W124 Belt Tensioner Failure Hypothesis

[Jeremy5848]

The 124 diesels (and others?) have a somewhat complicated arrangement of parts to keep the serpentine belt under tension at all times. Many of us, including myself, have had tensioner failures. I have discovered a possible cause. (Yes, there may be other causes.)

The attached diagram (from EPC) shows the collection of bits and pieces making up the tensioner. Common failure items (besides the shock absorber) are the lever (5) and its pivot pin (17). The lever is aluminum with a plastic-lined bore where the steel pivot goes through. The lever rocks back and forth on a crude "plain bearing."

What happens, I found in both my cars, is that the plastic cap (47) cracks or falls off and is not always replaced ("it's just for decoration"). Also, the seal ring (29) is not always replaced when the assembly is serviced. Why, I don't know, but both my cars were missing this seal.

The result is that road water coming through the radiator carries dirt into the space between the lever and the pivot pin and turns the lubricant into grinding compound. Under constant pressure from the pull of the spring and the serpentine belt, the parts wear and gradually tilt. Hopefully the owner notices the noise and takes action before more damage is done—we've all read about what can happen.

My conclusion, then, is that everyone whose engines have this type of tensioner should make sure that cap 47 is present and undamaged. When time is available, it wouldn't hurt to take the tensioner apart and make sure seal ring 29 is also present. The cap and seal are not expensive parts—a few dollars each. It's also a good opportunity to clean the "plain bearing" and apply a light coat of fresh grease.

The alternative is to wait until the lever and pivot pin are damaged and then replace them (about $100 for the lever and $50 for the pin). Your choice.

Jeremy