-88 560SL OBD1 error messages

Hi folks,

trying to search in this forum for this problem, but with no luck.

Problem: Check engine lamp is lit
I have a code reader, and found this from the OBD1:
Plug hole 3: Constant light
5: (empty socket)
6: Code 3, 5, 6, 8 and 9
7: (empty socket)
8: (empty socket)

I have tried to reset the OBD by pressing the knob #2, with no result. The lamp at #4 is lit constantly.
The car runs normal, except for uneven idle.
I read at Google that error messages from socket nr 3 is for SRS systems only? Other places have other explanations.
Any advice will be highly appreciated.

Thanks.

Rgds Erik