93 300se automatic belt tensioner

I replaced my drive belt on my 300se,taking the old belt out was easy,by pulling on the belt tension and slip it out of the pully.But when I try to put in the new belt the belt was too loss. I cannot get any tension from the auto tensioner and the adjuster cannot help either.Iwonder is my auto tensioner need replacement.Need helpTHanks I use mercedes original belt,I try putting back the old belt still, too loose.I used the edit button because I do not know how to sent another message .Sorry. thanks for your fast respond.need more help