Neutral safety switch!!!!
 

Where is the darn thing??? got an old 82 300 TD... so many more things to do and this is the next!!!! who knows the expert on old Benz??? Thanks!!!

The search function can be your friend.

http://www.peachparts.com/shopforum/diesel-discussion/299044-81-300sd-where-neutral-safety-switch-located.html

If you replace it there is a hole on the Side of it and a matching hole in the Transmission Housing. You insert the the proper sized Drill Bit into it to line it up correctly when you install it.

Above the accelerator pedal is a 4-wire plug with a harness that runs down to under the accelerator pedal and to the transmission tunnel. Unplug it, jumper across the two purple wires in the female plug and try to start again. If it starts, you need a NSS.”

I do not know if your cars wiring is the same but I got part of this quote from another Forum concerning a no cranking problem with an 84 300D.