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bluesman123 11-20-2005 06:50 PM

Welding Question
Just a quick question. Someone told me not to weld on my truck as it will blow the diodes in the alternator. Is this true? Can I just disconnect the battery ground and the VCM and get away with it?

Thanks for your help,

Dave :thumbup:

Henry Highrise 11-20-2005 07:05 PM

All the welding books that I have ever read say to disconnect the battery cables before welding on a vehicle. But I have welded on vehicles for years without doing that. Never bothered anything. Just be careful around the gas tank, be sure you do not have any leaking fuel lines etc. Just use common sense.

bluesman123 11-20-2005 07:31 PM

Thanks for the quick reply, Henry. I'll try it.


oldred 11-20-2005 08:24 PM

Blues, Just make sure that your ground is on the part you are welding on and you will be ok. It is very much possible to run into problems if you, for instance, put your ground clamp on the body and then weld on the frame. In a situation like this if you are welding at fairly current and the body to frame ground is poor you could burn up a lot of things including the whole car! If you are grounded to ANY part and welding on another you need to make sure that the current is running through a good ground and not your wiring or some other critical component. :)

bluesman123 11-21-2005 10:07 AM

Thanks Red! I'll do it.


MARTINSR 11-21-2005 11:17 AM

Unless you hook the ground cable to the alternator I can not imagine anything happening, another "old husbands tale".

I do know that with late model computer controled cars, you should always disconnect the battery. But honestly, it is a CYA deal, I have NEVER seen any kind of damage what so ever concerning this.

I have however seen exactly what Oldred is talking about. I have seen hinge pins welded because the guy gounded the body and welded on the door. I have seen a throttle cable burn right in half because he grounded the exhaust and welded on the body! Put your ground as close as you can to the weld and ON THE SAME PART.


redlightning 11-21-2005 04:43 PM

I am a honda mechanic.They ask me to go to a body shop down the road and program a key for them on an accord.I took our scan tool and programmed the key and it would not start.I ask him did it start before.He said yes ,and all he did was some welding on the exhaust system.They towed it to the dealership and I found no output from pcm.I installed a known good pcm and then got a map code and found the map sensor open.I replaced both of them it ran fine.To take both those out.I really wondered if he didn,t have the key on maybe listening to the radio or something.
Brian,I lurk around the body forum often.I am not questioning your knowledge or experience.This one might have been one in a million that got zapped. I don,t know these guys so I don,t know what else they did.

MARTINSR 11-21-2005 07:58 PM

I'm with you Carl, I don't doubt for a second you can't smoke something, but I think you have to go out of your way to do it.:) I mean, you have to want to do it. :)

I use a "surge protector" when ever I weld on any computer controled car. Or unhook the battery of course. Even if it is a CYA thing, so is wearing seat belts and I do that everyday as well.


bluesman123 11-22-2005 09:41 AM

All sounds like good advice. On my truck it is so easy to disconnect the battery AND the VCM that I would do it anyway.

Welded hinge pins?? That must have been a shock! :mwink:


lowROLLERchevy 11-22-2005 10:14 AM

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Originally Posted by redlightning
,and all he did was some welding on the exhaust system.

key phrase right there

all my exhaust welds are done with a 90* bracket and a hose clamp that when on the pipe, give me a handy place to connect the ground to the pipe

ive seen MANY MANY techs slap the ground on the frame while welding a new exhaist tip or muffler on .... pretty easy for power to travel up the ehaust (because chances are its rubber mounted) and into the ECU through a coolant sensor or something

incredibly rare, but possible

i dont see how disconnecting the battery raly makes a difference tho, all your doing is decucing the # of possible power supplies from 2 to one ...

firebird_red 11-22-2005 03:18 PM

The key is correct placement of the ground clamp. If you run welding current from the exhaust pipe, through the engine back to various ground connections, it won't help much that you disconnected the battery. The ground needs to be next to the welding so the current flow path isn't through the engine electronics.

oldred 11-22-2005 05:48 PM

Firebird, I think the same thing, I just can't see how unhooking the battery will help all that much but it seems that a lot of people say just unhook the battery and it will be ok. I think not.

milo 11-22-2005 10:20 PM

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There is a do-hicky from Matco Tools that can help protect also. The idea is you won't have to reset clocks and radio stations etc. by hooking to the battery during welding operations.

If you let the smoke out of some computers it'll kill em. :nono:

oldred 11-23-2005 07:38 AM

The problem is that body to frame grounds are not designed to carry a weld load of over a 100 amps or even more, or at least I think this is the case.

kringold 11-23-2005 07:42 PM

There is no such thing as a ground clamp or ground lead. It is a work clamp/lead. It does not ground the circuit, but completes the circuit.

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