Having gone on a welding course two years ago and bought a mig welder I then had a change of heart and bought an MK Indy chassis.

Having now got all my running gear and wiring in my MK chassis and started the car I now need to weld a couple of extra pieces in the chasis.

This will be the first time I have put my skills to the test, but I've just realised that I don't know anything about what (if anything) I need to disconnect on the car to prevent it being damaged in the welding process.

Can I keep the electrics live whilst I'm welding or do I need to disconnect and isolate current susceptible items such as the battery, ecu (Pinto 2.0efi), coil, alternator, fuses, relays, etc.?

disconnect the earth strap from the battery!

unless you fancy a big bang

I have welded my tintop with the ECU etc connected, just removed the battery...


Whenever im welding on the locost I switch the battery off and unplug the alternator...

i would disconnect the battery earth lead as a minimum before welding, Although i have also seen both battery leads removed from battery and joined together.

remove that battery as nib says, dont want any big bangs

I have also heard it's a good idea to unplug the alternator, dunno how important it is though

I'm not suggesting it wouldn't be good practice, along with possibly the alternator and other electronics, but....
WHY In theory at least should the battery need disconnecting to weld the chassis?

[Edited on 2/6/08 by ReMan]

Because the battery earth will be a 0v and the chassis will be at whatever the output voltage of the welder is. I suppose it would give the same effect as short circuiting the terminals.

I always unplug the alternator and disconnect the battery. I've seen a few alternators damaged by leccy welding with then connected.

I'd also unplug any ECUs from the looms. It is easy to do and they're expensive to replace.

I wold (and do) remove any components fitted with semiconductor devices - from your list this would be ECU and alternator.

disconnect both battery leads.

don't forget to disconnect any electronic guages just to be safe.

[Edited on 2/6/08 by 02GF74]

On this topic

Is it possible for 2 people to weld simultaneously on the same car using 2 mig sets or are you likely to fry each others migs / or the car?

I'd say yes It is possible for 2 people to weld simultaneously on the same car using 2 mig sets

and I'd also say you don't need to disconnect the battery.

Think about it, you are connecting the mig ground to the chassis
Battery Ground is connected to the chassis

Both are nominally Zero volts...

So lets say you do a weld 6inches from the earth clamp...
Where do all the volts and amps and sparks go when you strike the arc? In the shortest path back to earth clamp, they don't go anywhere near the battery +ve terminal or anywhere else, so I dont see the problem.
Likewise, 2 migs are only ever going to do the same, the welding is with respect to the welders earth, nothing else.

Now as said good practice for sensitive electronics maybe, alternator maybe as they can be sensitive, but this is due to transient spikes of voltage that may be induced into the car, the same as lightening making the radio crackle, but there's certainly nothing that should break a battery.
In fact there is a case for leaving the battery connected, as it may well absorb some of these spikes better than taking it out of the circuit............

ReMan I was thinking the same. I was welding up my exhaust last night. Put the earth clamp on the outlet, and welded a foot or so from the manifold. I figured the current runs straight to the earth clamp. For good measure, I always disconnect the ECU and earth, as I am a scaredy pants. I might now have to add alternator to that list. No wonder everything takes so bloody long!

The reason for disconnecting the battery ground is nothing to do with the different potentials of the welder and chassis, the chassis is effectively floating with respect to the welders ground.

The problems comes from the large potential differences within the chassis that welding currents can create. This can cause large ground currents to circulate through any components that are now at different ground potentials, which can damage semiconductors.

Disconnecting the battery helps, but does not provide 100% protection if you have grounding points scattered throughout the car. The best way to avoid problems is to keep the welders earth clamp as close as possible to the point which you are welding, and ensure the ground connection is clean. This will discourage stray earth currents.