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Old 11-12-2007, 08:20   #1
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Alternator died. Cause? motor ran w/batt off?

Someone told me that running the motor with the battery cables disconnected will cause the alternator to malfunction.

True of False?

Secondly; If it is "true" then will the same thing happen if the battery master switch is turned "off" while the motor is running?

My alternator was in excellent condition, now has died...

I discovered my "Guest" switched the master switch position in error to "OFF" instead of the requested #2 battery. Could it be related or not?

I just don't want to have the problem re-occur if it is true.
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Old 11-12-2007, 08:35   #2
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That sounds like fried diodes.

The alternators contain semiconductor diodes that convert the alternators AC output to DC. It's not running an alternator disconnected that can destroy the diodes, it's switching a running alternator from connected to disconnected that will do it.

Yes, turning the battery master from "on" to "off" does it.

There are after market "zap stop" devices you can get to prevent this, they run between $20 and $40.

Or you can just not turn off the master switch while the engine is running!

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Old 11-12-2007, 09:05   #3
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As Bill indicates, switching the Battery Selector Switch through “OFF”, with the engine running can fry your Alternator Doides, as will disconnecting the Battery cable (at either end) from the Alternator.

Many Battery Switches also have a pair of Alternator Field Disconnect (AFD) terminals, which help prevent potential alternator damage from inadvertently turning the switch “Off” while the engine is running. Of course, you need access to your alternator field windings for these to help.
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Old 11-12-2007, 09:15   #4
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Gord/Bill Thanks fellows. That confirms my fears.
Now I guess I have to find a rebuilder to fix the alternator.

I have fair fix anything skills EXCEPT almost devoid in 12 volt systems..What are the "ZAP STOP" Devices and where do I get them?

I just know this is going to happen again, if I don't find a way to prevent it.
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Old 11-12-2007, 09:23   #5
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Xantrex and a number of other companies make them. Check at your nearby Chadlery. They should be under $40.
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Old 11-12-2007, 09:26   #6
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Update:

West Marine has them for $30. See:
West Marine: Zap-Stop Alternator Protector Product Display

Cheers,
Bill
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Old 11-12-2007, 09:33   #7
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Awesome help Bill.

Thanks much!
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Old 11-12-2007, 10:25   #8
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You stated the Alt is in excellent condition. If the diodes have failed, you won't have to rebuild the Alt. Just the diode (rectifier) block needs replacing.
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Old 11-12-2007, 11:23   #9
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I am curious if this happens instantly or does it take a few seconds?

I mention this because I believe that wife accidentally shut off switch and I immediately put it back on.. within a few seconds and I believe my alternator still works as it did before. Is that possible?
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Old 11-12-2007, 11:30   #10
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Sometimes you're lucky and sometimes not. I believe that if the diodes are going to go, it's an immediate thing. Doesn't seem to happen every time though.
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Old 11-12-2007, 11:42   #11
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You wouldn't by chance have a Yanmar diesel in her?

A while back the alternator on my port engine just stopped working. When I got around to looking at it I found that it was spinning just fine but the volt meter indicated zero output. Before I ripped it off the engine I picked up the Yanmar engine manual and flipped to the electrical schematics. I found that Yanmar in their infinite wisdom installed the charging indicator lamp in the field circuit. Now if that lamp were to burn out...... yep.... the alternator stops working. It cost me a $1.00 for a replacement lamp at Radio Shack, and it's been fine ever since.

Sometimes, you do get lucky....
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Old 11-12-2007, 11:58   #12
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The reason the diodes fail is because the alternator is normally charging the batteries and the field winding is being supplied current by the voltage regulator to supply the magnetic field for normal operation. If the output of the alternator is disconnected by turning off the battery switch or other means, the regulator will cut back on the current being fed to the field winding. Unfortunately, the magnetic field doesn't collapse immediately. This means the output voltage of the alternator rises significantly. How much it rises depends on the engine rpm and how hard the batteries were charging in the first place. There's no damage past the alternator because its output is disconnected, but the increase in voltage exceeds the maximum reverse polarity rating of the diodes (usually about 25 volts) and they self destruct.

Steve B.
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Old 11-12-2007, 12:47   #13
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Hey all

Wheels, Thanks. As you now are certain....I am underqualified on electrical systems, I will attempt to locate someone who knows to put a diode or 3 in place with out pumping the costs up 10X's for a complete rebuild.


Rick, no I am sorry I do not have a Yanmar, although I would love to replace a $1.00 bulb that would relight my electrical charging system again.


Steve B "You R the strongest link." I am embarrassed because I don't even understand what language you are speaking in...Am I in trouble?


Thanks guys,
I really appreciate the input and I am seeking out local assistance, and will never chance ask someone to touch master switch, without my personal look, again.
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Old 11-12-2007, 14:28   #14
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False!...Running the engine with the alternator disconnected from the battery will not kill the alternator unto itself. What will kill the alternator regulator is any voltage spike caused by the alternators field collapsing when you either connect or disconnect the alternator while the engine is running. Disconnecting the alternator when the engine is running is especially bad!

A field wire with no voltage will not create a field..and therefore no current when the engine is started with the batteries disconnected. In other words, an alternator will do nothing if you run it while it is disconnected from the battery.

You can also kill an alternator by running it with the voltage sensor wire disconnected. The regulator will think the voltage is too low and keep increasing the field (the voltage) until the alternator destroys itself.
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Old 11-12-2007, 14:37   #15
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Quote:
Originally Posted by defjef View Post
I am curious if this happens instantly or does it take a few seconds?

I mention this because I believe that wife accidentally shut off switch and I immediately put it back on.. within a few seconds and I believe my alternator still works as it did before. Is that possible?
Not likely, a complete field collapse causing a voltage spike which then destroys the diodes is almost instantaneous....like a few milliseconds.
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