It's interesting that we rarely seem to heard of tensioner failure in service, but it did happen to a friend's '88 190E 2.6 back when the car was fairly new. He drove about 20K miles a year and offed the car when it had 90K. I can't remember the mileage it failed, but the belt broke, took out a power steering hose, which made a mess in the engine compartment and the combination of the broken belt and oil spray took out the hood pad. The culprit was the belt tensioner, but the dealer did the repair, so I never saw anything.

I had my belt replaced apriori when the dealer changed the water pump under warranty at 45K miles. That was circa 1992. As you say, the belt itself appears to be very high quality and long lived - maybe good for up to 100K miles and inspecting it is fairly easy. On the subject 35K mile 300E the belt had numerous transverse cracks on the inside ribs, and I attribute this to lots of time at low speed in hot weather (Palm Springs). The belt on my car is now about 13 years old, but only has about 30K miles, and it looks like new.

Job number 13-342 in the old 103 engine manual under step 11 has the following note:

"As from November 1986 the alternator is attached at the bottom to the bracket with a 13 mm collar bolt. If this collar bolt is removed, the V-belt can be slackened by swiveling the alternator without slackening the bolt (16) [which is the tensioner lock bolt in the photo]."

I would suggest that, especially in a pinch, replace the belt by using this method as I think most old tensioners may not be reuseable, however I have to add that this "trick" is NOT mentioned in job number 13-3420 (belt replacement) in the CD, which was written later in time.

The rubber bushing does provide torsional damping, especially when new, but this probably decreases as the rubber bushing ages. Also, there is a job in the CD "Retrofiting Tensioner Damping Device" or similar verbiage that implies the external damper was not part of the original design, and early 103 engines may not have had it.

In any event, I'm sure many of us could come up with a better design architecture, and it's too bad for us that the aftermarket has not done this.

I just placed the 190 in summer storage and brought out the '91 MR2, so I have six months to think about it.

Duke