02   #4

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Re: 12 volt Blues

I would recommend doing a voltage drop test on the charging path. I found a connection that looked good on my starting circuit that was causing a 2.5 volt current drop and slow cranking. Heres instructions...

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Old 29-03-2014, 04:46   #5
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Re: 12 volt Blues

Everybody that posted a reply has the same answer because that's what's going on.... Inspect every connection, wire, cable from source to load... Something's loose, green, blackened, melted, undersized.... forsail posted a quick way to discovery...
In the harsh marine environment, something is always in need of repair...

Mai Tai's fix everything...
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Old 29-03-2014, 22:18   #6
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Re: 12 volt Blues

First, have you taken a reading right at the alt while it is running to verify you are getting nothing out? Also are there other wires lose that should be hooked to the alternator like your reference volt wire? If there's only one wire attached the diodes could of fried for going to full field and poof. If you are getting around 14 volts then work your way from there and see where it cuts out. It's harder to work from the end of the line to the beginning.

Going just from the tach I would not even consider that.

wondering around with no destionation
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Old 03-05-2014, 09:00   #7
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Re: 12 volt Blues

Just to amplify what others have said: You have an impaired electric path somewhere. It is most commonly on the ground side. It the alternator does not have a separate ground wire, install one, using the same size wire as the output. clean all contacts to and from the alternator, and major grounds, such as the engine block or a common buss. Examine all wires for continuity...a simple ohm meter will work fine. If someone used auto grade wire somewhere in the system, it may have corroded internally, but very likely at a terminal or two. Give a light tug to each wire where it goes into a crimp connector. If your current situation continues, you will soon blow the diodes in the alternator'

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