Merc600sec, do you have a little LED and pushbutton near your diagnostic socket? If you do, there is a way you can read codes using the pushbutton. The LED will flash codes.
I did a lot of research on OBD and OBDII when looking for a scanner for my 95 S600. My car is OBD. The systems on this car have analogue and digital components. Once the next version of OBD came out, it was called OBDII, which retroactively started people referring to the original OBD as OBDI.