Welding on a modern car bumper

My SIL is a good guy and treats my daughter good. He can't back a trailer for crap, though. He put two nice dings in my truck so far, and I've had to repair two trailers.
He comes to me with his Yukon Denali. It has a small 4" x 4" plate where the plug plugs in that has somehow (?) been ripped off the bumper right along the original MIG welds.
No problem, sez I. Sez he, "Aren't you going to disconnect the battery so it doesn't foul up the computer?" I really don't think it's going to put any electricity into the system, and it's not welding near any computer type component, so I sez no. But, I tell him I'll check with the pros.
I've heard to disconnect the battery, and not to disconnect. When is it (is it) appropriate to disconnect?
Steve

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Why take a chance?

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I've heard anecdotal stories from people claiming that somehow welding on a vehicle without disconnecting the battery caused computer problems. If somebody would come up with a plausible explanation for how this is possible (even if only for a particular process, like TIG), when the ignition system doesn't do it, I'd love to hear it.

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I've seen that as well and can't figure out why that would be true.
It seems to me that keeping the battery connected is more likely to protect all the electronics in the car than harm them while welding. It acts as a voltage regulator so in case there are any currents induced in any of the wires, it's more likely to clamp the voltage levels to 12V instead of allowing them to rise to higher levels that could do some harm. The only thing I would expect to be protected by disconnecting the battery is the battery.

"Curt Welch" wrote: (clip) It seems to me that keeping the battery connected is more likely to protect
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^^^^^^^^^^^^^^^^^^^^^ Actually, following that logic, you should turn everything on--radio, ignition, lights, global navigator, etc. Otherwise, the 12v clamp isn't there. ^^^^^^^^^^^^^^^^^^^^ The only thing I would expect to be protected by disconnecting the battery is the battery. ^^^^^^^^^^^^^^^^^^^^ And it will carry more current without harm than most welders will put out.

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Yeah, I was thinking the same thing.
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Just about anything somebody would regard as "modern" electronics that might be damaged by the welder is actually still powered-up (albeit in a standby mode) even when the ignition is turned off -- that certainly includes the engine controller and radio. Haven't checked, but I'd be pretty surprised if that wasn't also true of the transmission controller, GPS, etc etc.
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Right.

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I wouldn't worry too much about the battery for the reasons stated above. The point of disconnecting the battery is to protect the electronics. Most electronics are not designed for reverse polarity. The possibility exists when welding for forcing current back through the "wrong" side of the battery - not normally a problem as that's how battery chargers work. The issue is the potential for allowing the current through the electronics first.
We could argue for ever about it and only the auto electronics designer knows for sure. Given the cost of replacing the electronics it seems like cheap insurance to break the electrical path through the electronics (i.e. remove the battery cable). I think that everyone would agree that if the conductive path through the electronics is removed then the likelihood of damage is significantly reduced. hope this helps

"Kelly Jones" wrote: I wouldn't worry too much about the battery for the reasons stated above.
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^^^^^^^^^^^^^^^^^^^^^^^^ Do you disconnect the battery before attaching a charger? Try to visualize a path that would send current backwards through an electronic component. Say your radio is turned off, but receiving 12v standby current. You start welding. Because you have not properly routed your ground return cable, a surge reaches the battery ground through the frame and pushes current BACKWARDS through the battery and out to the radio. Your current surge would have to be more than 12v for this to happen. That's probably more than half your arc voltage being spent in the return circuit. You wouldn't be welding--you would be scratching your head.

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I've done it both ways, main thing is to ground to what you're welding, if you're welding exhaust ground to the exhaust. If you're welding on the bumper ground to the bumper. If you're welding on the exhaust and you ground to the frame, you never know what's rusty and what path the current will take. If a nice path by way of the o2 sensor through the ECM and back to the frame is the way the current travels then you just smoked the ECM.
Wire brush or grind a clean spot for the ground clamp.
Last car I welded on (mine) had an air bag, I disconnected the battery and shorted the battery cables together. I don't know if this was the best thing to do but I wanted no chance of the air bag circuit to get power.
Anyone shows up or calls me to weld on a car I send them to a body shop.
Thank You, Randy
Remove 333 from email address to reply.

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Finally, an explanation for how welding can damage electronics that makes sense! Note that the way to avoid it has nothing to do with the battery.

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Yeah, that makes a lot of sense. If you weld the exhaust, which will likely have a very good electrical connection to the engine, but might have a poor connection to the frame because of rusty brackets and rubber engine mounts, and you ground to the frame instead of to what you are welding, the path the current finds from the exhaust to the frame might well be through many places you didn't want it to go including your drive train and bearings as well as through sensitive electronics. I could see how that could be a common error people could make which could lead to bad issues if the engine wasn't well grounded to the frame.

Aside from all the issues about damaging the electronics, there is an overriding reason to make a good ground connection near the weld. You'll get a better weld. Welding currents are high, and arc voltage is low. Voltage drop in the return path is going to have a bad effect.