welding on cars with ecu

As a minimum I would disconnect the negative battery cable. You could also pull the ECU fuse out. Don't mess with the ECU connectors unless you know how to unlatch them, they are easy to break if forced.

Blacksmith

besides those any other people have any experience with welding directly on the car

We routinely weld on new vehicles (trucks from 1/2 ton to tandem and 6WD plows) all with electronics to control the engine as well as electronics to control the salt/plow gear without disconnecting the battery . We use a device that connects across the + and - battery terminals to shunt any stray surges. We get ours from Snap-On but I would imagine there is more than one manufacturer, our slang name is a 'zapper' the actual name escapes me at the moment.
No matter what you use, be sure to have your ground as close as possible to the weld area...Mike

Two big things to think about when welding on modern automobiles.

1) While a good practice when welding on anything anywhere, keep the work lead (also called the ground clamp) as close to the actual welding as possible. I have seen a few damaged bearings and transmissions from folks not thinking about total current path! Bearing and transmissiona are particulary vulnerable components as the current makes a small arc across the metal surface as it travels from one piece of metal to another. This often leads to premature failure.

2) If using GTAW (AKA TIG or Heliarc welding), most systems initate the arc with superimposed high frequency. This is particularly damaging to electronic components. If you need to use GTAW, use the "scratch start" method, or the "lift arc" method if your machine is so equiped. William Roth, PE, CWI