big 40 problems

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i have a late 80's miller big 40 that im having some trouble with.  It will not arc when i touch the rod to the ground clamp.  all connections are tight and the lead is brand new so its not arced out. The machine has 3400 hrs on it and it ran and welded fine until the last 2 times i used it. I noticed a couple circuit breakers coming off of fine control box, but am not sure if those would affect the weld process. i hope the board isnt messed up, how would i be able to check that?  Im not super knowledgable with these machines so any information would be helpful.
Reply:Hello smw, I also have a Miller Big 40. On two occasions I have "lost" the arc. What I found is that there is an adjustable resistor under the sheet metal near the end of the fuel tank about mid-machine,that can lose continuity. It is cylindrical in shape and approximately 6" long with a metal clamp that can be loosened and slipped along it to vary the resistance values. It has a wire attached to one end of the cylindrical part and another wire connected to the clamp portion. Carefully remove and note the position of the clamp piece and then carefully clean it and re-install it. Both instances where I lost the arc I found that this was the culprit. Hope that this is all that is wrong, good luck and best regards, Allanaevald
Reply:thanks i will check that out. that may explain the first time it happened i didnt have arc then it came back.  So maybe i lost continuity, it came back, then lost it again.
Reply:thanks aevald for pointing me in right direction.  It was the black wire off that part that was not making good contact on the terminal.  Cleaned it up and it welded fine.
Reply:the big 40 is nice welder I have one myself. if for some reason that dont fix it give me haller ill give you a number of a man that will tell you whats wrong with it.  does yours have the old f163 red seal in it?
Reply:Glad to hear that it was a relatively simple fix........ WELD ON! Best regards, Allanaevald
Reply:Glad to hear it worked those are great welders to have . hang onto it
Reply:Originally Posted by smwIt will not arc when i touch the rod to the ground clamp.
Reply:Originally Posted by duaneb55Why do guys trash their work clamps but checking weld output this way?
Reply:Originally Posted by duaneb55Why do guys trash their work clamps but checking weld output this way?