Coming back from OH years ago in my 6.3 300 SEL, I stopped to get the kids some ice cream and the car wouldn't start. After a while I realized I couldn't hear the whine of the fuel pump. I tapped the fuel pump with the hammer from the MB tool kit and the fuel pump started to run. Now we all know what the hammer is for in the Mercedes tool kit. I had to do this 2 more times to get home.
I took the fuel pump apart and found it had bad bearings. I had another fuel pump that was electrically bad so I was able to make one good fuel pump out of the two bad pumps.
Cost: about 2 hours labor. Sure beats paying $500.
P E H