Welding and ECUs

[AdrianH]

I need to do a couple of tiny bits of welding on my Quattro – one of the exhaust hangers under the boot and a small hole in the front wheel arch where the first owner fitted mudflaps. I'm aware that MiG welding will fry the electronics if proper precautions are not taken – what are these precautions? Is it as simple as disconnecting the battery and isolating the terminals, or is it ECU and dashboard out? Any other gems of advice?

Thanks in advance

[tractor Dave]

Disconnect battery terminals, remove the main ecu and abs ecu if fitted. Remove the instrument cluster. Everything else should be ok. Some also disconnect the alternator and starter but I've never done this and not had issues.

[K Simmonds]

Normally just disconnecting the battery will be fine but if you want to be sure do Dave says and unplug anything your at all concerned about, the clock's and both electronic control units are really the only things to worry about.

To honest I've worked for over 12 years in the motor trade and in that time all I've seen damaged by MIG welding is a single Sony head unit that didn't seem to like it and that's without disconnecting anything. Most mechanics just get stuck in and get the work done.

Though given how expensive and hard it would to replace the three items both Dave and I mentioned I'd disconnect them or remove them completely if your really paranoid.

Some mechanics argue that leaving the battery connected is safer as it acts as kind of a capacitor protecting other bit's of electronics from the surges in voltage, personally I'm not so sure about that one.

[BackintheFold]

I just read this, which seems sensible, but I am not recommending anything. Dave's suggestion also seems sensible-to remove the three most important delicate electronics.

QUOTE::
Yes, the batt add protection from spikes. If you disconnect the negative, jumper it to the positive terminal to prevent the main line from just floating.

What is more important is proper placement of the return/work lead. I have repaired cars where someone grounded a welder to the frame then welded the exhaust. As the exhaust is rubber mounted and then usually bolted to engine with rusty bolts, the only return path is though the oxy senor and the ECU. This type of issue is main cause of damage due to welding

Don't consider everything is ground. When I was doing factory service; I have seen rear ends that have no electrical ground then cause radio noise due to static (that was a bugger to find.) Same for doors, hoods (common problem), dashboards (often shock mounted).

Welding doors should have the work clamp connected to the door. In not, you can A: weld the hinge pin or B: set the grease in the hinge on fire.

It has been many years since I worked as a service tech. I read lots of service bulletins from GM & ford. I never saw any on disconnecting the batt for spike issues. It was always for safety. Weld the frame with the starter cable on the opposite side and you will see how 450 cold cranking amps makes one hell of an arc welder

[K Simmonds]

We always make sure the ground for the MIG is clean and as close as possible to the work area, perhaps that's why I or anyone I've worked with has never damaged anything despite not disconnecting anything.

Welding an exhaust with the earth clamp attached to the body is simply retarded, that person probably shouldn't be using any power tools at all.

It was the guy who does our MOT's I know that (allegedly) popped the head unit I mentioned doing some welding on a mates MK2 Scirroco, still don't really get why that happened, perhaps it was simply coincidence.

[mikes2]

On an Ur, I would 100% remove the digi dash and engine ecu.

[Isimmo]

When ours was restored the dash survived but the ECU failed. Not enough for it to be a non-runner but it had all manner of boost/control issues.

If I did it again I’d do what Dave suggests, they’re all too valuable to risk.