Jack:

I've been down the road you're on now. I too have a '91 300-SEL.

To verify the fuel pump relay circuit in the MAS was the issue, I removed it and jumpered pins 1 & 2. They are closest to the outboard side of the MAS. When they're jumpered, you should hear the pumps run and if the car starts, that part of the MAS(fuel pump relay) was bad.

Be careful when you jumper them as it's a hot circuit and if the jumper wire gets just a hair caddywompus, you'll get a spark. I usually pull the grd. strap off the battery first, then set the jumper wire in place, then reconnect battery.

The MAS is a costly gadget. I'd like to find someone who could open and repair them.

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