Welding and Plasma Cutting

[Don4107]

The problem is that even if you put the welder's ground connection close to the weld and the parts are not directly connected close to the weld, when you strike the arc if it hits the part on the other side of the gap from the ground, the current has to flow around the gap through the rest of the structure. This can induce unwanted current in unsuspected places.  

To illustrate this, when the starter on our first bus failed, it was drawing a huge amount of current from the single 8D.  When it didn't turn the engine I went to the rear controls. When I hit the starter switch the tail lights appeared to light.    I don't claim to know what the path was to cause this, I just know that the two systems are only connected by the chassis/engine/body. The ground from the battery is directly connected to the engine block with a ground strap to the body. The positive to the starter solenoid.  

When the starter was rebuilt everything worked normally.  No computers/inverter/anything in 1968 GMC.

That said, I too have welded on vehicles without disconnecting anything without any apparent damage.  Mostly non-computer.