If it's a Sperry-Vickers pump, there's an 6mm allen screw, just to the right of the 2 fluid lines, facing up. Loosen the 2 bolts holding the pump to the bracket, which you probably already have. Turn the socket head screw and you should see the pump move up/down to adjust the belt. I recently removed one from my 560SL, 117 engine, but lots of interchangeable stuff and common adjustments on our Benzes....

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1986 560SL
2002 Toyota Camry
1993 Lexus