My post (#11) was not in regard to the tumblers/key cylinder. It was in regards to the rest of the locking assembly. In my case, it appears that the return spring couldn't overcome the worn parts, so it would bind a bit after starting. I replaced the key cylinder and lubed the lock as best I could, but it didn't help. I could move the switch with the key to the Run position after starting, which was a minor annoyance but an easy fix as far as I was concerned. A year or two later I ended up replacing the entire lock assembly when it started to jam up trying to move it out of the 'lock' position. The key cylinder is not the culprit in this situation. It is the lock assembly. Thus my advice to ensure your lock assembly is in good order, and that it returns the key cylinder solidly on its own to the run position after starting

And as far as the return spring, that too is part of the lock assembly. I have a new switch assembly in my parts bin, and it has detented positions, but no spring that returns the assembly to the run position.

Granted, this all applies to a 123, but charmalu's post indicates the assemblies are very similar. The main difference seems to be the different mechanical connection for the switch. So that's the basis for my post. YMMV.

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)