What is this? 1989 300SE US
 

Hey - Can you all help me identify this part? My fingers are pointing to it - it has a screwtop on the right side of the engine bay. What ever it is has a white cord which runs to the front of the engine and it apparently blew off while I was driving...and now my 300SE is running but idling very rough. Any help would be great, there's probably a post out there somewhere on this...

That's the X11 diagnostic socket.

Thank You! I have a sinking feeling that because it broke isn't the worst of my worry - sounds like clanging and metal on metal...taking her in this week for an exam.

OK, the round plug is the diagnostic port yes, BUT the copper coiled wire in his hand is at the other end of the wire attached to the port and attaches somewhere to the front of the engine and has come loose apparently also with the bracket that holds it, what is that? Crankshaft position sensor thats been torn open maybe?

it's just a timing pickup for the factory tool. not needed for running. good luck, chuck.