When you say the motronic EMC I take it you mean the motronic control unit (DME ECU) if so:
I think you’re loosing a main feed or earth to the to the DME ECU or the crank sensor signal.
I’m a Brit so I don’t know how the engine management light works on an E30 but it sounds like the DME ECU isn’t turn on. Volt drop test the main feeds and earth at the DME ECU. I can give you pin numbers etc but BMW use DIN standard wiring so the Feeds will be heavy wires main red ands the earths will mainly brown. To test the crank sensor you should get a 2-5voltage AC approx from the sensor at cranking speed, with out that AC voltage the DME ECU won’t turn on