Is he turning the Engine Clockwise as viewed from the front of the Engine?

The OM617.952 Engines have a Timing Chain Tensioner that has a ratchet that keeps the plunger and spring from easily being depressed but even then the Manual has to only turn it in the direction it normally rotates when running.

The Engines with no turbo have timing chain tensioners that if you rotate the Engine opposite to the engines normal rotation you can collapse the spring and plunger on the timing chain tensioner relieving the tension and there is a chance of the timing chain skipping over gear teeth.

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84 300D, 82 Volvo 244Gl Diesel