Originally Posted by martini
I've never heard of any problems caused to ECU's, tbh I rarely bother disconnecting batteries, alternators or anything else and I've never had any trouble.
Now I've said that watch what happens next time I weld on a car
Neither have I, and i have done more than my fair share of welding.
As long as the earth for the welder is near the piece you are welding then
its impossible to do any harm to anything, especially with a MIG as its DC so unlikely to cause eddy currents.
Is a good idea to disconnect the battery though for safety,
a mate of mine was welding a hillman imp on a 4 post ramp, he melted the wiring loom, the car started itself, it was in gear, it ended up on top of another customers car which was parked behind the ramp
both written off.