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Old 06-04-2016, 16:43   #1
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Alternator warm after a month gone.

Recently returned from a month off the boat. When I went to start my deisel engine the alternator was warm to the touch. My batteries have not been performing was well as they should and now I'm wondering if some of the energy I'm using is not going the alternator in some fashion. Any ideas why power is returning to my alternator? Thanks
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Old 06-04-2016, 16:49   #2
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Re: Alternator warm after a month gone.

If a problem has developed with the regulator allowing the alternator field to stay energized, even when off, then the alternator might feel warm and it might also drain the battery bank.

? Make of alternator and regulator.
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Old 06-04-2016, 17:27   #3
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Re: Alternator warm after a month gone.

Sounds like the exciter circuit is active, faulty circuit? If so it's amazing that your battery bank and alternator have held over the month
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Old 06-04-2016, 17:35   #4
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Re: Alternator warm after a month gone.

Likely that current is flowing thru the alternator from batteries or another source (hopefullly DC so not dangerous). I was recently aboard a boat where the alternator was hot (HOT not just warm) to the touch while at the dock for several days, but not my problem to diagnose fortunately.

I think most likely its a path to ground via the alternator from the batteries. I would start with the most likely path (whichever bank it is wired to directly) and check for current flow when engine is off.

Check main battery lead and field wire as suggested too.
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Old 06-04-2016, 17:47   #5
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Re: Alternator warm after a month gone.

The two options are pretty much field current and backfeed through the (failed) diode bridge.

Typical full field current at 12V is 5A, so you have a 60W heater if the field current is on. Most likely source for field current when not in operation is a failed ignition switch, field current generally passes through the ignition system only after the engine starts, but a failure in the switch leads to field current all the time. If the battery voltage is below 13V then the regulator has no reason to limit the field current and will let it run forever (unless you have a fancy regulator with some special functions).

The other option is backfeed from the positive connection through one or more failed diodes in the rectifier bridge. Since diodes have an unknown resistance after failure this could be a small current or a large one.

If you don't have a current meter, do you have a battery switch that disconnects alternator positive from batteries? Assuming everything shut down, you can open the switch and see if that kills the heat, in which case likely bad diodes. On the other side, disconnect the field wire (which may be hot) and test the same way. Or test by voltage, field wire should not have voltage with ignition off, if it does something is not right.
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Old 07-04-2016, 09:03   #6
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Re: Alternator warm after a month gone.

Is your alternator internally or externally regulated? Is the regulator getting power when the ignition is off? If yes you are turning the rotor into a little space heater. If it was a failed diode in the rectifier you might hear a howling or singing when running and it probably would not produce power.No smiley face, Just good business.
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Old 07-04-2016, 09:36   #7
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Re: Alternator warm after a month gone.

Maybe? something about a solar panel in the circuit even with batteries on off
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Old 07-04-2016, 12:35   #8
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Re: Alternator warm after a month gone.

Quote:
Originally Posted by Steve Mason View Post
Recently returned from a month off the boat. When I went to start my diesel engine the alternator was warm to the touch. My batteries have not been performing was well as they should and now I'm wondering if some of the energy I'm using is not going the alternator in some fashion. Any ideas why power is returning to my alternator? Thanks

I had exactly that issue just a few weeks ago. It turned out to be a faulty voltage regulator (internal).

Other symptoms were the alternator becoming hot to the touch, volt meter on occasion read 15+ volts.And of course the batteries discharging when turned off.

Replacing the voltage regulator was ridiculously expensive so I bought a new alternator for only a $100 more.
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Old 07-04-2016, 15:46   #9
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Re: Alternator warm after a month gone.

Diodes in rectifier!
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Old 07-04-2016, 23:03   #10
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Re: Alternator warm after a month gone.

Thanks for all the great advice and insight. Seems it is bad diodes I determined from the help above. Thanks again.
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