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Old 26-06-2017, 12:18   #1
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Did I ruin my alternator?

I accidentally turned the key off while my Yanmar 2gm20 was running (I have two of them on my cat and turned the wrong key off). I kept it running in neutral and I increased the RPM to about 2000 for 5 min or so before I noticed the key was off, yikes! I shut down the engine, turned the key on and it energized and the engine started just fine.

What is the likelihood that I ruined my alternator? What is the best way to check to see if it is charging ok? If so, is it an easy fix, i.e. replacing diodes, or am I looking at a new alternator?
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Old 26-06-2017, 12:32   #2
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Re: Did I ruin my alternator?

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Originally Posted by Jerry Woodward View Post
I accidentally turned the key off while my Yanmar 2gm20 was running (I have two of them on my cat and turned the wrong key off). I kept it running in neutral and I increased the RPM to about 2000 for 5 min or so before I noticed the key was off, yikes! I shut down the engine, turned the key on and it energized and the engine started just fine.

What is the likelihood that I ruined my alternator? What is the best way to check to see if it is charging ok? If so, is it an easy fix, i.e. replacing diodes, or am I looking at a new alternator?
Put a load on all circuits, crank up and see if it is charging.
Back in the day it was a death sentence to run them like you did, I would think modern stuff have some kind of built in protection..?
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Old 26-06-2017, 12:34   #3
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Re: Did I ruin my alternator?

Only way to find out is to restart the engine and see what the output voltage is. If it is 14.4 DC and shows 0 on any AC measurements, it's OK. Diodes may be weaker now, but it is working.

If you pick up any AC on the output, it is no good. Also, if you disconnect the battery and use an ohmmeter across the alternator output to ground, you should see near zero ohms in one polarity, and near infinite in the other. If you don't, then at least one diode has blown. (Alternators can work in a dregraded state with just one or two diodes blown.)

Don't have a multimeter? OK, good time to spend $20 and buy a basic one. (Or over a hundred for a basic Fluke, a much better instrument.)

If that's too complicated...off it goes to any auto chain, where they'll bench test it for free. Try to get "the old guy" to test it when things are slow, because sometimes the newbs blow them out by improperly testing them. (Yeah.)
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Old 26-06-2017, 20:34   #4
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Re: Did I ruin my alternator?

AFAIK, that isn't what destroys the diodes.

I thought it was disconnecting the load--the battery and equipment on-- from the alternator while the alternator is pumping out electrons that does the damage.
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Old 26-06-2017, 20:54   #5
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Re: Did I ruin my alternator?

The key won't do anything.

It's the battery switch that kills them.
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Old 26-06-2017, 21:11   #6
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Re: Did I ruin my alternator?

Key off... no output.no blowing diodes . Key on running ,output ,disconnect battery. output blows diodes.
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Old 27-06-2017, 03:36   #7
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Re: Did I ruin my alternator?

Output current doesn't go through the key switch. No harm!!
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Old 27-06-2017, 03:38   #8
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Re: Did I ruin my alternator?

If I had a dollar for every time I'd done this . . .


Never hurt my alternator. Doubt if it hurt yours. Don't worry; be happy!
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Old 27-06-2017, 06:09   #9
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Re: Did I ruin my alternator?

...no charterboat would have a working alternator...none!
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Old 27-06-2017, 07:44   #10
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Re: Did I ruin my alternator?

Your fine. Key doesnt disconnect alternator load (batteries). Sail on.
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Old 27-06-2017, 09:46   #11
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Re: Did I ruin my alternator?

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The key won't do anything.

It's the battery switch that kills them.
That was going to be my question. Why should the switch being off hurt anything? The alternator current doesn't go through it to charge the battery.
All the key is for is to provide juice to the starter and maybe accessories.
I wouldn't worry about it.
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Old 27-06-2017, 09:58   #12
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Re: Did I ruin my alternator?

The switch only kills the instruments and idiot lights, no harm done!
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Old 27-06-2017, 12:28   #13
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Re: Did I ruin my alternator?

The key also kills regulator power, so alternator goes to sleep.
No harm, no foul.
Just don't turn off the battery switch when there's an electrical load, the engine's running with the key on.
As said previously, that's what blows up alternator diodes as the output voltage will spike before the magnetic field in the alternator has sufficiently collapsed.
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Old 27-06-2017, 13:47   #14
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Re: Did I ruin my alternator?

Thanks everyone, I feel better now. Literally every boat I've ever chartered, the checkout includes a lecture to ALWAYS shut the diesel down by first cutting the fuel (push the button or pull the lever), then turn the key off, with dire warnings that I will destroy the alternator if I turn the key off first. I guess this is one of those instances where, if it's said enough times, it must be true!
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Old 27-06-2017, 15:04   #15
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Re: Did I ruin my alternator?

Jerry-
In theory, shutting the key first shouldn't hurt anything. But consider this: If some PO did some oddball wiring...By shutting the engine first, and stopping all alternator motion first, and only then shutting off the power? What can that hurt? Nothing.
I don't remember why we were all advised to do it this way, just that there was some reason for it. Can't hurt to check the diodes anyway, five minutes with the multimeter.
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