When MIG or arc welding on a car always disconnect the alternator by removing the wiring carefully and labelling each wire as you take it off to ensure it goes back onto the correct terminal. There's no need to remove the alternator completely, just remove the wiring from the terminals at the back of the alternator. Strictly speaking, you don't need to, but I always disconnect the car battery as well just to be on the safe side. The reason you need to disconnect the alternator is in order to prevent damage to the alternator from the electric current which has to pass through the vehicle when MIG or arc welding is carried out. You need not disconnect the alternator though if you plan to use gas welding. But gas welding, of course, throws up its own separate challenges, mainly in that the heat diffuses and spreads much more than with MIG and you have to be extra careful if welding anywhere near fuel lines or the fuel tank. In these circumstances, shielding may sometimes be sufficient, but by far the best option is removal.
