OKAY! Thanks for everyone's replies to my original question. I've got a lot of good stuff to work with.
NOW, another part of the question...

When the tensioner unit "wears out", which part is it that keeps the tension on and actually needs replacing?!?!?!?

I've got a Performance Parts catalog for my wife's 190E 2.3 and the exploded diagram shows a "belt tensioner shock". Is this the part that wears out and fails to keep the tension on the serpetine belt? If so, is this the part that needs replacing (at a cost of $35 instead of $172) rather than the whole unit?

What advantage is there to replacing the entire assembly instead of only the shock?

I hope that these are "good questions" and that many people will benefit from the answers. Thanks in advance!!



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Jay Yambrovich
1989 300 E
1992 190-E 2.3

Jay, the tensioner assembly has an inner and outer sleeve. The outer sleeve is attached to the tensioner pully. The inner sleeve is secured to the tensioner bracket by that BIG nasty bolt in the middle. There is a small amount of space between the inner and outer sleeves that is filled with a rubber compound not unlike that found in control arm bushings. This rubber is vulcanized to both of the sleeves. When you are cranking the tensioner bolt. you are putting a load on this rubber and this is what keeps the tension on the belt via the pulley. This is the part of the tensioner that is most important in keeping tension on. When the rubber separates, time for a new one. The shock is there to absorb harmonic vibrations only.

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Jeff Lawrence
1987 300e
1989 300e
1987 BMW 325