Ground clamp arcs when attaching to running diesel welder frame...WHY??? Won't Weld!

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I'm trying to weld cable racks on a trailer upon which my Miller Big Blue 251D diesel welder is mounted. I welded about six inches of weld perfectly fine and when I went to weld the opposite end of the mount I couldn't strike an arc. This is on a piece of I beam I am welding to the trailer as a base for bottles next to the welding machine frame. The ground clamp (attached to the work) will spark when dragged across the work and will arc violently when you open it to remove or attach it to the work.  I cannot strike an arc with the electrode.  I CAN however move the ground clamp to a piece of scrap unattached to the trailer and weld a bead perfectly fine on the scrap metal.  WHAT GIVES???? Why would it work initially and now not work?  Thanx in advance for any suggestions Last edited by TexasCowGrower; 04-24-2015 at 05:52 PM.
Reply:I'm going to guess you're shorting out your machine. I bet that half of the trailer is connected in some different way to your welder than the other half. Got a picture of the setup?John 3:16(2) Miller Pheonix 456(2) Millermertic 252Dynasty 210DXHobart 210MVPDoringer D350 SA Cold SawScotchman 350LT Cold SawWebb 10x50 MillWebb 15x40 LatheGeka Bendicrop Ironworker
Reply:When the ground clamp is attached to the work/trailer that the machine is bolted to I can ground across the cable lugs at the machine and get no response whatsoever.
Reply:I can get a picture, but I have no clue how to post it here    The machine is bolted to a small trailer frame that looks to have been made especially for this purpose. The trailer is comprised of two outer 3" channels that are cross connected with 3" angle iron. It's a skeleton and there is only 4 bolts holding the machine to the frame.  It welded fine for the first couple of welds, then went haywire.
Reply:Here's a scenario that would explain the problem;The first half you were welding was not electrically connected to the rest.   Rubber isolation pads would cause this.  So would a good layer of rust.  If the welder frame has been  grounded internally (incorrectly) then you will have an electrical  path from the ground clamp to the generator.Dan----------------------------Measure twice.  Weld once.  Grind to size.MIG:  Lincoln SP100 TIG/STICK:  AHP Alphatig 200X
Reply:The rod is pointing at the initial weld. The failure is on the edge next to the gloves in the pic. The engine and generator are mounted to the frame via the rubber motor mounts visible in the pic and to my knowledge everything else is wired by the factory. I cleaned the area next to the gloves and was unable to get an arc with the ground clamp attached as in the picture. I couldn't get an arc by grounding the rod to the ground clamp either. The ground lug wire goes into the generator and is connected to the field lugs inside. Not sure where to look for further ground issues.
Reply:Your lug isolator is probably shot and making a connection to the welders chassis. Pull them both apart and see where the problem is.
Reply:Ok...Took both lugs apart and cleaned and tightened connections...nothing...Not really much to fail.
Reply:So you successfully completed 1 weld on the trailer, and couldn't complete the other side of that beam, but you can successfully ground and weld something off the trailer?John 3:16(2) Miller Pheonix 456(2) Millermertic 252Dynasty 210DXHobart 210MVPDoringer D350 SA Cold SawScotchman 350LT Cold SawWebb 10x50 MillWebb 15x40 LatheGeka Bendicrop Ironworker
Reply:Yes...Exactly.  I actually began tacking the piece next to the glove with a wirefeed suitcase.  I did about two inches next to the glove and an inch next to the outside of the frame on the glove end. I then went to where the rod is pointing and welded the entire length with a 7018 rod. When I went back to the other end to finish I couldn't strike an arc.  I can clamp to a scrap ten feet away from the trailer and weld all day ON THE SCRAP as if there is no problem.  It's perplexing.
Reply:Can you weld where you originally used the 7018?John 3:16(2) Miller Pheonix 456(2) Millermertic 252Dynasty 210DXHobart 210MVPDoringer D350 SA Cold SawScotchman 350LT Cold SawWebb 10x50 MillWebb 15x40 LatheGeka Bendicrop Ironworker
Reply:No.  With the clamp as it is in the pic I can't strike an arc anywhere on the I beam or ground clamp.
Reply:Originally Posted by TexasCowGrowerOk...Took both lugs apart and cleaned and tightened connections...nothing...Not really much to fail.
Reply:Sounds like a field winding has become compromised and getting into the case internally causing a short to the frame. This can come and go as the winding heats and cools. The work lead, (clamp) should be electrically isolated from the case but I reason the field winding getting against the frame of the unit is electrically joining the pair though the leads hence no potential between the work and stinger. This is a direct short but field excitation voltage can be as low as 5VDC so it may not be noticeable without test equipment. If I am accurate in my thoughts the welder would attempt to work normally when the work lead is only on the work piece which is not connected or laying on the metal trailer. My understanding is this is what you portray and the machine works seemingly normal?SlobPurveyor of intimate unparalleled knowledge of nothing about everything.Oh yeah, also an unabashed internet "Troll" too.....
Reply:I never touched them until the suggestion.  They seem to be hard plastic with a hex head bolt recessed into the back that protrudes through the bushing. There is a flat L shaped bracket that folds over the bolt on the outside and goes through the bushing where the leads from the generator are attached. It looks to be a pretty unique and effective fail safe design. The bushings appeared to be in good shape. The machine only has 2600 hours and hasn't been used in years. FWIW  I also cleaned the surface where the brushes ride on the end of the armature. I does appear voltage could be leaking to the frame but I don't get shocked touching it barehanded while it is running.
Reply:EXACTLY  Slob!  So....How do I find the short? When I attempt to remove the clamp it arcs pretty good so I bet it's more than 5 volts!
Reply:Can I verify with a volt meter without blowing it up?
Reply:Originally Posted by TexasCowGrowerI never touched them until the suggestion.  They seem to be hard plastic with a hex head bolt recessed into the back that protrudes through the bushing. There is a flat L shaped bracket that folds over the bolt on the outside and goes through the bushing where the leads from the generator are attached. It looks to be a pretty unique and effective fail safe design. The bushings appeared to be in good shape. The machine only has 2600 hours and hasn't been used in years. FWIW  I also cleaned the surface where the brushes ride on the end of the armature. I does appear voltage could be leaking to the frame but I don't get shocked touching it barehanded while it is running.
Reply:If so, am I correct in thinking it's dangerous to use without fixing? I REALLY need to build some corrals...and they're not attached to the trailer
Reply:Originally Posted by TexasCowGrowerEXACTLY  Slob!  So....How do I find the short? When I attempt to remove the clamp it arcs pretty good so I bet it's more than 5 volts!
Reply:Originally Posted by TexasCowGrowerIf so, am I correct in thinking it's dangerous to use without fixing? I REALLY need to build some corrals...and they're not attached to the trailer
Reply:Good advice....not exactly what I wanted to hear, but good advice. Thanx
Reply:Can I ask a dumb question? I am not electrically smart and just wondering, could someone get electrocuted by walking up and touching the trailer or welder while it's running with this problem?Thermal Arc 210 - Tweco 211I - Cutmaster 52
Reply:Originally Posted by mxpapaCan I ask a dumb question? I am not electrically smart and just wondering, could someone get electrocuted by walking up and touching the trailer or welder while it's running with this problem?
Reply:OK...With ground clamp ON the I beam I found -0.215 Volts DC to ground (metal 4x4 set in concrete at the edge of my building). Remove ground clamp from I beam and get 15 volts DC to trailer frame from machine. Not lethal (fortunately) but perplexing how such low voltage could disallow welding to frame.  Probably indicative of my ignorance in this area.Remove Welder from trailer.Place a piece of plywood on the trailer.Place the Welder on the plywood completely isolating any connection the the metal frame of the trailer.Multimeter between welder and trailer frame.Try again and see how it welds.Your 4x4 metal post test really doesn't help because that really isn't the "Ground" for the system- that's why you get such a low reading. You need to test for voltage between Welder and Trailer frame.Ed Conleyhttp://www.screamingbroccoli.com/MM252MM211 (Sold)Passport Plus & Spool gunLincoln SP135 Plus- (Gone to a good home)Klutch 120v Plasma cutterSO 2020 benderBeer in the fridge
Reply:Broccoli1 If I do that I can weld on the trailer just as normal.  I THINK I create a similar test situation by grounding the work clamp to a scrap metal away from the machine. DC voltage from scrap (ground clamp) to machine reads -88.5 volts DC, and 6.5 volts AC.
Reply:Check for a short as follows; Turn the engine off.Use an voltmeter set to OHMS to check for resistance.   Connect one probe to an unpainted spot on the frame and the other to the stinger.  If an analog meter the needle should not move.   If a digital meter, it should read the same as when the probe is not touching.   Reverse the probes to eliminate the presence of a  diode.    Repeat  this on  the work clamp to the frame.It seems that the frame is electrically connected to the lug that the stinger is attached to.   Once you verify this, you can trace the wiring back, disconnecting one connection at a time until the short goes away.Dan----------------------------Measure twice.  Weld once.  Grind to size.MIG:  Lincoln SP100 TIG/STICK:  AHP Alphatig 200X
Reply:Took Dan's advice and found continuity between Positive lead and trailer/welder frame.  Traced wiring and found THIS where the wire from the Positive lug goes into the generator. It had grounded out on a bolt head protruding into the access hole.  Four or more 2/0 wires going into a 2" hole and Miller sees fit to put a BOLT in the way...BAD DESIGN... Fortunately a cheap fix!  Many THANX to Dan, Slob and the others without whom I might have been prone to set fire to the machine in frustration!
Reply:Wow that looks like the bolt was threaded right into it, not just rubbing on it. The good thing is its a cheap fix.
Reply:Good work!   Dan----------------------------Measure twice.  Weld once.  Grind to size.MIG:  Lincoln SP100 TIG/STICK:  AHP Alphatig 200X
Reply:Glad to hear that was an easy fix.
Reply:Plans for building welders from air craft generators always cautioned not to weld on the welder itself.I was thinking welder cables were grounding to the frame but didnt post it soon enough.