For the second time in about a year and a half my serpentine belt was shredded to pieces and it seems to be due to the pulley being worn out while likely led to the belt slipping.
The first time around it was genuine Mercedes parts and the second time around it was from Amazon https://a.co/d/iYhc8Hc
I imagine that the usual failure of this would be due to the belt wearing out, but the pulley looking badly damaged makes me wonder what was the cause and what was the effect. Did the pulley go bad and destroy the belt or did the belt go bad and in the process of being shredded, destroy the pulley?

Or is there another underlying root cause to all this? Could a faulty engine mount cause excessive vibration and cause this? I don't feel any extra vibration while driving.
Or is an oil leak leading to the belt wearing out quicker?

Very painful to face this problem twice within such a short time.

Have any photos of the damaged pulley? Which pulley was damaged?

I have the M272 engine and I have a 2404mm belt in there and it seems perfect, runs smooth and the wear mark on the tensioner is right at the beginning where it should be.

First cause i'd look for is installation error. Were the bolts properly torqued when installed? Loose bolt could lead to a pulley wobbling then failure of the belt.

I bought and installed INA upper and lower idler pulleys, Gates brand tensioner and Continental belt.

I cant comment on the Amazon parts. How are the pulleys now, are the bearings gone?

I would also look at all the other pulleys for issues. Inspect the crankshaft pulley rubber damper for cracks.

What exactly happened when the belt failed, any noises, vibrations, or just out of the blue?

Below is the picture of my pulley. It is completely chipped and destroyed. There are shavings/debris all over of the belt and the pulley. It is the idler pulley similar to the one shown in this video The pulley is close to the part above it. Is is possible due to vibration it hit the part above and got chipped?

No engine vibration is gonna cause that. If no oil is on the belt then that’s not an issue. The likelihood of 2 belts breaking because there poor quality is low. Has to be the pulley or alignment. I would get a new pulley and belt and verify alignment it correct.

I do have an oil leak but since the belt is shredded I don't know whether there was oil on the belt. The last time I got it serviced by the dealer there was a recommendation to change the cam magnets. I'm guessing that is the cause of the oil leak?

Did you ask why he recommended changing the cam magnets?

I’m doubting oil anyway. Has to be something with the pulley or alignment. Spin all the pulleys and make sure they spin freely. Get a new idler pulley and belt and install.

You have an accessory that's momentarily freezing, spiking belt tension for fractions of a second. Given it's the ribbed one that's busted, and that the belt rotates clockwise when facing the engine, the upstream component is the alternator.

How many miles on this car?

I am quite sure it was for the oil leak.

I did all that (including the tensioner) just over a year ago.

Does that imply that the alternator may be the problem. Car is about 125K miles.

Spin the alternator by hand with the belt off. You should not hear grinding noises. It will be obvious when the bearings are shot as it should be smooth and silent.

Alternator spins very smoothly without any noise. Idler pulley (now chipped) has a slight wobble if I tug its rim.The pulley itself is solidly connected but the rim has a slight wobble but no sound as I rotate it.

If it’s wobbly you have to start with that. Could have been the cause. If all pulleys(crank, PS, idler, alternator, water pump and tensioner) are spinning smoothly have to start with the idler and belt. Where are you located? What state?

Des the alernator pulley have a clutch and is it working ?

You can look at the alternator. If the alt pulley has a cap, it has a clutch. If no cap, non-clutched. My M276.957 3.5 liter (non-turbo) alternator has a clutch. My M276.825 3.0 liter (bi-turbo) does not.


C350 Alternator Pulley (M276.957)

SL400 Alternator (M276.825)

It appears that the 2009 C350 alternator does NOT have a clutch.

It has been over a week now. I thought I should at least start the engine to ensure the battery does not die. The engine did start but it sounded like something was hitting something else. I though maybe a remnant of the belt is hitting something else but was unsure and let it run for a few more seconds to investigate. Within 20 sec the engine automatically stopped. I was so worried that I did not start the engine again. Sounds ominous

Yeah I did clear out the mangled belt and as many pulley bits as possible. But there are a lot of finely shredded pieces at multiple nooks and cranies in the engine area. I didn't see any obvious big pieces that should have caused such a sound. Maybe I was wrong.

Used the serpentine belt toolkit from FCP Euro to replace the belt idler pulley another pulley and tensioner. However the sounds continued. Checked oil and was very surprised to see that oil level was very very low. Added over 2 quarts of oil. But even then sound did not go away.
Has the timing chain gone bad?

I used this kit https://www.fcpeuro.com/products/mer...-272273beltkt1 for serpentine belt with pulleys and tensioner
Now I hear a rattle like maybe a bit louder.

Does this indicate that the timing chain has gone bad? Or would changing the camshaft adjuster magnets solve the problem?