I checked a duty cycle in my 260E 1987 - 97% on idle (pin 3 and pin 2 as a ground).
I was not able to change it by a screw for adjustment airflow, only O2 reading was adjustable.
The O2 sensor reading is constantly changing from 0.2 to 0.8 V. When I push an acceleration pedal the O2 goes for a moment lean to 0.06-0.02 but then jump to 0.8. The car does not response quick enough on acceleration.
When drive on highway the reading is in same range 0.2-0.8. When I release the acceleration pedal and leave the car to run by inertia O2 reading goes to 0 V, even in minus.

As I understand a duty cycle has to be about 50% and O2 - about 0.5 V ??

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1987 MB260E 187K