Oil Cooler Seals: The oil cooler on the OM642 is not a radiator-style cooler like in the previous engines. The oil cooler sits in the bottom of the V between the cylinders and the oil gets cooled by the engine coolant. There are two $5 gaskets underneath the oil cooler that fail. The original gaskets are orange. The replacements are purple. They changed the material. Changing the gaskets themselves is easy. Getting to them is the hard part.
A journey into the the infamous Om642 oil cooler leak!
I did this, my story begins on page 4. But read from the beginning as the original poster has lots of photos and insight.
If you have decent mechanical skills and the right tools (good set of e-torx sockets and the special fuel line wrench) you can do this job in a long weekend. It is not nearly as daunting as I thought it would be.
The reference to the bell housing has to do with the fact that there is a drain hole in the area of the oil cooler that drains down near the bell housing/engine block junction. Thus leaking oil tends to show up there if the oil cooler seals are bad.
DEF is short for Diesel Exhaust Fluid. It is basically ammonia in deionized water. My grandma used to clean her house with ammonia so the smell makes me think of my grandma
The process is called Selective Catalytic Reduction or SCR for short. It reduces the oxides of nitrogen present in the exhaust.
https://www.dieselforum.org/about-clean-diesel/what-is-scr
The fluid is generic (you don't need to buy the special MB branded fluid, any fluid that is compliant with ISO 22241 is acceptable). Peak BlueDef is available at Sam's, Walmart, and most auto parts stores.
It goes in a separate tank, not the fuel tank. On the sedans (for sure on 2013, not sure on the other years) the tank is in the trunk so you get no spare tire (the car has MOe run flat tires). The SUVs (again I speak of 2014 because that is what I have) have a tank above the rear axle and you fill it through a second port behind the fuel door.
The transmission on W211:
Pre-facelift (2003-2006, including the CDI) use a 722.6 which is the same transmission used in the 1997+ W210s. This transmission is electronically controlled. The computer is external to the transmission. On 210s it is in the relay box on the passenger side of the engine bay. Not sure where they put it on the CDIs. The transmission has solenoids and sensors that communicate with the computer. They are placed in an assembly known as a 'conductor plate' to make it modular and easy to install. The conductor plate sits on top of the valve body. On the 722.6 the conductor plate has no intelligence, there are I believe 13 wires back to the computer, each solenoid and sensor is individually wired to the computer.
Post facelift (2007-2009 including the Bluetecs) use a 722.9 which is also the transmission they use on the first generation W212 (up to 2013 sedans) and W166 (up to 2014 SUVs). The conductor plate on the 722.9 has integrated electronics and no separate transmission computer elsewhere in the car. The electronics in the conductor plate have to be coded to match the other computers in the car and this can only be done by the dealership. This job is just south of $2k as the other poster mentioned.