Reading Codes on 1990 300E

[shertex]

Have been having an intermittent (not very often) check engine light on the 1990 300E I just acquired. Since it's OBD1, Autozone and the like can't read the codes for free. I had hoped the car would have a built-in pulse counter so I could read the codes that way, but I don't think it does. All I see by way of a diagnostic module is a little black box with holes numbered 1-16.

I've been doing lots of searching on the forum but still not sure what to do. Can someone point me in the right direction.

Have spent virtually all my time over on the diesel forum....this is my first foray over to the gasser side!