This is just informational, not a problem per se:
I have a '79 240D, '82 300TD, and '81 300SD. They are all quirky in their own ways but there is one common quirk I've always been curious about. If you turn the key to crank the engine and it doesn't start (or starts and dies, or you turn it off) you have to turn the key all the way out to the lock position take it out and put it back in to make it crank again. There is some kind of lock that prevents you from just turning it back to crank.
Why?