The W123's series 300D actually had a ECU in the later models! What did it do?
 

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?

really now.. i have an 85 and in the passenger side right side kick panel their is a door that i am missing and can see a round thing that has EGR on it.. i dont have an EGR anymore or an ARV..

ill czech out that one in the drivers side

Do you remember where this ECU is?
Is it near the gas pedal, kind of above the pedal and in front of the defrost vac pod by any chance? There was a little square thing there with wires attached and I was wondering what it was. It looks like it might have been mounted to something, but naturally it's no longer mounted since I did some experimentation with that vac pod last weekend.

the 85's are a weird year. the vac system is completely different, the IP has a box on it that watches rack travel and other weird things

My ECU is on the passenger side, side kick panel. I know it has something to do with the tach operation also.

i dont think it could be called an ECU since its not really managing the engine. maybe just some sort of computer for the sensors and stuff that are on the engine.

Its not really an "ECU", the W126's have black box's over their for the air bag and SRS.

The E300D's have an ECU.

The only thing that the '85 ECU does that is IMPORTANT (in my car at any rate) is drive the Tach. It replaces the Nasty small garbage can looking VDO tach module used in pre-85 diesels.

The rest of it is used to switch the EGR valve and emissions stuff

well i dont have an EGR , or ARV... so its only driving the tach.. tach computer thingamabob it is

Just count your lucky stars that there is no check engine light. :D

nope none.. only one i get is the one about the brakes.. not working.. but they work