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Unread 07-06-2010, 07:15 PM   #1
g4x4xgeoff
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Miller bobcat welder idles, but to arc

So guys,

I've got a miller bobcat 250 series multiprocess welder. It hasn't been used in a few years, and a couple days ago i started (well jump started) it for the first time. Engine idles, but when a load is applied (arc is attempted to be struck, or a tool is plugged into the generator) nothing happens. I'm clueless.

Anyone have any idea, or someone in the area who would like to take a stab at it?

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Unread 07-06-2010, 07:45 PM   #2
Jerry Bransford
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Moved to the Fabrication forum where the pro welders who might be able to help you hang out.
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Unread 07-06-2010, 07:46 PM   #3
g4x4xgeoff
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Thanks jerry! I wasn't too sure where to put this, so i figured GD was the safest.
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Unread 07-07-2010, 09:54 AM   #4
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Pull the covers off and see if any rodents have eatin through the wires or anything,also check and see if a connection somewhere has corroded off
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Unread 07-07-2010, 10:43 AM   #5
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I had the covers removed when i went to restart it for the first time, because i had to replace the fuel filter, and check a few connections for ignition.

I took a look around, traced wires and looked for busted fuses and loose connections but really couldn't find any. I've repaired some electric welders, but this is the first gas powered welder i've owned, and it's, well, frightening. 28horsepower vtwin powering an alternator the size of a tool box and two capacitors the size of masonry bricks.
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Unread 07-07-2010, 11:17 AM   #6
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does it arc at all? when you attempt to strike an arc, does the machine rev up? does it stall? when you put load on it, the engine should rev up to full throttle, then idle back down a minute or so after you remove load.
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Unread 07-07-2010, 11:19 AM   #7
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It doesn't remove from idle, which is what concerns me. Whenever I attempt to apply a load like striking an arc, or using a power tool/light, I hope for the engine to kick up, but it refuses.
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Unread 07-07-2010, 11:36 AM   #8
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the engine is still coupled to the generator?
did a shaft key get sheared?
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Unread 07-07-2010, 11:39 AM   #9
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98GP

I'll take another look this afternoon, if i remember correctly, it appeared that the alternator was turning, which means it should be generating electricity right?

shaft key? im not familiar with this, but again, I'll take a deep look at it this afternoon. It's kind of a pain to have such a great welder with unbelievable power sitting around being useless. It even generates enough to power my house in the event we lose electricity.
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Unread 07-07-2010, 11:48 AM   #10
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the shaft key is the square/rectangle piece found in the keyway/slot on the output shaft of the gas engine and input shaft of the generator. These shafts may have splines. IDK...just throwing out questions.
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Unread 07-07-2010, 11:57 AM   #11
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RIGHT DUH.

I knew it as the "key slot" I dont know why i couldn't put 2 and 2 together. I'll take a definite look at the engine/generator attachment, and see whats up. I'll probably have to jump this sucker again!
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Unread 07-07-2010, 11:58 AM   #12
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its possible the exciter is cooked or the brushes are arcing out on the genorator
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Unread 07-07-2010, 01:23 PM   #13
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Some of these welders need the battery connected or the jumper cables left on -- they use the 12 battery current to energize the field. Another thing to check are the large multi point switches used to change settings making contact I have had them stick ---- If none of these could be brushes -- have you tried it in high idle -- look over the IC board if it works in High idle

The motor to Alt. attachment is most likely not the problem I also doubt anything is cooked from just sitting.

Last edited by bryanm; 07-07-2010 at 01:34 PM..
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Unread 07-08-2010, 08:13 PM   #14
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Hey guys,

took a look at the welder today, confirmed that the shaft key is intact, the alternator does crank, and the brushes at the alternators output appear to be in working condition. I did find a small ceramic fuse that was cracked, I'm going to try and replace that tomorrow with a new one. I doubt this was the entire cause, but who knows, maybe its something.

Bryanm, what do you mean by having the jumper cables left on to energize? I've never heard of anytihng like that.
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Unread 07-08-2010, 08:56 PM   #15
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The jumper cables or battery supply current to energize the field in the welder if the field is not energized no arc --the small Lincoln (G9) welders work this way I believe the Millers do also. Usually there is a separate wire it hooks to the battery along with the battery cables. I said leave the cables on the get the current to energize the field normally the battery takes care of it most large machines do not require this but the smaller machines do. You can get them to start but if you don't leave the 12 volt current connected you will not be able to weld. Did you check the adjustment switches (the big dials) sometimes they stick and do not make contact this will yield the results you describe.
You can PM me and I will give my cell number maybe talk you through if the resistor does not fix the problem I have one of the machines you are working on at the shop but I never use it.
mccollum.bryan@gmail.com

Last edited by bryanm; 07-08-2010 at 09:14 PM..
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miller bobcat welder , welder repair , welder , no arc , idle , miller welder

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