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Why didn't I blow my alternator??

 
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smckean (Tosca)



Joined: 18 Jan 2014
Posts: 974
City/Region: Guemes Island (Anacortes)
State or Province: WA
C-Dory Year: 2005
C-Dory Model: 25 Cruiser
Vessel Name: Tosca
Photos: Tosca
PostPosted: Tue Jul 06, 2021 3:10 pm    Post subject: Why didn't I blow my alternator?? Reply with quote

I've been analyzing the AC and DC configurations on my boat because I'm pretty sure I have some wiring errors (mostly surrounding how the batteries are configured and charged). Yesterday, I did perhaps a foolish thing. Can anyone explain it?

While underway with both the starter and house battery switches on, and my eye on voltage read-outs for both battery banks, I got the bright idea (not!) to try an experiment. I wanted to test whether the installed ACR/VSR unit combined the batteries regardless of the state of charge of either battery. Somehow I convinced myself that I would learn something if I switched off the start battery. Surprise!! As soon as I set the starter switch to "off", the engine instantly quit. It dawned on me that I may have just killed my alternator in the OB since I've heard that, on inboard engines, if you disconnect the starter battery, even for a fraction of a second, you permanently destroy your alternator. Thankfully, I turned the starter battery switch to "on", re-started the engine, and voila the alternator was fine and supplied normal charging voltage to all the appropriate places.

So my question to this august group is: was I just lucky; or do OBs have some sort of protection system that detects this situation and cuts the engine before the alternator blows; or is it false that one can blow an alternator so easily? Also, unless the 2nd option is true (which would explain it), why did my engine quit when I switched off the start battery?

P.S. Replacing an alternator on a standard engine isn't normally very hard or expensive, but I assume it could be very expensive on an OB. Anyone know?

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Purchased Tosca in 2014
Re-powered to Yammi 200 in 2015
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T.R. Bauer



Joined: 17 Nov 2007
Posts: 1726
City/Region: Wasilla
State or Province: AK
C-Dory Year: 1993
C-Dory Model: 22 Cruiser
Vessel Name: C-Whisperer
PostPosted: Tue Jul 06, 2021 3:50 pm    Post subject: Reply with quote

I do - yeah they are expensive! I'm glad nothing bad happened, but I think all you probably did is just cut the power off to everything - hence the engine quitting. And since there was no feedback loop from the alternator to power itself because they probably intentionally wired things in the outboard that way, it didn't have power and quit running. No idea if any of that makes any sense.....probably have to look at your outboard's wiring schematic to figure it out for sure.
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thataway



Joined: 02 Nov 2003
Posts: 20778
City/Region: Pensacola
State or Province: FL
C-Dory Year: 2007
C-Dory Model: 25 Cruiser
Vessel Name: thataway
Photos: Thataway
PostPosted: Tue Jul 06, 2021 5:40 pm    Post subject: Reply with quote

As TR Bauer said. By cutting the engine at the same time you disconnected the alternator, you avoided damage.

Your engine stopped, because the computer on modern engines need 12 volts. The only way they get it is thru the battery. The modern 4 strokes will not run without a battery.

You proved that when you disconnect the engine from a 12 volt power source, it stops the engine. There is no "internal" feedback, as you might find with a 2 stroke, which will run without a battery.

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Thataway
Thataway (Ex Seaweed) 2007 25 C Dory May 2018 to Oct. 2021
Thisaway 2006 22' CDory November 2011 to May 2018
Caracal 18 140 Suzuki 2007 to present
Thataway TomCat 255 150 Suzukis June 2006 thru August 2011
C Pelican; 1992, 22 Cruiser, 2002 thru 2006
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smckean (Tosca)



Joined: 18 Jan 2014
Posts: 974
City/Region: Guemes Island (Anacortes)
State or Province: WA
C-Dory Year: 2005
C-Dory Model: 25 Cruiser
Vessel Name: Tosca
Photos: Tosca
PostPosted: Wed Jul 07, 2021 3:10 pm    Post subject: Reply with quote

OK, I get it now. When I turned off the start battery via the switch, I basically did the same thing as tuning off the ignition key.....so wonder the engine quit Embarassed.
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