My particular 85 300D has a Brain in the lower driver side kick panel.
Back when my dash caught fire and I didnt trust the wiring, I reran the entire harness and one afternoon, it all made sense about how the ECU tied in to the system; its more of an emissions ecu, I mean you can't modify the software map and tune your ECU like in a gas car and get 350 hp from your diesel thru your PC.
Since then I have completely forgotten its function..(Hey I am in my 40's now!) Does anyone recall what the ECU does and what its software or algorithims had loaded into it?