key cylinder?
 

I have an '86 300E that doesn't want to start. All the lights on the dash are on. Turn key to start and nothing happens. I have to disconnect the battery to turn everything off. Out of curiosity I wired up a remote start button. Engine turned over fine, but I could not shut it off with the key.
Do I need a new key cylinder or should I check something else?

others can weigh in...but it could be a neutral safety switch issue

Fixed!

I tried everything and finally bought a whole steering column from the JY. Disassembled the key structure and went to swap the cylinder when I decided to pull out the whole key steering lock assembly. Turns out, the aluminium rod from the key cylinder to the switch had snapped. Replaced that piece and it was good to go.