Iv had the entire locking mechanism including the barrel. The key turns but the part at the back of the barrel won't.
I had a flat battery a couple of months ago and needed to use the key, it was quite stiff to turn but it suddenly went with a click and unlocked, couple of days later i opend the door without unlocking with the keyfob and the alarm went off, it locks now with the keyfob but the key in the barrel wont do anything, hens new key barrel
Have you tried dismantling and lubricating the barrel , its unusual for a door lock barrel… that is usually operated by remote fob… to fail for any other reason other than corrosion due to lack of use. The W202 in the link below may be very similar to your ML.
See bottom of this page… W202 Ignition key won’t turn past 1 or 2 - Mercedes-Benz Owners' Forums
If you open the door without first disarming the alarm with the remote key fob then the alarm will go off, that’s normal, the metal key blade is for emergency locking/unlocking only, it won’t disarm the alarm.
If i lock the door with the keyfob i can still open the door lol, very odd but yes i think the the barrel was'nt used and has seized up.
i'll have a look at the links and let you know.
thanks for the help Dec, much appreciated