The vehicle: 2015 Mercedes Benz ML-250 Bluetec diesel. The car has symptoms of grating clicking sound when the car accelerates or is going uphill. Additional symptom is leaking oil. Many of the online sources for this are very good on YouTube. The problem is that there are very few videos about the ML-250. Most are about the ML-350. No problem, these in fact turned out to be nearly identical. As usual, I won’t rehash what is already in the videos.
- Remove front and mid undercarriage covers (plastic skid plates)
- Mark the front and rear drive shafts so that you can re-assemble them easier.
- Remove or loosen some of the heat shields covering the rear drive shaft so you can get at and remove the stabilizer that holds the drive shaft. You need to do this to get enough slack to get the shaft off of the transfer case.
- On mine, the drive shaft bolts were easy to remove. No lock-tite was present.
- Then there is a wide aluminum brace that holds up the transfer case. Before you can remove it, you should support the transmission on another jack. The ML-350 videos show that as a steel crossmember, but mine is a flat piece of welded Aluminum. That bell-shaped gizmo with the two bolts sticking out is one of the mounting points. It is more like an engine mount in that it have rubber components inside to isolate vibrations.

The picture below shows a rectangular weep hole, pointed at the screwdriver tip along the seem in the case, that is the source of the oil leak. The point of that weep hole is to let leaking oil escape when the interior seal fails. When that happens, you must remove the transfer case from the transmission to get at that seal.

Then to remove the transfer case, there are 8 special star-hex bolts. These are a number 12. I had one that had a mildly rounded off head and a number 14 was a better fit. The two bolts at the top of the case are pretty much inaccessible. To get to them, you need a wrench that turns back onto itself. I tried using a crows foot. of course, they don’t make those in star-point sockets. The 10 mm crow’s foot has a close fit, but couldn’t really grip the bolt and only served to round it over. Internet guys recommended this doohicky from China Freight.

Total fail. Then I remembered that I had a welder and could create tools at a whim.

A very cheap set of crows foot wrenches from China Freight sacrificed themselves for the cause of science.

First three attempts failed, but the fourth attempt resulted in the device below, which when hooked to 2 long extensions and a regular ratchet wrench got the job done. I cannot emphasize enough that all 8 of these bolts were EASY to remove. If they have been over-torqued, lock-tited, or corroded, they would not have been possible to remove without dropping the whole transmission.

Opening up the case as we can clearly see the chain is very loose and has stretched over time. That is the cause of those clicking sounds. Under tension, the chain slips over the top of those gears. You will also note that the remaining oil in that case is brown. It should be red. This transfer case is supposed to be filled with ATF fluid. What is in there smells like 30 wt motor oil.


The new chain fit in there very snuggly with no drooping at all. When I put this in, I was careful to ensure the white marks on the case were still lined up. The upper half of the case has a magnet that need to be cleaned and put back in. I used a smidge of RTV to keep it from falling out when I re-assembled the case.

Then, as the instruction said, put everything back together, using RTV (silicone sealant). Follow the instruction on the package. Then torque the case bolts back on at 22FT-lbs. While this was on my bench, I also filled it with about 1/2 L of ATF fluid.

Now I need to remove and replace that leaky seal. In the picture below, you can see the leaky oil inside the case. That is confirmation that this seal is what was leaking. You can also see the snap ring with the blue and green marks on it. Normal sized snap ring pliers or needle nose will not work. Just do like I did and run back to China Freight for the big wall of pliers and grab the big-ass snap ring pliers. I bought the last one, but they will likely restock at some point.


Here you can see the difference in flexibility of old and new chains. (new on is on top).


Then after some lifting toting and grunting on my back, the resealed and rebuilt transfer case gets put back in where it belongs and all is well. Once installed, I removed the fill plug to check the proper level of fluid was present. 1/2 L was just the right amount. The parts I skipped over were not very important and the YouTube videos covered them very well. It is also worth noting that having the rear wheels off the ground will assist you in re-aligning the drive shaft bolts. If they are off by just a little bit, they will not go back together.
Total cost of parts and special tools was about $100. The MB dealership will not do this work. It doesn’t price out enough labor to make it worth their effort. Their repair is to replace the entire transfer case for estimated cost of about $10,000. They won’t replace that $20 seal or that $60 chain. Their motto is: If you can’t afford it, you shouldn’t have bought a luxury automobile. Go back to Chevrolet, Peasant.



