I had a similar problem with my 88 300E as the tensioner adjustment was stripped and did not work. It is not easy to see the 19mm bolt that locks the tensioner down. It is located between the tensioner body and the power stearing pump pulley. It may be covered with grime if your car leaks like mine did. You
will have to remove the tensioner if the threaded rod is damaged but can sneak a new nut in there with a lot of work if you want to. It is easier to see what you are doing and access to the bolt is better with the
fan and shroud off. While fixing mine I found the crankshaft pulley bolt loose, removed it with my fingers. So you might want to check that out too. I have a Technibook manual and it is not too good in that area either.

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Dave
88 300E 130K