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Old 15-06-2009, 18:21   #1
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Alternator Drains Batteries - Engine Off

The alternator on my Yanmar 2GM engine is killing my batteries. It may be a generator and not an alternator because it has one small brush visible when I take the back cover off.

What happens is when the engine is off, and the battery selector switch is off as well the alternator gets hot and completely discharges my batteries.

I can't find the source of the current leak. It must be something simple, but I don't know what to look for. Could it be a diode? And if so where I wonder.

Any ideas of where I should start looking, or what to test?

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Old 15-06-2009, 20:38   #2
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I suspect that the regulator (internal or external) is being powered and it in turn is powering the field wire. The regulator may get its power from the ignition switch or from an oil pressure switch. If you have an external regulator, pull the two conductor plug from the back of the alternator (usually has a white and a blue wire) and measure the voltage from the blue wire to ground. If there is a DC voltage there, check the alternator regulator to see why it is being powered. An internal regulator is a bit more difficult to troubleshoot so check what you have and come back with a bit more info.

Hope this helps.

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Old 15-06-2009, 21:04   #3
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Hi vic and welcome to the forum!

Charge the battery, remove the wires from the alternator and see if the problem persists. This will rule out other sources of the current drain.

If your troubleshooting skills are not as up to date as your mechanical skills you could remove the alternator and take it to a shop for testing.

If the alternator checks ok then troubleshooting, with the assistance of a wiring diagram and perhaps a manual is the next step.
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Old 16-06-2009, 06:28   #4
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With everything apparently "Off" lay a screw-driver or other ferrous metal object against the pulley on your alternator. If it's magnetic, the field current has not been shut off and the Regulator is at fault. We had a similar problem with a Xantrex "smart regulator" last year and replaced it with a Balmar AR5 (and keep a second as a spare) and solved our own similar problem.


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Old 16-06-2009, 09:41   #5

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You didn't say how quickly the batteries get discharged, or how hot the alternator. You may have one or more shorted diodes.
Disconnect the fat wire (+) from the alternator while leaving the others connected. If that stops the discharge, you probably have bad diodes in the alternator. If the discharge continues, as previously posted, the field may be
still getting power even though the key switch (bad/miswired?) is off.
Check with a voltmeter on the field winding to see if there's voltage when the key is off.

Steve B.
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Old 23-06-2009, 13:55   #6
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Thanks for all the replies everyone, I finally figured it out. Ended up taking the alternator off and bringing it to a starter and alternator place. They tested it and said that the diodes were blown, and the regulator on the back was the wrong one. Apparently someone put the wrong regulator on. They rebuilt it for $175 with new diodes, and regulator. It's set up to be self exciting so all I need to do is hook up the ground and the positive charge wires. Haven't had a chance to put it back on and test yet. I'm not sure why the diodes blew. There was a lightning strike on a tree near the boat a few weeks ago. Might have done it.

Thanks again,
Vic G.
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Old 23-06-2009, 16:03   #7

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"It's set up to be self exciting so all I need to do is hook up the ground and the positive charge wires." That's typically called a "one wire" alternator, as opposed to a conventional "three wire" alternator.

There are some pros and cons to that.

The good thing, is that it is simple.

The bad thing, is that it is crude and has no way to actually sense the battery voltage, so it can never optimally charge the batteries. Never.

With a conventional 3-wire alternator, one lead is a battery voltage "sense" lead. That goes directly to the battery, so the regulator can tell the battery state. The problem is that if you have dual battery banks, one sense lead can't go to two places. So, either you need to switch the sense lead as you switch banks (not impossible, but adds cost) or you fall back to a "one wire" system that abuses the batteries.

Depending on your installation, you may want to consider converting to 3-wire if you can. Your batteries will thank you for it.
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Old 12-07-2009, 23:02   #8
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Had a similar regulator problem from a lightning near miss about 2 months ago. Very carefully check all your electronic gear. At first glance we had no problems, lights came on, electronics powered up etc. But then we noticed things weren't working properly. Eventually replaced our regulator, stereo, SSB, VHF, Autopilot computer and even a handheld GPS. Thankfully our insurance company were great.
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Old 13-07-2009, 04:31   #9
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Hellosailor - Electrical is not one of my strengths, so please bear with me.

Is it possible to have the sense lead go to the common post on the battery selector swith, so the alternator would sense, and more properly charge the bank that was selected at the time.

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alternator, battery, engine

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