Originally Posted by Crossbow39

It is the fuel pump relay. We had the same problem...sometimes start every time for days, then a no start, wait a while and then starts just fine. No rhyme or reason; dealer could not find anything wrong and no codes so take it back home and hold your breath. Then the no starting increased in frequency in addition to the car stalling out while driving on the highway (nothing like losing all power without warning and coasting down at 60 mph+)! It was completely unsafe and untrustworthy...you never knew when it would work or not.

After talking with Mercedes owners and checking the Mercedes bulletin boards (type in "fuel pump relay" and read all their posts and problems!) I replaced the fuel pump relay and the problem has not happened again. We now have @ 91,000 miles on the car and the relay was replaced at @ 43,000 miles.

Evidently, the soldered contacts on the printed circuit board that control the fuel pump begin to fail and corrode over time. When the contact is broken, the fuel pump shuts off, and the car will not turn over or crank. You still have all your lights, gauges, and fan...so it is not a battery problem like everyone's first instinct. The computer is not receiving a signal from the fuel pump, so it will not crank the starter (part of the anti theft I believe). Plus, the computer sets no code because it is working like it should...the pump is not running so no crank is needed! Meanwhile, you are sitting there feeling quite "frustrated".

The contacts get pitted/eroded so the circuit is broken, usually at first as the heat rises after driving it a ways: that is why it usually starts at home, but strands you somewhere else. When the circuit board cool down, the metal solder shrinks, contact is made, and the car starts. What is bad is it will continue to get worse. Everytime the contact breaks , the spark erodes a little bit more of the solder joint and your problems will happen more and more often...and still no codes will be set in the computer.

The relay costs about $150 at Mercedes (I bought it there rather than Chrysler) and my part number was A 170 545 03 05. You need to replace the whole relay and not just the fuses on top, because the circuit board is inside the box (after replacing we took apart my old relay and you could see the burned and pitted solder joints...but only with a jeweler's loupe...that's how small we are talking about). It only takes about 15 minutes to replace. It is located up by the battery in the main fuse/relay box. This relay has the big orange 40 amp Maxifuse, one 20 amp yellow and four blue 15 amp regular fuses on top of it and also 4 (I think) harness connectors that plug in into the top of it. You undo the harnesses, switch in the new box and that is it. Easy enough to do on the side of the road if you have the part.

Sorry for the long post, but I understand the frustration from this happening and no one finding anything wrong with it or acting like you are doing something wrong in starting the car. Like I said, check out the Mercedes boards for fuel pump relay, then go get a new one. Best $150 bucks I ever spent.

Crossbow39