Big blue 251D shut off problems

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My dad got a Miller Big Blue 251 D not to long ago, everything  worked great, but all the sudden I went to turn it off and turned the knob to the off position and it wouldn't kill the engine.  It didn't flinch or anything.  You can turn the the switch and still engage the starter, but when you go turn it off the red light comes on very dimm and it stays running.  I finally had to shut the fuel off to kill it.  Ater repriming it, it acts like normal, the lights come on full brightness and it starts up then lights go back off runs fine, but then I have to shut the fuel off to kill it cuz the switch won't do it.Any help would be appreciatedOne other question. I know you can reverse leads for reverse polaritys on this welder, but is that what causes sparks to fly if you touch the ground lead to the trailer even if the electrode or electrode lead is not touching the trailer, or is there some kind of short or something else touching inside the welder?By trailer i mean its mounted to a trailer.I appreciate anybodys helpThanks
Reply:Sounds to me like the alternator is backfeeding into the ignition.  Try a diode in-line on the exciter wire to the alternator.  As a test, you can pull the little plug on the back of the alternator after you turn off the ignition switch and see if the unit stops.  It won't hurt anything in the alternator if you pull that plug.  Just don't ever disconnect the big battery lead from the alternator with the engine running.  If it stops, you have found the source of the running-on.  I have never run a 251D, but I know some construction type diesels have to have the fuel shut off to kill them.Miller Syncrowave 350Millermatic 252/ 30A spoolgunMiller Bobcat 225g w/ 3545 spoolgunLincoln PowerArc4000Lincoln 175 Mig  Lincoln 135 Mig Everlast 250EX TigCentury ac/dc 230 amp stickVictor O/AHypertherm 1000 plasma
Reply:The part about getting sparks on the trailer if you touch the work lead to it is troubling.  That should not happen.  I suspect Duane will be along to do the real troubleshooting.Miller Syncrowave 350Millermatic 252/ 30A spoolgunMiller Bobcat 225g w/ 3545 spoolgunLincoln PowerArc4000Lincoln 175 Mig  Lincoln 135 Mig Everlast 250EX TigCentury ac/dc 230 amp stickVictor O/AHypertherm 1000 plasma
Reply:Originally Posted by DougAustinTX I suspect Duane will be along to do the real troubleshooting.
Reply:most diesels are killed by shutting off the fuel at the injector pump, either by a manual valve (think "PULL TO STOP" type knob) or an electric solenoid (key switch). Look on the injector pump for the linkage or a single wire on it.Sounds like you have and electric solenoid type, check to see if there is still voltage going to the solenoid after you turn it off, could possibly be a bad switch. If there is voltage at the solenoid when running and not when you turn it off sounds like a stuck solenoid valve, might give it just a little love tap to see if that might jar it loose while it is running.miller... 225g, s32p, 250x, 304, 12vs, MSW41     victor o/a thermal dynamics cutmaster 50 lenco panel spotter        hobart hf-boxG3 Farms.....raising cattle, hay, kids and hell, ...oh yeah I'm a fire sprinkler contractor by trade.
Reply:Thanks, I'll check those things out.Is there an easier way to repost to Miller forum or do i need to just retype it out in a new post.Thanks4thahelp
Reply:copy and pastemiller... 225g, s32p, 250x, 304, 12vs, MSW41     victor o/a thermal dynamics cutmaster 50 lenco panel spotter        hobart hf-boxG3 Farms.....raising cattle, hay, kids and hell, ...oh yeah I'm a fire sprinkler contractor by trade.
Reply:Taking one thing at a time, depending on the age of the unit, it may already be equipped with the diode that DougAustinTX mentioned above and it sounds as if he's already directed you to the likely cause of the problem - back feed into the RUN circuit from the battery charging alternator field exciter wire.If you haven't tried it yet, do as he suggested and pull the small wire plug on the alternator and see if it shuts down properly.  If it does this confirms his diagnosis and then it's just a matter of determining if your unit has the referenced diode already in place and has shorted or if not then the alternator will need to be looked at.What is the unit Serial Number?  This will allow determining if it should be equipped with the diode or not.MM200 w/Spoolmatic 1Syncrowave 180SDBobcat 225G Plus - LP/NGMUTT Suitcase WirefeederWC-1S/Spoolmatic 1HF-251D-1PakMaster 100XL '68 Red Face Code #6633 projectStar Jet 21-110Save Second Base!
Reply:BTW (that's By The Way ), do you have the unit manual or wiring diagram to follow along with?MM200 w/Spoolmatic 1Syncrowave 180SDBobcat 225G Plus - LP/NGMUTT Suitcase WirefeederWC-1S/Spoolmatic 1HF-251D-1PakMaster 100XL '68 Red Face Code #6633 projectStar Jet 21-110Save Second Base!
Reply:Yes I do, I'll see if I can get that # todayThanks
Reply:Just to let everyone know, I pulled the wire on the alternator and it kept runnin.  I used the manual shut off lever to kill it this time.  I got a feelin, pretty sure, that toolman is right about the fuel cut off solenoid.  The #'s on the machine are kh304549, stock # 903162.  The schematics on the side plate, if i'm reading em riight show the diode comin off the d+.  But its been a while since ive done any schematics troubleshootn.  I'm going to put a multimeter on the solenoid and double check.Now if i can figure out the arc comn from the ground.Thanks for the info everybody.
Reply:Oh and by the way, I got a lincoln 250 gxt that has some problems, so I'll be gettn back with ya'll pretty soon.Thanks again
Reply:If the wiring diagram on the machine is the same as the one in manual OM-480M that the SN brings up, the diode you refer to should be flashing diode D1.  If so, I would say it is leaking voltage back into the RUN circuit sufficient to keep the fuel solenoid held in with the ignition switch OFF.You can check it with a VOM that has a diode setting or just disconnect it from the circuit and see if the machine shuts off as it should.  If disconnected before starting the engine, chances are the unit will not produce weld/auxiliary power as initial field excitation voltage won't be present with diode D1 out of the circuit.MM200 w/Spoolmatic 1Syncrowave 180SDBobcat 225G Plus - LP/NGMUTT Suitcase WirefeederWC-1S/Spoolmatic 1HF-251D-1PakMaster 100XL '68 Red Face Code #6633 projectStar Jet 21-110Save Second Base!
Reply:Originally Posted by DougAustinTXSounds to me like the alternator is backfeeding into the ignition.  Try a diode in-line on the exciter wire to the alternator.  As a test, you can pull the little plug on the back of the alternator after you turn off the ignition switch and see if the unit stops.  It won't hurt anything in the alternator if you pull that plug.  Just don't ever disconnect the big battery lead from the alternator with the engine running.  If it stops, you have found the source of the running-on.  I have never run a 251D, but I know some construction type diesels have to have the fuel shut off to kill them.
Reply:In this case "ignition" is a generic term and would refer to the RUN circuit.  I have no doubt that Doug knows the subject machine is a diesel reading the entire post. MM200 w/Spoolmatic 1Syncrowave 180SDBobcat 225G Plus - LP/NGMUTT Suitcase WirefeederWC-1S/Spoolmatic 1HF-251D-1PakMaster 100XL '68 Red Face Code #6633 projectStar Jet 21-110Save Second Base!
Reply:Use a voltage tester to see if the alternator is putting out while the machine is running. Put the leads on the battery you should see about 14 volts if it is charging.miller... 225g, s32p, 250x, 304, 12vs, MSW41     victor o/a thermal dynamics cutmaster 50 lenco panel spotter        hobart hf-boxG3 Farms.....raising cattle, hay, kids and hell, ...oh yeah I'm a fire sprinkler contractor by trade.
Reply:Thanks everybody,  for your help.The fuel cut off solenoid was the problem. After replacing it, she fired right up and when the switch was turned to the off position it killed the machine.Thanks again