Originally Posted by Jim B.

It could be possible that OBD I codes are there. I have a '91 coupe and
someone did run them for me once. But it was long ago. I vaguely recall a small black box near the middle of the outer firewall that held the secrets- but not sure of it. The codes would flash small red lights in a specific sequence to identify the fault.

I DO remember the last time the car was in the car, last month, the mechanic told me that he would rather run the codes in the W126 because there were only 9 of them whereas he would rather avoid running them on the W140 S class (I think he said there are 32 of them) and would prefer not to wrench the W140 series cars at all

As I said , I'm not sure of all of this, those are just my recollections, but I think they are on the right track