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Old 05-01-2014, 07:57   #1
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Turning off Ignition?

One of the things I was taught early and continue to tell people who have less experience than I do is never turn off the ignition key while the engine is running but not being a mechanic I'm not sure why.


What damage does it cause? Alternator, regulator, batteries; what?
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Old 05-01-2014, 08:11   #2
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Re: Turning off ignition?

Alternator becomes unregulated by loosing ignition feed

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Old 05-01-2014, 09:27   #3
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Re: Turning off ignition?

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Originally Posted by wmcruiser View Post
Alternator becomes unregulated by loosing ignition feed
And then what happens?
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Old 05-01-2014, 09:32   #4
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Re: Turning off ignition?

Nothing happens. When you turn off the ignition the regulator stops sending current to the field winding and the alternator stops putting out current. No damage though.

You are probably thinking about another alternator no no: never turn the 1,2,all,off switch to off while the engine is running. That leaves the alternator with suddenly no where to put its juice and the voltage spikes and blows the alternator diodes. A Zap Stop diode will usually protect from that happening however.

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Old 05-01-2014, 09:45   #5
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Re: Turning off ignition?

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Nothing happens. When you turn off the ignition the regulator stops sending current to the field winding and the alternator stops putting out current. No damage though.
David
The school/charter company made a big deal about it when I was starting out I assumed it burns something up otherwise why emphasize it?

Better safe than sorry I guess.
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Old 05-01-2014, 09:56   #6
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I imagine, on most boats, oil pressure alarms and such would cease to function as well.
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Old 05-01-2014, 10:02   #7
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Re: Turning off ignition?

This all depends on how the alternator is wired. On our boat, all that happens is the power to the alt regulator is cut, rendering no output. On some small OEM installations (not too common) the alternator output ran through the ignition switch, turning off this switch would then cause the alternator to see an open in the path to the batteries, and as a result the rectifier diodes would burn out. IMHO this was a really dumb way to wire the output anyway......
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Old 05-01-2014, 10:02   #8
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Re: Turning off ignition?

Not a problem.

This is one very good way to force the external regulator back into Boost mode when it has prematurely dropped down to Float mode. Try this as soon as the regulator has fallen to Float and see the difference in charging current when the voltage goes from 13.2v to 14.4v.
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Old 05-01-2014, 10:21   #9
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Re: Turning off ignition?

Is it possible turning off the ignition and motoring for an extended period of time can burn up the alternator (bad) or burn up the regulator which then results in overcharging and damaging the batteries and then burning up the alternator as well (worse)?
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Old 05-01-2014, 10:26   #10
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Quote:
Originally Posted by OldFrog75 View Post
Is it possible turning off the ignition and motoring for an extended period of time can burn up the alternator (bad) or burn up the regulator which then results in overcharging and damaging the batteries and then burning up the alternator as well (worse)?
Again depends on how the boat is wired, but normally all that happens to the alt is that it is turned off. I could motor for days with the ignition switch off and suffer no ill effects.
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Old 05-01-2014, 10:41   #11
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Re: Turning off ignition?

On most boats when you turn off the ignition you also turn off the oil pressure sender and the temperature sender too. Not a big deal for a short while and if you are careful but under some conditions could ruin the engine.
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Old 05-01-2014, 10:54   #12
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Re: Turning off ignition?

On some boats, the stop solenoid is energized by the ignition switch, so it won't work if you turn off the ignition. Otherwise its no big deal.
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Old 05-01-2014, 12:06   #13
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Re: Turning off ignition?

Guess i am just curious, why would you WANT to shut off the key only? What would be the benefit to this? Sounds and seems like way too many risks associated...
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Old 05-01-2014, 12:25   #14
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Re: Turning off ignition?

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Guess i am just curious, why would you WANT to shut off the key only? What would be the benefit to this? Sounds and seems like way too many risks associated...
Read my post #8.
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Old 05-01-2014, 12:33   #15
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Re: Turning off ignition?

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Not a problem.

This is one very good way to force the external regulator back into Boost mode when it has prematurely dropped down to Float mode. Try this as soon as the regulator has fallen to Float and see the difference in charging current when the voltage goes from 13.2v to 14.4v.
This is probably regulator brand/model dependent. On our two different brand/model regulators, if the batteries were in float and the regulators turned on and off, both go to 14.8V, but drop back down into float within a minute or two, and put in almost no current in that short time before float.

What would make a regulator prematurely drop into float? Another current source at a higher set voltage than the regulator?

Mark
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