I have a 2006 ML 500 (4JGBB75E56A053890) that out of the blue didn't start after work one day, after sitting for about 8 hours. It would crank but not fire. Just happened to have my scanner in the truck and had absolutely no faults on anything in and around the engine! A couple for things on the back right door as I'm working on the lock and have things disconnected at the door. Called for a tow truck, while waiting, about 45 min, I looked things over again and pressed in the valve at the end of the fuel rail to see if there was any fuel present, there was. Then I tried one more time and it fired! Ran perfectly smooth, no hesitations, missing, nothing but a perfectly smooth running engine. Drove home, shut it off and it restarted right away. Again scanned it and nothing! Next morning, fired right up, so I went to work. Nothing for the next several days, everything seemed fine just like it has for the past 3 years..
Ran for about 2 weeks and then, bam, cranks and won't start again after work! Again, no codes! Looked everything over, nothing seemed wrong, but when I checked the fuel rail, no gas this time!
Tow truck home, scanned again at home, no codes. So since there were no codes, no fuel on the fuel rail, checked relay O and fuse 57 in the back fuse panel in the passenger side luggage compartment. Both good. After pulling up the access cover for the right fuel pump, I had voltage to the pump wire (RDWH) on the connector to the cover to the pump on the tank. I opened the access cover as I was pretty confident it was going to be the fuel pump and had already ordered a fuel pump.
So now that the new pump is in, it still isn't starting, cranks but no fire! Still no fuel at the valve on the fuel rail on the engine!

What simple thing might I be missing now that I swapped the pump?

 

Do you have a Star diagnostic system? A full set of data will help in this case.

Do you hear the fuel pump running?

 

No on the Star System.
The fuel pump is not running.
Good ground at the connector at the top of the fuel pump cover. Have voltage for about 2 sec after the ignition is turned on. Then have it again when you try and start, the whole time it cranks. Pulled the cover off the top of the tank, pulled the fuel pressure line off the pump and turned on the ignition. No fuel coming out of the pump! Pulled the pump and I put 12v to the pump directly and it runs. Pulled the fuel pump relay put 12v on the coil and measured the contacts were made.
I'm starting to suspect the contacts on the relay are arced up allowing the meter to read voltage but not allowing enough current to drive the fuel pump.

 

Yep, it might be time to throw parts at it until it works. Relay seems like the easiest one on the list.

 

Have you found your problem yet?

 

Yes, it's running again.

After changing out the fuel pump relay I still had nothing! I took the old relay apart just to find no pitting or arching on the contacts! I used the old relay to jumper 12v directly to the pump to see if it would run. At first it didn't. I tapped the wire on the pin in the open relay several times to make sure I had good contact and it finally started pumping. I put the new relay back in and the car now started! It's been running ever since!

I was talking to a friend who's a certified master mechanic for several European car makes and has worked most everything from Europe. He mentioned that most European manufacturers used aluminum connections in their fuse panels. As in a house with aluminum wiring, heat and the dissimilar metals is a bad combination over time. He said that they would get cars in where they had different electrical problems that ended up just basically being bad connections in the fuse box especially in older vehicles. They'd pull the fuses and relays, use contact cleaner on all the plugs and put a dielectric on them as well and put everything back, solving most of the problems. Being the fuel pump relay is on the whole time the car is running it gets warm. This warming and cooling over time with the aluminum and coper eventually can cause resistance to build up. Thus a meter will read the voltage, like I saw, but there is insufficient current capability due to the build up, which also creates more heat adding to the problem. In your house, unchecked, that can lead to a fire!

One good thing, in my opinion, Mercedes is switching the 12v on and off to the fuel pump unlike those Fords that were catching on fire because they always had power on the fuel pump and switched the ground on and off!