I had this issue with a used starter I put in my rebuilt OM617. It ended up being the starter went bad, I am not very familiar with the internals of starter motors, but the gear was stuck in the engaged position and exhibited the exact same symptoms as what you are having. It's a lot easier to pull the starter on your OM616, I would pull the starter and have it tested at a AutoZone or other parts store near you if you don't have any loose connections.

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Milan Brown 1979 240D, rebuilt OM617.952 turbo diesel, rebuilt 722.315 transmission (my only daily driver)

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