Ignition off, diesel still running, Bad? - SailNet Community

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Go Back   SailNet Community > On Board > Gear & Maintenance > Electrical Systems
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Old 03-23-2011
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Ignition off, diesel still running, Bad?

I've always been under the impression that turning the key off while the diesel is still running will fry the alternator. Recently we accidentally did just that. I think the motor had been running for quite a while with the key off when I finally discovered it. Tested the alternator and it seems fine. According to my schematic four wires leave the alternator. One wire leaves the alternator then goes to the battery selector switch, the key switch, the push button starter switch, then to the starter; one to the temp light/switch; one to the charge lamp/switch and one to ground. I don't see a shunt on the alternator, as Casey suggests adding. It appears to me that energy goes to the batteries as long as the battery switch is closed, and that the key only allows power to get to the push button switch for starting. After that, that circuit is open anyway, except for the battery switch. Seems like I could take the key out after starting. Is that correct? Is the shunt a good idea in case the battery switch gets turned off accidentally?

Last edited by L124C; 03-23-2011 at 01:20 PM.
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Old 03-23-2011
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It's probably OK.

What will fry the alternator diodes is interrupting the output circuit between the alternator and the batteries.

Rather than a shunt, I'd consider first some better onboard discipline and maybe a sign, like "DO NOT TURN IGNITION SWITCH OFF WHILE ENGINE IS RUNNING" :-)

Bill
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Old 03-23-2011
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Whether this will fry the alternator depends on the alternator's internal circuitry, and the boat's circuit wiring.

Typically, if the voltage sense lead (field lead) is disconnected, an alternator's regulator will say "Oh my, there's no output, I must work harder!" and immediately go to maximum output power. If there's no load attached, it will burn out in a very short time.

On newer "smarter" alternators, they are designed to protect themselves from this kind of failure, and they'll say "Oh my, I must work harder! But no harder than this" and they'll shut down if necessary to prevent overheating. How to find out? Contact whoever makes your alternator, and ask them directly.

The mixed blessing is that sometimes the protection components themselves can fail, so the smarter alternators are (brely) more expensive and arrguably less reliable since there's more stuff to fail in them. In reality...modern equipment uses regulators with what is called a "heroic" mode that protects against failures of all sorts. Boats don't always have "modern" in them though.

So you got lucky, either your boat or your alternator/regulator was smart enough to protect it from failure. In general, the rule is to never leave the alternator running while disconnected, because MOST of them will indeed burn out very quickly in that situation.

There's probably some simple way to alarm that condition, like installing an audible low oil pressure alarm on the engine. Engine is off, oil pressure is zero, alarm will ring if the power circuits to it are still active. Or, like they say in Ghostbusters, "DON'T TOUCH THAT SWITCH!" (VBG).
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Old 03-23-2011
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Not really.

The reason the alternator didn't burn out was simple, and you're right L124C: the alternator output (AO) still was running through the 1-2-B switch and was not interrupted.

The very best thing you can do is to remove the AO from the C post of the 1-2-B switch and relocate it to your house bank.

Many discussion links, some back to this forum, can be found here: Electrical Systems 101
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Old 03-23-2011
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Quote:
Originally Posted by btrayfors View Post
It's probably OK.
What will fry the alternator diodes is interrupting the output circuit between the alternator and the batteries.
Rather than a shunt, I'd consider first some better onboard discipline and maybe a sign, like "DO NOT TURN IGNITION SWITCH OFF WHILE ENGINE IS RUNNING" :-)
Bill
Believe me, I've tried! Every time before we leave I show everybody how to operate the motor (in case something happens to me) , and stress not to turn the key off until they have pulled the kill handle and hear the low oil pressure alarm (indicting the motor has stopped). I have someone steer the boat, and someone else run the motor (both, under my guidance) when we leave the slip. Despite this...I actually had a guy hand me the key while the motor was running, thinking he was helping! The key is in a bad place anyway, in a tight cockpit. It gets bent and this last time someone inadvertently turned it off with their foot! So, if I can simply take the key out while the motor is running, it makes life easier altogether. If I'm reading the schematic correctly, it doesn't make a bit of difference, as the Battery's and battery switch T off to the left, and the ignition switch, starter switch T off to the right. the right side of the circuit is open anyway unless I'm pressing the start button.

Last edited by L124C; 03-23-2011 at 05:21 PM.
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Old 03-23-2011
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Quote:
Originally Posted by Stu Jackson View Post
Not really.

The reason the alternator didn't burn out was simple, and you're right L124C: the alternator output (AO) still was running through the 1-2-B switch and was not interrupted.

The very best thing you can do is to remove the AO from the C post of the 1-2-B switch and relocate it to your house bank.

Many discussion links, some back to this forum, can be found here: Electrical Systems 101
Thanks, for the link. I'll check it out. In the meantime, I can take the key out while the motor is running and the system doesn't know the difference. Correct?
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Quote:
Originally Posted by hellosailor View Post
Typically, if the voltage sense lead (field lead) is disconnected, an alternator's regulator will say "Oh my, there's no output, I must work harder!" and immediately go to maximum output power.
The voltage sense lead and the field lead are two entirely different things. Disconnect the field lead and the alternator cannot put out anything. Removing the field with alternator output will do no harm either. In fact, that's what your regulator is doing. That 2-3 field volts you typically read with your digital multimeter is just an average reading caused by the field voltage being rapidly turned on/off from full voltage to zero volts. Ever look at the field voltage with an oscope? Turning the key switch off with the engine running typically just kills power to the stock engine wiring harness and causes no harm other than killing the gauges, stock alternator sense voltage and alarm circuitry. It will also kill power to the engine stop relay if you have one.

Eric
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In many boats, the engine instruments are powered by they key switch, and this includes the hour meter. If you turn the key off, the hour meter no longer runs and doesn't track how long your engine has actually been running. The opposite problem is perhaps worse (though more ethical) when you go to sell, that is you left the key on when the engine wasn't running and your engine shows way more hours than it was actually used.
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An engine hour meter should be powered through an oil pressure switch. Hour meters that are integral to the tach will not count with zero rpm.

Eric
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L1, I've got to agree with you. Discipline, training, relying on human beings...We're fallible, and when a physical system is built in such a way that it CANNOT be misused, that removes the chance of "pilot error" and that's almost always the best way to go.

Safeties, interlocks...whatever. Because there's always a day when someone is overtired, seasick, distracted...and if "ooops" physically CAN'T happen, that's when it pays off.

Eric, thanks for clearing that up.
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