I had a very similar issue with a '91 300-SEL.

Fortunately for you, the 88 & 89 models have a non-integrated fuel pump relay which may be repaired as mentioned above or replaced for not too much. The 90-91 models have about 6 or 7 circuits integrated in a MAS unit to include the fuel pump relay and it's about $450.00. You're lucky.

When I got the car to start and run successfully after jumpering pins 1 & 2 in the MAS, I replaced the FPR and the problem went away. There may be a similar approach available for the FPR in your car. I believe it's near your battery.

About a yr. later, the car would stall after heat soak(car fully warmed up and shut off-then restarted 5-10 mins. or so later). The vehicle would die pulling away after restart. Removed OVP, did the shake test - I heard rattling inside - replaced - no more stall after heat soak.

I'd bet a buck it's your FPR or OVP or a combination. Dig around the SEARCH facility for ways to diagnose the FPR in your vehicle and remove/shake your OVP. If you hear rattling replace. Just becuase it doesn't rattle when shaken does not means it's well. I believe the ABS light will also come on if the OVP is bad. Again, the SEARCH function will keep you busy for days reading about the OVP.

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"