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I have a 2008 C300 4Matic and I am having a problem where nothing seems to stop making my tensioner bounce.
I have put on both new OE idler pulleys, new genuine belt, and now two belt tensioners. The newest one is an INA OE manufacturer as well.
It still bounces whenever I put it into drive. No other pulleys have any noticeable play or noise, besides the alternator pulley which has a *very* small whining noise, but no wobble to the pulley at all. I’m honestly stumped on what else could be causing this. I have a vibration at idle that went away 100% when I ran the engine without the belt, so I’m certain that the tensioner is the root cause of the vibrations. I just doubt I got two faulty tensioners out of the box in a row, though not impossible.
I’m truly stumped, any help at all would really greatly be appreciated.
Last edited by hmclaughlin67; Jan 23, 2025 at 03:00 PM.
I found an old forum on here saying some of these W204’s take a 2390mm belt and some a 2404mm belt. However, the belt I bought is a 2309mm according to the website, but I bought it directly from Mercedes and it says it’s an exact match. Could this be the problem?
According to mbpartscenter.com (Mercedes-Benz or Scottsdale) they sell two belts that fit my car. A 2392mm and a 2409mm. Should I order the smaller one instead? Is there any way to tell which one I need? I already checked the owners manual and it says 2404mm but even Mercedes themselves don’t sell that anymore.
If your belt is correct have a good look at the alternator. These models have alternators with clutched pulleys that allow the alternator to freewheel when the engine RPM decrease and at idle, preventing belt slippage and reducing strain on the serpentine system. If the clutch is seized, which is common, that might be worth investigating.
If your belt is correct have a good look at the alternator. These models have alternators with clutched pulleys that allow the alternator to freewheel when the engine RPM decrease and at idle, preventing belt slippage and reducing strain on the serpentine system. If the clutch is seized, which is common, that might be worth investigating.
Thank you. If it is seized, can it be replaced seperately or is the only solution to get a new alternator?
In my 2010 operators manual it says the poly V belt size is 2401mm. I have the M272 engine. I bought and installed the 2404mm belt and have no issues.
I think I remember you posting a few videos about pullies and i did specifically look at the tensioner marks to see where the belt length was showing and it did look within the lines, but I could be wrong.
In my 2010 operators manual it says the poly V belt size is 2401mm. I have the M272 engine. I bought and installed the 2404mm belt and have no issues.
I think I remember you posting a few videos about pullies and i did specifically look at the tensioner marks to see where the belt length was showing and it did look within the lines, but I could be wrong.
Hi Tim, you’re correct that was me. This is the weird stuff I’ve had to wrack my brain through recently though. MB has the belt I had installed as 2409mm. I ordered directly from them. The part number matches, however when I removed the belt today it actually says 2397 on there. I’m not 100% sure that’s the length in mm, but it’s close enough that it makes me wonder.
Today I picked up the 2390mm belt just to give it a shot and see if it fits better, and it says 2392 on there. And sure enough, my wear indicator you mentioned on the tensioner is now exactly on the “tight”-most mark without being over it with the new belt. I’m going to run it for a few days and see how my car likes it, while I keep the old in the trunk just in case it breaks it.
Also worth noting if you remember Tim, my tensioner had a lot of jumping around, and when I installed this new belt to try, it was much more of a “wobble” than a jumping now. Which obviously isn’t abnormal to see on these automatic tensioners. I’m going to give it a few more days experiment and if it doesn’t work I will just bring the belt back to the dealer and put back on the old one, but I’m certainly a little confused on what dimension the “old” new belt I put on actually is.
Thank you. If it is seized, can it be replaced seperately or is the only solution to get a new alternator?
I understand that they clutch can be changed generally on MB's but I don't know specifically about yours. Changing it is typically a short term solution. Others will know more about the success of changing it.
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