Belt Tensioner Problem

[TuckerD]

Ok, So heres the deal my Smog pump goes out and I go to replaece it, and all is still wonderful and beautiful in the world. I forget to mention that this a 1992 300E. I got the fan and belt out of the way in record time and got the new pump in. I go to retension the belt and I got nothing and it feels like the tension adjustment nut is stripped. So I enlist the help of a friend to tighten the secureing bolt ( the 19mm one) while I with a long 2X2 in hand apply pressure to the tensioner pulley (the ungroved one) in the attempt to restore at least a little bit of tension into the system. At first this revised tensioning method show some promise bescause my pushing gets quite a bit of tension back into the belt. Then my world crumbles!! My friend tightens the bolt sufficiently and I ease off my 2X2 in hand., and the belt slacks up. Can anyone help? What's going on internally in that tensioning device that makes it slack up even when that so called securing bolt is tightened? Come to my rescue.

Thanks, a violinmaker that loves to work on his car,
Tucker