George,
I have a question...You wrote that the dealer said that most likely your airpump was bad. How did he come to that conclusion?
Did you talk to him/her on the phone, or did you go to the dealership? I certainly am not a professional Benz mechanic, but the O2 sensor (was) would have been the first item that I would have examined with a critical eye,...NOT the airpump.
While I am ranting just a little...airpumps usually last a helluva lot longer than O2 sensors. (what I mean is that had I been the service advisor I might have suggested the sensor be replaced...hopefully the next thing that would have happened is that the tech involved in replacing would have noticed the harness being off and said so)

sorry...just venting I guess

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03/83 300D 184k
07/73 280 160k