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26-06-2017, 12:18
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#1
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Registered User
Join Date: May 2008
Location: Punta Gorda/Kentucky
Boat: PDQ 32 LRC
Posts: 508
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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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26-06-2017, 12:32
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#2
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Now on the Dark Side: Stink Potter.
Join Date: Aug 2003
Location: Palm Coast, Florida
Boat: Sea Hunt 234 Ultra
Posts: 3,962
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Re: Did I ruin my alternator?
Quote:
Originally Posted by Jerry Woodward
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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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..?
__________________
Life is sexually transmitted
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26-06-2017, 12:34
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#3
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֍֎֍֎֍֎֍֎֍֎
Join Date: Apr 2006
Posts: 15,136
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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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26-06-2017, 20:34
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#4
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Registered User
Join Date: Mar 2009
Location: Southern Chesapeake Bay
Boat: Norseman 430, Jabberwock
Posts: 1,411
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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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26-06-2017, 20:54
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#5
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Registered User
Join Date: Jun 2013
Location: canada
Posts: 4,642
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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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26-06-2017, 21:11
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#6
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Registered User
Join Date: Dec 2015
Location: Nanaimo BC
Boat: modified Spray 56' oa
Posts: 378
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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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27-06-2017, 03:36
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#7
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Registered User
Join Date: Sep 2009
Location: Houston
Boat: ‘01 Catana 401
Posts: 9,626
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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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27-06-2017, 03:38
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#8
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Moderator
Join Date: Mar 2009
Location: Denmark (Winter), Helsinki (Summer); Cruising the Baltic Sea this year!
Boat: Cutter-Rigged Moody 54
Posts: 33,764
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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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27-06-2017, 06:09
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#9
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Registered User
Join Date: Feb 2016
Location: forest city
Boat: no boat any more
Posts: 2,510
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Re: Did I ruin my alternator?
...no charterboat would have a working alternator...none!
__________________
...not all who wander are lost!
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27-06-2017, 07:44
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#10
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Registered User
Join Date: Nov 2015
Location: Fl
Boat: Wauquiez Hood 38
Posts: 1,187
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Re: Did I ruin my alternator?
Your fine. Key doesnt disconnect alternator load (batteries). Sail on.
__________________
Keth
Boat Vinyl Lettering and Graphics
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27-06-2017, 09:46
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#11
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Registered User
Join Date: Dec 2008
Location: SC
Boat: None,build the one shown of glass, had many from 6' to 48'.
Posts: 10,208
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Re: Did I ruin my alternator?
Quote:
Originally Posted by smac999
The key won't do anything.
It's the battery switch that kills them.
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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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27-06-2017, 09:58
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#12
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Registered User
Join Date: Apr 2008
Location: Punta Gorda, Fl
Boat: Endeavourcat Sailcat 44
Posts: 3,173
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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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27-06-2017, 12:28
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#13
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Registered User
Join Date: Aug 2003
Boat: Dragonfly 1000 trimaran
Posts: 7,145
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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.
__________________
The question is not, "Who will let me?"
The question is,"Who is going to stop me?"
Ayn Rand
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27-06-2017, 13:47
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#14
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Registered User
Join Date: May 2008
Location: Punta Gorda/Kentucky
Boat: PDQ 32 LRC
Posts: 508
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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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27-06-2017, 15:04
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#15
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֍֎֍֎֍֎֍֎֍֎
Join Date: Apr 2006
Posts: 15,136
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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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