The car is a 1992 190E 2.3. It has been parked for about 1.5 years and in april I put it back together. During the 1.5 years, I did a lot of repairs on it like the suspension and also some welding to remove rust. I had the battery out of the car when welding and I was putting the ground strap close to the areas I was welding on. The car has a new battery since I "resurrected" it and I changed the alternator's voltage regulator/brushes 2 years ago but the car only covered about 10k miles since then.

The car started all the time with the new battery. I didn't drive it much, but it always started. This week I finished all the tuning and solving all the little bugs and I drove it to get it aligned. The shop had to jump start it to get it out of the shop, and I had to jump it again to drive it back home. It drove the 10 mile drive home OK. As soon as I parked it and shut the engine off, It wouldn't start anymore. Not even one turn or anything.

I tested the battery/alternator today after jumpstarting it:

Alternator output at idle (after revving the engine to about 3K once): 13.1 V
Battery voltage at idle: 13 V
Battery voltage after turning car off: 12.8 V
Battery voltage about 15 minutes after shutting the car down: 11.9 V

I tested parasitic drain and it looks good (almost nothing)

I tried disconnecting the battery + while the car was running and although the idle dropped for about a second, it was OK.

I had to have my jumper car run for about 10 minutes to get the Merc to start. As soon as I shut it off, it wouldn't start back up again.

Any thoughts?