See post #14 in the following thread:
http://www.peachparts.com/shopforum/diesel-discussion/348483-russian-epc-back-again.html
The title says "Russian.....". Later into the thread whunter provides pointers to U. S. based web sites for various older MB doc.
Free 107 - 124 - 124 - 126 - 201 service manuals.
Beware - some downloads may be very slow. The "Circuit Diagram Index" located within "Electrical Trouble Shooting" for a '91 300-SEL took 13 mins to download and no, I am not using dial-up or DSL.
You'll need the circuit diagram to trouble-shoot your problem.
Querying thru the "Maintenance Manual" section for a '91 300-SEL returns "Document not found".
Hard to knock free.