I've done this a half dozen times or so - just went through this replacing an alternator.

There's a belt routing diagram in the owners manual and the MB maint. manual. It's easier if you make a loop to shorthen the belt and work from the center pulley routing around. I always end up at the PS pulley(last pulley to slip belt on). Make sure the belt grooves are in the pulley tracks. I then put a 6 point 27mm socket on the end of the crank pulley and rotate clockwise as seen from the front and thoroughly examine the tracking of the belt at each pulley.

Once I'm sure that's ok, I start tightening the tensioner adjuster clockwise with a 13mm socket until the belt is good and snug. Finally I tighten down the 19mm that's buried in between two of the upper pulleys.

Piece of cake.

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"