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I would disconnect the alternator. I have welded on vehicles without disconnecting anything, and not had a problem. I have also fried an alternator. Just my experience.
I agree with you- disconnect the batteries. Make sure no sparks can get to the batteries, or anything else flammable, for that matter. Connect the ground as close to where you are welding as you reasonably can.
On my truck using a MIG welder, even with the battery disconnected all the new bulbs in the back where I was working failed within a week of the welding. I thought just bad bulbs until I welded the other side and the new bulbs on that side failed shortly afterwards.
So, now I totally disconnect both battery cables from each battery and pull any bulbs near where I am working. My alternator failed about two months later, but, it was three years after I bought the truck and I have no idea how old it was before that since it was painted Ford blue. But, I do not think it would hurt to pull the alt. plug since it is so easy to reach on a Ford.
Better safe than sorry, so disco the battery. I am guilty of leaving them connected on my Mustang when I was welding on the exhaust and never had a problem.
Agree with what others have mentioned so far. I would have on standby, a fire extinguisher, or a garden hose already on with a on / off nozzle. Also if welding near a painted body surface, wet rags, to cover the painted surface, and maybe a water bucket with rags to cool off your welds, if welding on the backside of a painted surface, or undercoated surface. (keeps from starting a fire where you cannot see.)
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