You haven't fully loosened the alternator.
There is one nut and two bolts that you must loosen before you tighten the long adjusting nut on the tensioning screw.
https://www.startekinfo.com/StarTek/outside/12265/disc_2/program/Engine/617/13-340.pdf

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022