Fuel pump relay repair
I have an '85 500SEC. I have repaired my FPR. It took all of 15 minutes.
What you probably have is a bad ground/support.
Look on the backside of the larger of the two solenoids, at the bottom(pin end) of the circuit board.
The main bracket for the coil of copper wire is also the electrical ground, for the fuel pump relay.
I found mine to be cracked and corroded. Dissimilar metal.
I took a small stiff bristle brush; a little flux and my $15 soldering iron, and re-soldered the joint.
It hasn't missed a beat since(6mos).
I did buy a second FPR, prior to fixing the original. I found it to have the same problem. I carry it in the boot, just in case.
As a side note, the smaller of the two solenoids is the over-rev cut-out, I believe.
Good luck, John F.