I just ran into this same problem with my 83 300Dt, 296K on the clock. After reading and searching this forum for information, I replaced the tumbler (from local MB dealer $109.00, keyed to the VIN). This solved my problem for three days, then back to being unable to turn the key to start the car.

After re-searching the posts I knew that I was in the .5% group that needed to replace my steering cloum lock, so I removed the tumbler while I could and called Phil to order the VACLO lock and overnight the shipment.

Total time to remove and install was around two hours, I did have to push out the inst. cluster, remove he speedo cable, and remove/drop the steering colum (two 13mm nuts) to access the colum lock.

Now the WIFE and I have no worries that MY car will start and I can take her to the hospital when our daughter arrives in Oct/Nov.

IMHO, If you think your tumbler is going south, replace BOTH the tumbler and colum lock You will be time saved and hassle free, with great peace of mind that the old girl will start, everytime!