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  #11  
Old 04-09-2012, 10:50 AM
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Current follows the least resistance, but the forcing potentially is distributed. So grounding near the weld will reduce current elsewhere but the voltage is mostly uniformly present everywhere on the circuit. The battery is a shunt, but only to current near by and only to fairly low frequency currents. They are generally fairly high impedance to anything over a few KHz.

So the battery will likely be the next lowest path if you're grounded by the weld, but if the sensitive electronics are between the weld and the battery, the stray currents will flow past and some are bound to find their path home across the ECU or whatever.

The ECU will have shunts to protect against EMI/ESD. What happens is those guys are doing their job but repeatedly subjecting them to transients will eventually ruin them. The amount of energy is really very low, but the TVS diodes aren't designed for sustained surges. This is why the starter crank position turns off electronics, which would normally be tough on the shunts.

I'd physically disconnect the ECU and any important devices, although I would leave the battery in the circuit. It can tolerate anything a welder will throw at it and will, like Bill says, act like a filter.
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Old 04-09-2012, 11:28 AM
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BJ Goodheart came over to do some welding on Saturday, and had a suppression device designed for shunting transients when welding to vehicles. Looked intriguing. Easy too.
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Old 04-09-2012, 11:36 AM
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All this and no one has asked "Nak, whatcha' welding man?" I am interested in what you are up to?
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Old 04-09-2012, 11:36 AM
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Quote:
Originally Posted by DaveInDenver View Post
The ECU will have shunts to protect against EMI/ESD. What happens is those guys are doing their job but repeatedly subjecting them to transients will eventually ruin them. The amount of energy is really very low, but the TVS diodes aren't designed for sustained surges. This is why the starter crank position turns off electronics, which would normally be tough on the shunts.

I'd physically disconnect the ECU and any important devices, although I would leave the battery in the circuit. It can tolerate anything a welder will throw at it and will, like Bill says, act like a filter.
Um, the starter crank position doesn't take the ECM out of physical connection, does it? I thought the starter crank position turning off electronics was mostly to conserve current capacity... ?? And maybe keep voltage fluctuation damage from happening to that nice stereo due to draw down while cranking?

Taking the ECM out of physical connection would make welding to one's own vehicle a bit difficult in the field... with an on board welder.
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  #15  
Old 04-09-2012, 01:05 PM
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Quote:
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All this and no one has asked "Nak, whatcha' welding man?" I am interested in what you are up to?
there's a cross member for the Asfir skids that is pretty weak to me how it attaches to the frame. So I'm staring at it thinking of what kind of new bracket I could make, or should I drill/tap into the frame, then starting thinking huh I could just zap it on for good and call it done? Even adding grinding it off later to the equation, when/if I had to swap a t-case or something, I'd still be time ahead.
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  #16  
Old 04-09-2012, 01:19 PM
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Originally Posted by Red_Chili View Post
Um, the starter crank position doesn't take the ECM out of physical connection, does it? I thought the starter crank position turning off electronics was mostly to conserve current capacity... ?? And maybe keep voltage fluctuation damage from happening to that nice stereo due to draw down while cranking?

Taking the ECM out of physical connection would make welding to one's own vehicle a bit difficult in the field... with an on board welder.
The ECU obviously can't be taken out of the circuit when cranking, so that's why they put TVS diodes in them. Most electronics do not have (or have insufficient) protection and that's why they're put into a safe condition. I guarantee no manufacturer have ever bothered to consider the effects of an on-board welder on their electronics. Remember we're talking peak power, not average. Sure, you can roll the dice, just giving my $0.02. Take a close look at the datasheets. They're going to tell you a TVS is capable of, say, 5KW, but that's at very low duty cycle (like 0.01%) with a repetition rate of probably on the order of 10 hits within 1ms or so.
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  #17  
Old 04-09-2012, 01:42 PM
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I'm pretty sure the manufacturer never considered the use of a solid front axle, soft top, cut down doors, high capacity power steering pump, 3.4L V6 swap with supercharger, R151 tranny, double transfer case, 4.88 gears, and 35" beadlocks in their mechanical engineering analysis either, so if that is the safety rail I'm already off the highway and hanging over the abyss...

As evidenced by the need to beef up the front crossmember or fracture the frame. Typical mod.

Let me ask another way... how many ECMs have been zapped by on board welding (following basic common sense cautions)?
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  #18  
Old 04-09-2012, 01:51 PM
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In the spirit of clarity, when you say "on board welding" are we still talking about welding something onto the actual truck? Or are you talking about firing up your Mobi Arc out on the trail?
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  #19  
Old 04-09-2012, 04:07 PM
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Reminds me of a "funny" story. Back in the day, we had a history of tackling HUGE amounts of fab projects for club members the weeks leading up to GSMTR. We often took the whole week off in anticipation of the event, though the weekend before was always welding up something new and crazy into the wee hours of the night. Last time I remember doing one of these all-nighters, we decided to get the fab guy's truck up and running. It was a spring over, it was late....welding clamp went on the frame, made a few welds for some linkage mounts, then decided to weld on the rear axle. Neither of us remembered to move the grounding clamp to the axle. Well, short story, the braided SS brake line became the ground...and it lit on FIRE. He threw in the towel for the night after that, and I'm not sure he ever got the truck running either.
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  #20  
Old 04-09-2012, 04:33 PM
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Not by a mobi arc or the like, But I have personally seen a ecu fried by smaw to the frame. of course, that involved ac and a stuck rod and an idiot, but none the less.
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