I just noticed that the second link in my sig was not working and fixed it.
The second attachment in that link:
http://www.peachparts.com/shopforum/attachment.php?attachmentid=24327&d=1109717988
...is exerped from MB service CD and indicates that, to recognise the control unit the Key On reading should be 70% (federal) or 85% (California) ,,,
I have assumed that If the test method gave the correct reading at this stage it was via that method that it would give the correct duty cycle reading.
Reading through many posts on this subject however has not assured me that one method was universal or that there is a well defined MY change-over to a different output method. Perhaps 9/89 is the date.
90's were built in 89... what is your build date? (driver door panel)