My daughters '90 190E 2.6 did the same thing many many moons ago, it wasn't getting fuel. I found a relay behind the battery, it's one that has a locking device on it to keep it in place (a twist knob on top).I replaced the relay behind the battery...it still didn't work. I took the cover off and found a small relay on the board that would enable the fuel pump. I fashoned a tiny wedge of wood to slide into the relays frame to get me to the dealer.
Took it to the dealer and they worked on it from 7:30 AM till 7:00 PM, I stayed there being it was a hundred mile one way trip, I'd go into the shop about every half hour to see what was going on. The mechanic had help from every other mechanic in the shop. I saw every kind of meter and piece of test equipment possible on the fender at various times. He even had another mechanics 190E pulled in next to it and was swapping parts and comparing data. The service writer came out several times and apologized for taking so long. At the end they claimed that there was a burned wire in the loom that goes from all that control stuff behind the battery over to the left front fender area. I've since looked for rewrapped looms and repaired wires to no avail.
Not too long ago it quit on her all of a sudden while driving along. But different to your problem, it would'nt start, battery was too low. Everything checked OK (bat & alt), I drove until I finally proved the alternator was intermittent, was going to swap it out till I found out (through this forum) that the brushes were an easy fix.
Hang in there , you'll get it, it's a good tough little car.