When you say the motronic EMC I take it you mean the motronic control unit (DME ECU) if so:
I think youíre loosing a main feed or earth to the to the DME ECU or the crank sensor signal.
Iím a Brit so I donít know how the engine management light works on an E30 but it sounds like the DME ECU isnít 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 wonít turn on