Hello All.

Lurker for a while, posting for the first time.
Proud Owner of a Canadian 2012 W204 4D with 2.5L V6 4Matic

My serpentine belt has been fine, but I decided to change it because I don't know its age.
To know the correct length, I went to my Indi guy who used my Vin that he already has on file in his software to retrieve it and then took a picture of it (below). The part number according to that is A0039937396 and length 2390mm.

Serpentine Belt Part number from Indi Mechanic

I bought a Continental belt and installed it. However, it was too tight and also quite sqeaky. I also noticed that it's hanging a little bit from the tensioner pulley. The tensioner pulley mark can be seen well outside the two marks between which it is supposed to stay. Image of the new belt below.

New Continental Belt too tight and squeaky and hanging from the Tensioner pulley

Ok, so now I have put back the old belt which is working perfectly. the tensioner position is about right in middle of the two marks, there is no hang, and no squeak. Image below.

Old belt gives correct position to tensioner and there is no hang from the tensioner pulley

Now my question is, did my guy give me the wrong part number? He pulled it from the software that has my VIN. (I assume, I wan't looking at the screen)
The old belt is definitely a little bit longer than the Continental one I bought.
VIN is WDDGF8FB2CA697910
I should add that this model (2012 W204 4Matic with M272 2.5L) was sold only in Canada, and not the US. In the US, 2012 W204 4Matic started with C300.

-Altaesar

 

Bought my 2010 C300 4matic sport used and changed the tensioner due to it bouncing, changed the belt also. In the Operators manual in the glove box it even lists the belt size.



Mine says 2401mm. The place I bought the replacement from, EEuroparts , had the Continental 2404mm for cheap so 1 got that, fit great.. Funny enough the belt I removed was a 2390mm. The Mercedes branded belt i was finding was a 2397. The difference is only a few MM. .

I dont know but I want to say maybe the new belt wasnt installed right somewhere. Try installing it again.

Ill add if you plan on keeping the old belt its a good idea to mark the way it was installed, i read the belts wear a certain way to the pulleys over time. Not sure how much it really matters but its what I did. I keep my old belt in the trunk where the spare tire is.

 

Thanks Tim.

I got mine used too. Unfortunately, no manual in the glove box.
I am quite certain I installed the new one correctly. I ran it for a few minutes. Then took it out and installed it and ran it again. It would surely have been shredded had it not been properly installed (I may be wrong here)
I compared the lengths and the new one is surely shorter, maybe by a centimeter.

My plan was, as you advise, to keep the old one as spare for emergencies. But it is now installed back to where it came from. Luckily, I had known about marking the orientation, and had done that when taking it out. Thanks for reminding !!

So, now, I will probably get the next higher size with Continental which is 2404 mm. Continental PN 4060945. But I am still wondering what the correct length for my vehicle is.

 

Pull them both out and stretch them out together and see if there is a difference.

 

I ran a 2012 C300 through a few websites and came up with part# 4060945, which is 94.5 inches. It says the 4060940 is 94 inches.

I just double checked and sure enough my old belt is the 2390. New belt is 2404. Thats not to say the 2390 belt is the right one, could be the wrong one and why the tensioner was banging around skipping and why I felt the need to replace it. The new tensioner and 2404 belt seem to be working great, smooth. been a year now no issues.

 

Great!
So 2404 is the one I will go for.
I had a similar thought that a super tight 2390 will f**k up the tensioner. maybe some pulleys and bearings too

 

You may want to replace the tensioner, as well. Remember, belts will stretch over time, hence the tensioner. Is there any play in the pulley part of the tensioner?

 

No play in any of the pulleys. I checked them all.

 

Does the tensioner pulley spin smoothly?

 

Yes, it does. There is no obivious or subtle resistance or grinding felt when I rotate it. Very smooth.

The entire system works perfectly with my old belt installed, My old belt just doesn't have any markings.
And I don't feel I can measure the length with accuracy down to 1-2 mm. Then there is stretch, so no point doing it anyway. It's definitely at least a centimeter longer than the 2390 that was recommended to me.

 

In my opinion your old belt looks fine at where its lining up with the tensioner markings. Unless its cracked. With mine you could see the pulley on the tensioner bouncing, the pulley wasnt staying in the same spot so I figured it was bad. When I replaced it I noticed how it looked newer, it wasnt as dirty as the surrounding engine. so it was replaced at some point. Carfax does show front end damage at some point but besides a few cracks and zip ties on the lower front bumper i cant tell what was done, every part in there is mercedes so it was fixed by Mercedes if so. I had researched and found out Gates makes the Mercedes tensioner so thats what I bought which happened to be less expensive than most. I replaced the belt because it was pretty cheap around $15. I also bought the ribbed idler pulley but the old one didnt seem bad and probably didnt really need to. The smooth idler pulley seems fine, no wobble or noise. Plus the belt wasnt the original belt since I could still see the markings on it, im assuming. Now im wondering if the 2390 belt made the tensioner fail prematurely. At first i had the mentality to replace everything i come across, but now parts are really adding up so i dont just replace parts for the heck of it. I would go on websites and start filling my shopping cart with parts i think i should replace, then id see the total and go back and ask myself if it really needs to be replaced at this time.


Same exact tensioner minus the Mercedes stamp.

 

mine is a 2010 c250 V6 the original belt says A0029933096 6K 2392