Hi Peaches,
Over the last four years I have bought a W202 C280, a W210 E430 and a W220 S600. Experience with the first two is guiding my preventive maintenance choices. I just changed my serpentine belt and all of the pulleys on the S600 at 125,000 miles. The belt tensioner felt coarse and rickety turning by hand; was going to fail sooner than later and the other pulleys were aged as well.
The belt tensioner failed and shredded the belt on a trip to New Orleans in the C280 a few years ago at about 12X,XXX. The same thing happened last year when my wife was getting groceries in the E430 that had about 12X,XXX miles.
I am not going to get stuck somewhere because of a belt getting shredded when a pulley fails this time. If your car has 120,000 or more miles on the original pulleys, it would be worth your time to remove the belt and check all the pulleys and belt tensioner. Replacing a belt and pulley(s) in your garage, at a time of your choosing beats a cold rainy day on some deserted road or highway! It is also less expensive to shop around for parts and choose from good alternatives instead of needing them on Sunday in the middle of nowhere when I was willing to pay anything to get home.
Thank you Peaches for sharing your wisdom with me over the years. I hope I can pay it forward by telling anyone who is curious about this expected failure that can be averted.
Best regards,
Jeffrey