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Tractor Talk Discussion Forum

Why does diesel run with key off?

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Quebec Bob

05-23-2006 09:01:52




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How is it that a diesel motor still runs even when you shut off the ignition? Like many tractors you have to shut off the fuel on my tractor to get the motor to stop. I remember Mack trucks were like that too.




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jdemaris

05-23-2006 12:56:39




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 Re: Sometimes the valve guides? in reply to Quebec Bob, 05-23-2006 09:01:52  
A diesel with mechanical injection usually has one of two different methods of shut-down. The most common being a fuel shut-off - either operated with an electric solenoid or mechanically with a cable or pull-rod. Some older diesels used an air-choke instead of a fuel shut-off - e.g. Mercedes diesels in cars, the emergency shut-off in Detroit Diesels, etc.
Back in the late 60s, there was a Mercedes dealership next-door to the Deere place I worked at. I was told stories by Mercedes mechanics about "run-way" Mercedes diesels that could not be shut off. In brief - the story went - the Mercedes diesel used an air-choke for shut-down - like in a 190D. When an engine got worn, sometimes when an attempt was made to shut the engine down, the engine would suck crankcase oil through worn valve guides and keep on chugging away. If it was a stick-shift it could be stalled out. But - with an automatic -not so easy.

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Jerry/MT

05-23-2006 09:46:50




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 Re: Why does diesel run with key off? in reply to Quebec Bob, 05-23-2006 09:01:52  
ecause it's not a "spark ignition" engine. It's a compression ignition engine. The compresion ratio is so high (16-18.5 compared to 8-9 on modern spark ignition engines)that the tempertaure rise caused by the compression of the air raises it's temperature above the auto ignition temperature of the fuel. At that high a temperature the injected fuel ignites. To stop the engine, you shut off the fuel.

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frankiee

05-23-2006 09:32:37




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 Re: Why does diesel run with key off? in reply to Quebec Bob, 05-23-2006 09:01:52  
On a diesel the key is more of a "electrical master switch" It does not cause "ignition" of the fuel like a spark plug in a car.
The fuel ignites by heating, by the compression of the piston going up. So all you need is the starter to crank the engine a speed fast enough.
Being it has a mechanical fuel pump as long as the engine is revolving then the pump is pumping fuel.
Why even put the ignition switch there????? ?????
I don't really know. Someone else will know.
I work with large 16 cylinder 150 liter engines and we don't have ignition switches.
On ship we start with compressed air so we dont need an electic starter or generator.

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Mike (WA)

05-23-2006 12:17:26




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 Re: Why does diesel run with key off? in reply to frankiee, 05-23-2006 09:32:37  
Was trying to troubleshoot a MF 135 diesel one time, and as near as I could tell, the only thing the "ignition switch" (actually, the key) did was ground a relay that turned on the power to the fuel guage.



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Bob M

05-23-2006 10:39:39




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 Re: Why does diesel run with key off? in reply to frankiee, 05-23-2006 09:32:37  
Why even put the ignition switch there????? ?????

The ignition switch is there for a couple reasons:

1 - So only authorized people (ie. someone with the right key) can start it.

2 - Provides a way to turn on and off the dash gages/warning lights, electrical accessories, the alternator regulator (in some cases), etc.



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Glenn F.

05-23-2006 09:22:38




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 Re: Why does diesel run with key off? in reply to Quebec Bob, 05-23-2006 09:01:52  
Diesels do not have an electronic ignition system. They simply compress the air/fuel mixture "tight" enough that it explodes without an electronic spark. Glenn



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J Schwiebert

05-23-2006 15:16:57




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 Re: Why does diesel run with key off? in reply to Glenn F., 05-23-2006 09:22:38  
Wait a minute here: All the new ones are fired by electronics. There can be up to 100 volts D. C. in that engine that makes them go. Questions?



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Bob

05-23-2006 09:31:41




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 Re: Why does diesel run with key off? in reply to Glenn F., 05-23-2006 09:22:38  
And...

Some diesels have an electric fuel shutoff in the injection pump, so they will shut down with the key.

Others have no electricals at all on the injection pump, and need to be shut down mechanically by pulling a cable or moving the throttle past the idle position.

Then, off course, there's the new stuff that satisfies the emissions nazis, which is ALL electronic, and sometimes won't even run WITH the key switch turned on!

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