I have a 1991 300te MB. Two months ago I installed a new belt. Recently it loosened and started slipping. It became so loose I could remove the belt. I have tried to adjust the belt tensioner but it will not adjust enough to tighten the belt. All the components seem solid...(no loose alternator, etc..) I am assuming I have a bad belt tensioner. Is this correct or should I look at something else before purchasing the tensioner???