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Old 07-06-2016, 03:02   #1
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Isolating alternator/starter

I have a Volvo penta MD22. My alternator is creating a ground circuit in the metal hull of my boat. Looking for possible ideas to isolate the alternator by means of customising it or some solution otherwise. I have more thoughts but will wait for some responses first. Cheers
Niki.
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Old 07-06-2016, 04:54   #2
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Re: Isolating alternator/starter

Balmarhttp://www.balmar.net/wp-content/upl...-60-120-DV.pdf

Ample PowerAmple Power Alternators

Prestolite ➥ http://www.prestolite.com/literature...tor_wiring.pdf
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Old 07-06-2016, 05:16   #3
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Re: Isolating alternator/starter

Try Googling "12 volt doctor's alternator book". Depending on the internal construction of your existing alternator, a competent alternator shop may be able to convert your alternator to an isolated ground alternator.

As an aside this excellent book needs to be updated. Its scope peters out around 1980.
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Old 07-06-2016, 05:30   #4
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Re: Isolating alternator/starter

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Originally Posted by wsmurdoch View Post
Try Googling "12 volt doctor's alternator book"...
Good book; but doesn't directly answer the "conversion" issue.
http://kb-kbh.dk/shipslib/el_ombord/...ndbook_ocr.pdf
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Old 07-06-2016, 22:25   #5
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Re: Isolating alternator/starter

you can buy an isolated case alternator if that is what you mean. then it has a pos and neg post, and no current goes to the engine mount.

just because it has a neg post doesn't mean it is. but some are.


you can also buy an isolated ground starter.
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Old 08-06-2016, 03:51   #6
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Re: Isolating alternator/starter

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you can also buy an isolated ground starter.
Then there are the high coolant temperature and low oil pressure switches and the coolant temperature and oil pressure transmitters that all normally use the engine as their negative terminal that will have to be dealt with...
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Old 09-06-2016, 20:45   #7
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Re: Isolating alternator/starter

Thanks everyone.

It's a tricky one. The boat has been wired up this way long before I owned her but while re-doing and upgrading a lot of the existing wiring I noticed this was the case, that being that the negative is running through the hull of the boat. This is not to say that it is grounded to the engine block by design like when boats use and engine mount as a ground terminal but that it is occurring through the alternator and starting cases. I only noticed as I got a shock, more of a tingle really, when I was tightening a cable on the positive battery terminal (starter battery) while holding the ss frame around the compass mount. Amongst the cruisers with steel boats I know most of them don't know whether or not their battery system is grounded through the engine block, intentionally or otherwise. This along with my research suggests it must be fairly common (not suggesting that makes it ok). I was hoping to find a way to customise my alternator and starter to isolate it from the engine block. If I can't figure out how to do this then I might look at forking out the cash to buy pre designed isolated ones ( although I would prefer to not have too).


Thanks for the help and thanks for the responses. I look into all that information more closely then get back to you all.
Niki
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Old 28-07-2016, 21:02   #8
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Re: Isolating alternator/starter

Quote:
Originally Posted by wsmurdoch View Post
Try Googling "12 volt doctor's alternator book". Depending on the internal construction of your existing alternator, a competent alternator shop may be able to convert your alternator to an isolated ground alternator.

As an aside this excellent book needs to be updated. Its scope peters out around 1980.
I just converted my alternator to an isolated ground alternator, many auto electricians will be able to do it. Not sure how to go about isolating the starter.
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Old 29-07-2016, 02:49   #9
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Re: Isolating alternator/starter

You may find it's easier to isolate the whole engine from the hull. Flexible engine mounts and a flexible coupling between the gearbox and prop shaft is the first step. I had to remount the engine controls on a gasket with plastic sleeves over the retaining studs and nylon washers under the SS and nuts. I also had to put an isolated signal ground system between the ATU and the hull and remove the transceiver ground.
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Old 20-08-2016, 22:46   #10
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Re: Isolating alternator/starter

Someone may have a solution to this. I'm in the process of isolating my engine so electrically it is above earth. I've had the alternator modified so it is isolated, replaced single wire senders/switches with two wire ones, installed a solenoid for the starter, and ran all earth wires what were using the engine for ground directly to the battery. So except momentarily when the engine is starting there is no electrolysis (steel boat). I used Seabis to find several additional engine-related sources of electrolysis which I can fix. Except one.

I have an external regulator (Balmar ARS-5). Rather than run the ignition wire to the alternator directly through the ignition switch (rightly or wrongly, I was concerned about the amount of current), I used a little auto-type relay. Strangely, the ground for the coil circuit in the relay is going to metal. It is showing electrical continuity with the hull as tested with my multimeter, as well as indicating a critical fault on the Seabis.

The relay itself is air mounted with a plastic case. The only thing I can imagine is that it is coming from the positive side of the coil circuit (some kind of reverse current flow) that can be fixed with a diode? Or remove the relay altogether or replace it with some king of isolated relay?

Thanks, Jim

Thanks, Jim
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