There are 2 styles of belt tensioners for the w124 that I can recall. Not sure which one your 91 has. My 87 has a tensioner that is also supported by both an external spring and shock at the 9 and 3 O'clock positions. My 94 is somewhat similar in design of the tensioner body, but does not have a spring nor shock, but rather a threaded rod and turnbuckle nut mounted by the water pump, this rod assembly is used to tighten the tension.

Both styles of tensioners rely primarily on a rubber bushing in the center of the tensioner to provide most of the tension and some give in the assembly. The life of the bushing is variable. I have noticed that MB units outlast the aftermarket ones, so gauging predictable life span is challenging. I would say if using aftermarket, Febi etc. you might want to change the tensioner with every belt change. If you use an MB unit, every other belt change would probably work.

Also note on the earlier style tensioners, the ones with the external spring, the spring is usually sheathed in a plastic liner, like a cigar, if this spring lets go (breaks) you will lose significant tension but may not have the belt slip off. In this case, the tensioner itself may still be good, and all that is needed is to replace the external supporting spring, The FSM will detail removal and installation procedure for the style that you have. It's not a difficult job, but could be tedious depending one what needs to be removed to get access.

I have not notice the need to change anything else out due to a failed tensioner, a snapped belt on the other hand could take out some wires, hoses and what not. Good luck

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