I was at the dealer for a software update in my 2015 E350, and after my 900 point inspection, where they try to sell you $25 windshield wiper blades, all they could come up with was a slightly noisy idler pulley. Their quote to replace two pulleys, tensioner, and the belt was a staggering $579.
Now, I don't expect a bargain at the dealer, but that was a cross between comical and criminal. I call it a laziness tax. After the detailed inspection by the dealer, they still didn't notice that my eco start/stop didn't work, but I don't really want it to.

I ordered the kit with all parts for $70 delivered, and tackled it yesterday. It was very straightforward, and took about 45 minutes. The pulleys come off super easy, and I used an allen wrench shaft to pin the old tensioner to access the bolt to take it off. If ever there is an easy diy, this is it. I found it easiest to relieve pulley tension and remove the belt first, rather than try to get the locking pin on the tensioner and then remove the belt..

One of my pulleys had a rough bearing, so I am happy when improvements are made. The hardest part was the silly clic hose clamp when removing the air filter box for easier access.



Next, I installed 2 caster adjustment bolts, but that detailed description will have to wait for another day.

 

Where can I get $25 wiper blades? Even the Amazon cheapies for my car are $45

 

leardriver, Thank you for the information. I've only had my 2011 E550 for about a month and the Princess has claimed it as her own.
Darel, check out Rock Auto for parts.. https://www.rockauto.com/en/catalog/mercedes-benz

 

Walmart has name brand wiper blades with the proper Mercedes attachment for $7.

 

My dealer just quoted me $900+ for the same repair on my GLK.

A little research and I learned what you said; not a tough job and parts under $100.

Where did you get your parts, BTW?

 

FCP Euro. $70-ish shipped.