The serpentine belt on new autos together with the tensioner that it uses appears to have been designed with one of the goals being to leave a motorist stranded on the side of the road when the tensioner fails catastrophically.

One the other hand maybe I was negligent in failing to replace my tensioner when my vehicle had 98K miles on it. I’m not really that familiar with tensioners and serpentine belts as I have only one other car with one; it’s a 92 300E and I did have that tensioner replaced at 88K mile because I could tell the drive belt was not that tight when I pulled on it.

About a week ago the tensioner on my ML failed catastrophically. It caused a belt with only 50K miles on it to shred. This is the first time in 34 years of driving I’ve had to have a vehicle towed because I could not get it quickly fixed on the side of the road sufficiently to get it home.

In the past if a drive belt would break I could just replace it, tension it manually and be on my way. Actually the only vehicles I’ve had that have broken belts are old Porsches and VWs. I did have an 83 240D that burned up a belt when the A/C compressor froze up. Not good, I had to drive without A/C but I could still drive it.

So bottom line should the tensioner be replaced based on mileage or age as part of the general maintenance on a vehicle?