I am about to replace the underrun bar on my truck. It has rotted away and I'm replacing it with a 400mm galvanised square hollow section. I need to tack weld the end plates insitu then remove it and fully weld the joints. Please can anyone advise me what electrical/electronic precautions I need to take as I was intending disconnecting the battery and possibly unplugging the engine ECU this was mainly due to possible EMF created. Does anyone have experience of insitu welding on their truck and whether there any risks.

 

Also, disconnect the alternator. Attach the weld ground to the piece you're welding and make sure you strike the arc on the piece the ground is attached to.
Grind the weld area to bare metal as well as the ground attachment point.
DO NOT breath the fumes (use a fan to blow them away from you).
Zinc fumes are very toxic.

 

Thanks for the info antichrist it is much appreciated. I didn't think to even consider the alternator but obviously it has diodes embedded. Thanks also for the safety tips as I'm usually a bit of a 'chancer' on my own (hence all the scars). I'm good at handing out advice to others but often ignore safety precautions myself

 

Just make sure you have a really clean and good ground CLOSE to the repair. We can debate for years about disconnecting electronics. Disconnect if it makes you feel safe or warm and fuzzy inside. Good luck!

 

As far as I know. Take the battery earth off. Ground the piece as close to the actual weld. Electric, path of least resistance etc. everything else doesn't feel a thing.

But, if it has a plug and your concerned and it's easy. Whip it off.

 

X2 on alternator disconnect, I have seen them go bad on many occasions & I have seen them survive in some cases, why take a chance!

 

battery ground should do it, it does on every other vehicle

 

Just re reading this. Yes. The biggest thing is the health risk from the galv. You can grind it off the bit that will take the heat, so it doesn't create fumes. But some will invariably still be produced. Also then the bit that you have ground will rust, so needs a coat of something.

You're only tacking it in place while under the vehicle. Earth off, really should be enough.

 

It can be difficult to grind it off inside the tube. That's why I said use a fan, so the OP can be upwind of the weldment and the fumes.

Tacking it is the most risky time because there's no bond between the two pieces.
Should be is the key. But when doing what he's talking about the current can potentially take the long way round to the ground clamp if the arc is struck on the other piece (I've had it happen). That's the reason I said as close to the joint as possible and strike the arc on the piece the ground is attached to.

 

Good advice guys I really appreciate your inputs.

I've ground a 1/4" mitre all round on the 40x40mm SHS tube but accessing the inside is a different and difficult game so I'll tack it up with a couple of 1/2" tacks one each side each end and do it in the open air with a fan blower across and away from me under the truck and ensure I strike the arc on the tube and not the end plate and then fully weld it out on a bench in the fresh air outside with the fan blowing across between me and the work piece, fortunately I have one of those full face solar activated mask thingies to direct the fan into. I will use some passivate and cold galvanising paint (galvafroid) when it's finished to avoid rust. These underrun protection bars have a nasty habit of rotting away with rust so I decided to fabricate this one as a 'heavy duty' galvanised replacement once and for all.