Experienced some long cranks to start the car. I had this kind of issues with my 2010 E550 and on that one replacing the starter fixed it all so I thought it is the same problem but the new starter did not help this 2013 S550.

So, focus turns into the Crank Shaft Position sensor. Searched these forums, youtube and the internet in general but found very little info about the sensor change on 2013 model. Lots of info for older engines but the 278 does not have much other than where the sensor is located.

In general the information I could find says the sensor is located in the back of the engine at 10-11 o'clock position (looking at the engine from behind), it takes E8 female TORX socket to open the one bolt and you can get access to it between the engine and firewall.

So, first to see the exact location of the sensor I used an old small security camera and found the sensor at area 10-11 o'clock position but for a big surprise the bolt was not of the type I had found information for. In my car the bolt is a pan head bolt with female T-30 end. No problem there, I have the tool. Bolt (machine screw) is M6 x 1 x 16.

But the problem is the access to it. The room between the engine and firewall is so tight I was barely able to put my arm down enough to touch the sensor and the mounting screw. At the top of the engine there is a plastic vacuum tube that runs to the brake booster. Luckily I could bend this tube enough but the firewall will not move and then there is a rubber hose clip with sharp edges...

Anyway, I got it done and how it went is I used an eight inch flexible extension from the T30 key. In the end of the flex extension I had a 10" solid extension. With these I was able to go around the back corner of the engine to the screw. Took awhile to figure this out as first I tried using the universal joints but was not able to keep the key on the bolt with these. Flex extension allows axial force use and you need that for this job.

Electrical connection has that Mercedes tab that you need to pull up first before you can press down on the lever to unhook the harness. This was another task as my finger nails were not strong enough to lift the tap. The camera I have was a great help as with it I could see the tap for directing my hand and very short flat screw driver in place to lift it.

For installing the screw back I used electrical tape to hold the screw on the tip of the TORX key. I put a piece of tape on the screw head and pressed the key thru it in the screw. The tape did not break, it stretched between the key and screw head and wedged the screw pretty well in place in the end of the key. With this I was able to get the screw back in the correct location as it was not possible for me to hold the screw in my hand and put it in place as I could just barely touch the screw in its location.

Engine starts with the new sensor are very much improved at least the about 15 I did after the job. Some of them with cold engine, some with hot. Did not experience any long cranks. Now engine like "explodes" to run at start every time when before this happened only occasionally.