Assuming you know (and knew) the proper way to adjust belt tension, then the tensioner is likely all that has failed. Tension is provided by torsion on a molded rubber bushing, and the bushing separates with deterioration from age. If you ever adjusted the tension without loosening the locking bolt, then you probably also broke the adjuster - ordered separately.

Steve

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'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7