I've had to replace two of those idiot things. One actually failed when all I was doing was replacing a worn belt. On some 124 chassis, the FSM recs loosening the alternator to get the belt off and on, to avoid touching the tensioner

Like he said, the internal rubber which holds the tension fails with age and engine heat. Why they designed this boggles my tiny brain, because the 602/603/606 diesel engines driving the same accessory device load use a simple heavy stiff spring and a damper attached to the idler pulley. That design fails very rarely and is easy to fix. Go figure.

NDM, but one can always mailorder parts with overnight delivery.