I quoted some M103 steps. The OM603 tensioner is simpler. Remove it by turning the big nut (24mm?), replace the crush washer, refit the tensioner and torque to 65 Nm. The manual talks about filling the tensioner with SAE 10 oil but if you keep it level and don't compress it while it's out it should be safe to put it back without further attention.

Sixto
95 S420
87 300SDL