Turning Diesel Key Off? - SailNet Community
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post #1 of 17 Old 09-05-2007 Thread Starter
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Turning Diesel Key Off?

I have a 1983 Islander Bahama 30 with a Volvo Penta MD7A 13 horsepower diesel. I have no experience with diesel engines and was told to use the pull handle on the engine cotrol panel to kill it and then turn the key off, which I have been doing. I did inadvertently turn the key off once (being used to turning off car engines) and when nothing happened turned it back on and then pulled the kill handle. Sometime after this incident I re-connected the remote battery charger monitor/status panel (which had been disconnected prior to the incident for relocation) and found that it is not working. Is this a coincidence or did I potentially damage something by turning the engine key off while running? I have been told that if you turn the batteries off while the engine is running it will damage the diode (although I don't know what diodes are).

So: Why does the diesel engine have this kill handle instead of just turning the key off? What exactly doeas this kill handle do to kill the engine? What can happen if you turn the key off while the engine is running? Did I fry the charger monitor by turning the key off (or did I fry the charger itself...please say no)? Where is the diode and what does it do (or not do when its fried)?

Thanks - Capn Santiago
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post #2 of 17 Old 09-05-2007
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Capn-
I'm not much help here except for i know that the "Kill Handle" stops the fuel from getting to the engine.

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post #3 of 17 Old 09-05-2007
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With a diesel engine that does not need spark the engine will keep running possibly and run all the fuel out of the lines and then you would have to prime it. this is what i was told. i am curious, what does it take to prime the engine?
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post #4 of 17 Old 09-05-2007
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The only way to kill a diesel, since it doesn't depend on electricity—having no spark plugs or ignition coil—is to cut off either the air or the fuel to the engine. On some engines, usually smaller ones, the air is cutoff... on most the fuel is shutoff. The only thing the key is required for on most diesels is to provide a safety mechanism to prevent someone from starting the engine unexpectedly. Hotwiring a diesel is pretty simple, since you only have to hotwire the starter motor for a short period of time...

Turning off the battery switch, unless you have a ZapStop device installed, or disconnecting the batteries will cause the alternator to burn out the diodes that allow it to function. On a diesel engine, this is a problem, but not as serious as it would be on a gasoline engine, which requires electricity to continue to operate...and will drain down the batteries as a result of this.

I don't believe turning the key off had anything to do with the remote monitoring panel, but it really depends on how things are wired. Unless your boat is wired in a seriously strange way, I doubt that you fried the alternator diodes.

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post #5 of 17 Old 09-05-2007
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Capn...

If the battery panel was disconnected prior to the incident, it will have known nothing about it.

The kill line simply cuts off the fuel delivery to the injectors and the motor stops. A diesel engine will carry on running, as long as it is getting fuel.

On a gasoline engine, the key turned to off will kill the spark, and it stops.

When you turned the key off with the engine running, you will have disabled all the sub-systems that are fed by the ingnition power supply.....rev counter, engine temp guage, and I suspect you will have disabled (not damaged) your alternator too.... it depends on the set-up. I really don't think you will have taken out the alternator diodes... the classical way to blow those is to disconnect the battery master switch with the alternator charging, and that is not what you have done !!!! I have done that a few times... someone's foot would hit the ignition key with the engine running. On my boat, it just interrupts the charging, and the charging ammeter betrays it.

Start the motor, and put a wee multimeter across the battery terminals to see if you are charging... it should read about 14 V.... never 12 V. If you are charging, then your diodes will be fine. A multimeter is about Ł15 and it's a classic diagnostic tool for your ship.

Your diodes rectify the AC output from your alternator to DC for your battery charging and the electrical loads. They are normally in the alternator, but not always.

If you get into this stuff you will find it interesting. It is not that difficult if you get into it... at least get into the basics. You will find an alternator to be an extremely simple and absurdly clever device, for example.... reliable as sin, and easy to maintain.

Be good....

Rocckter.
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post #6 of 17 Old 09-05-2007
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Diesel engines have either a manual fuel shut off/on like yours, or an electrically operated fuel solenoid valve. The manual valve requires no electricity and is therefore more reliable and simpler. With a solenoid valve you only need to turn the key off to stop the engine. Most newer diesels use a solenoid which is a mistake IMHO for marine use. I have a Yanmar with manual shut off and we have always turned the key off first and then pull the fuel shut off to stop the engine. I don't think it could have damaged anything by turning the key off and then on again while the engine was running, we have done it several times by mistake, but I am not familiar with your system. A Volvo dealer could give you better info. on that.

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post #7 of 17 Old 09-05-2007
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I was told always hit the kill switch before turning off the key. I think it is good practice but I also did turn the key off first by mistake once with no consequences.
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post #8 of 17 Old 09-05-2007
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Yes... stop the motor before ignition off would be the way I would do it.
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post #9 of 17 Old 09-05-2007
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Yes, shutting the key before the engine (and alternator) is off can cause things to fry. Some alternators are internally protected against that, most are not. What effect it would have on other electronics, would vary. You might not have blown the remote monitor panel--but blowing the alternator might make it look that way.

A "Zap Stop" (which is really just a special $2 diode that has been reasonably marked up three times and consumer packed to sell for $25) is good protection if you aren't sure about whether an alternator can protect itself.

Taking the alternator to an automotive shop for free testing is a good idea, most of the chains can test it out for you on the spot. The fun part comes if it is bad and they ask "What model car or truck did this come out of?"
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post #10 of 17 Old 09-05-2007
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Don't mean to steal the thread but I am confused by some responses. I am in the habit of turing the key to the off position after it starts. Is this wrong? Am I shutting down the charging process when I do this?
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