Tensioner fails when the the rubber bush inside, which acts as a torsion spring, breaks up. When this happens adjustment is futile as the adjuster twists the inner part of the bush so the torsion acts on the tensioner arm into which the bush is fitted. The danger is that attempts to turn the adjuster cause the rod to break either because the bush inner cannot turn because the big 17mm bolt running through it has not been released or because the crank has already rotated to the top.
Anyway it's fairly easy to take it all apart and see whats happened, you may have to remove the radiator to gain access. I did see a post about fitting a turnbuckle in place of the damper but if the bush is too badly broken up the pulley alignment will be wrong and cause excessive belt/pulley wear.
'08 Chrysler 300CRD (MB OM 642 engine)
'95 E220 estate
'89 230TE (R.I.P.)