You have asked some questions that really go into the quick with me. First, what happened: I had the crankshaft oil seal replaced on my car to stop a leak; that was the beginning of my problems. After the balancer came off the third time in the next 20,000 miles, I pulled the car home and called the area Mercedes Service Representative. (All the repairs had been done at dealerships.) I told him what had happened and that I was at a total loss as to what to do. He made a special trip to my dealer the next day. Ordered new parts by air and conducted a seminar with the service department techs on how to install the harmonic balancer. He had them strip away all the parts right down to the crankshaft and reassemble it with all new parts. It seems critical that the correct torque is used on the bolts and, I think, some Lock-tite was used. Once the dealer had done this, I took it upon myself to back off on the tension at which the belts were set. MB specs call for extremely tight belts. I reset them only to a tension at which they would not slip. I even instruct the dealer service shop not to set the belts to specs. They know instantly why I make that request. So far, there has not been another incident and that was about 55,000 miles ago. I still can't help wondering if it is going to happen again, however.
If the "incidents" resulted in crankshaft damage, I have been told that the only permanent fix is a new crankshaft. Let's hope that hasn't happened.
Now, for my opinions. From talking to people at the dealership and from reading this forum, I get the impression that it is not an unusual occurance. The problem? I think mine stemmed from the fact that no mechanic (technician?) who worked on my car knew the correct procedure for installing the balancer. I also get the impression that the design is not all that we are led to believe we are getting when we buy one of these expensive automobiles.
Anybody have some different ideas?
Hope this is the information you are seeking and that your balancer won't come off again. Good luck.