I ran across this recently when trouble shooting my emissions.
Kept getting 30% instead of 70% ignition on engine off
In reading thread after thread I found a reference to Mercedes being the reverse of the industry. To obtain the book value of duty cycle you have to reverse the leads (for me [86 300E] at the X11 diagnostic connector pin #3 the Lambda feed should be connected to the black or ground lead of the meter, and the red "com" lead is connected to pin #2, the ground pin).
This left me wondering for a long time if I had controller problems...
Not sure if the O2 sensor requires reverse leads or not.

__________________
-Marty

1986 300E 220,000 miles+ transmission impossible
(Now waiting under a bridge in order to become one)

Reading your M103 duty cycle:
http://www.peachparts.com/shopforum/831799-post13.html
http://www.peachparts.com/shopforum/831807-post14.html