Just completed a serpentine belt replacement at 63,000 miles. Some things I learned along the way.

Here’s a useful brochure on the M274 engine, with descriptive photos:
https://www.motor-talk.de/forum/akti...hmentId=758372

(All directions assume you are standing in front of the engine bay, facing the engine.)
Steps:
1) Remove air intake ducts and engine cover.
2) Belt tensioner is between the two large pulleys at the bottom of the engine. Visibility is limited, but the tensioner bolt is bright silver. I needed a flashlight (outside, in daylight) to spot it.
3) The tensioner release requires a 12 pt. socket. I tried three different 6 pt. sockets before I learned the trick. Tensioner rotates to the left.
4).The lock to keep the tensioner “open” is to the left of the tensioner release bolt, and on the perimeter of the assembly. I couldn’t see it, had to feel for the opening.
5) I used a 3mm Allen wrench to lock the tensioner open. 4 mm will not fit.
6) Removal/installation of the belt is straightforward. I worked from the bottom up, leaving the upper right pulley for last. It’s a tight fit, but doable.

 

@pkaia524 Thanks for posting. Is there a reason you replaced the belt? Mercedes does not specify a belt replacement interval, only a belt inspection from what I have seen.

Is the tensioner lock (3mm allen in your writeup) required? Is it not possible to just fully release the tensioner while removing the old belt and installing the new belt? Then re-tension the tensioner into position?

 

I replaced the belt as it started squealing at all speeds, but especially at startup. Upon removal the belt didn’t have any visible signs of wear

i tried installing the belt without locking the tensioner (since I couldn’t find the holes for the pin). After 20 minutes of wrestling with the belt and my temper while holding the tensioner open I listened to the better half of my brain. The belt has almost no play left even when the tensioner is open, and I couldn’t maneuver it over all the pulleys with the tensioner closed or one-handed with it open.

With the tensioner locked open it took me 2 minutes to fit the belt, working from the bottom pulleys to the top pulleys, leaving the tensioner for last.

if you had a helper holding the tensioner open you won’t need to find the pin’s location.

 

Hi there, thanks for posting this as I was also trying to see if any 6 point socket would fit (I was unsuccessful). do you know what size 12 point socket you used? Thanks

 

sorry, I don’t remember.

 

12 point , 17mm

 

When I did my our 2015 C300 4matic belt (squealing at 90k miles or so) the WIS did not give you any info on the tool for the tensioner and I could not see what it was. Finally I just used my cell phone in video mode and watched the video and it was like - oh ya a 12 point socket. Using a longer breaker bar makes it really easy solo job - no need to hold the tensioner via the pin.

Make sure to get the correct belt length! (use EPC to look it up - don't rely on web sites especially for the 4matics).

 

hello, can you tell me what can happen if the belt breaks ? No more alternator but car can still drive until battery is empty ?

Any other damage on the engine ?

Thank you.

 

I apologize for bringing up this old thread. I had a similar issue with my vehicle recently; I had a visible crack in my serpentine belt. I get a proper tool for the DIY then follow certain procedure on how to replace it. I came across an in-depth article that may be worth reading. Search for "Mercedes-Benz Serpentine Belt Replacement Guide on YOUCANIC" and you will see an article that goes over common causes and possible fixes. You can read about it yourself. I hope this information can assist someone else with a similar issue.