After having just gone thru a similar situation on my '90 300E, my guess is that your tensioner is shot and you will need to replace it and the adjuster rod (threads are probably stripped). The tensioner is about $180 from Performance Products. The rod is $14 and you might as well change the shock ($28) at the same time.
If you want to do this job yourself, it requires a special tool (holding tool-$9) and the removal of the radiator. Not too bad of a DIYer weekend job.


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Steve
1979 240D
1990 300E