I find myself having to do some welding on the exhaust of my GF’s street car (98 Nissan 200sx). What precautions do I need to take to keep from damaging the electronics in the car?
Where and what are you welding on the vehicle?
Exhaust pipe to muffler area. A flange broke loose. Crappy stuff to weld due to the rust, but it’s a cheaper alternative at this point to replacing the entire exhaust including the converter. Cheaper at least if I don’t blow out the computer or something.
If you were wondering about the gas tank, it’s fairly close, but I’m not really worried about it. Sparks are on the outside. Fuel vapors are on the inside. It’s all good. 
Couldn’t you remove the exaust parts from the car before welding?
Removing it would cause a lot of welding to put it back on the car, which kind of defeats the purpose of removing the exhaust. This is a northern car from states that have salt on the roads. All of the exhaust bolts are extremely rusty. They won’t turn out. If the exhaust comes off, new stuff is probably going to have to go on. The thing is, the pipe itself and the muffler both seem good. The flange used to connect the pipe to the muffler broke. I believe the metal to be solid enough to weld back together (if not fun). Thus, it seems to be the quickest and cheapest solution for a car that we aren’t likely to keep for more than another year.
I remember seeing a car having exhaust welded at an exhaust shop. They clipped something on or across the battery terminals that they said was to protect the electronics in the car. I have no such gizmo to clip to the battery, so I’m mostly just wondering if disconnecting the battery is good enough or if there really isn’t much concern in the first place.
I don’t think there would be much concern in the first place. Maybe to be safe I’d disconnect the battery, but other wise it sounds pretty straight forward…I’d just weld it up in there and forget about it.
Disconnecting the battery won’t do anything except potentially protect the battery. Remember that you’re putting electricity into the car with the welder power supply. You’d want to disconnect every bit of electronics that you have, which isn’t all that practical. Your best bet it to connect the ground connection as close as physically possible to the point(s) you’ll be welding at. This will result in the smallest loop for current, and at the kind of current you’re pushing, the loop could act as a transformer and fry stuff, too – so the small loop size is really a good idea. Maybe a bona fide EE can make an argument for turning on all of the electrical accessories that you can. If you consume the power that you feed it rather than try to resist the power, that may be helpful to prevent damages.
Whatever you do make sure that you do your welding in a well, and I do mean well ventilated place so as to ensure that gasoline fumes don’t reach the right fuel-air ratio for combustion.
This might be a rather remote possibility with current fuel tank/lines design, but the consequences are disasterous if there is a fault in the tank/lines system somewhere.
That’s for sure! The Howe fire-truck factory here in Anderson burned down several years ago. The fire started when an employee brought his own car in and attempted to weld on a trailer hitch. :eek:
Well the first question is just what type of welding are you going to do? Most exhaust welding that I have seen is done with a gas torch which, assuming you don’t set fire to the control unit, presents a zero risk to the control unit. 
Assuming that you are using some type of arc welding, the risk varies with the frequency used, where the welding is done, where the ground clamp is placed in relationship to the control unit and where the welding is done.
If you are welding on the exhaust, and keep the ground electrode on the exhaust, IMHO you are probably good to go.
When in doubt, disconnect the battery, and remove the control unit(s)
I worked on this last night. It took longer getting the car on and off jack stands than it did doing the welding. I used a mig welder. The ground electrode was on the exhaust pipe about 4" from where I was welding. I decided to leave everything connected. Other than not being able to weld 360 degrees around the pipe and flange due to obstructions, everything went fine. The car had about 60 miles put on it today with no problems. I’m not saying that leaving everything connected was the right thing to do. I may have just gotten lucky.
Anyway, thanks for the replies everyone.