Hi there,

There is a site that has some diagnostics that will help you determine the cause of your problem! Here's the URL for it:
http://www.fix.net/~gdl/testproc.html
They also rebuild the actuators and control units. I don't have any experience with these people, but did make good use of their diagnostic procedure to troubleshoot the cruise control system on my '82. There is also a very good description of the cruise control systems on various M-B vehicles somewhere in their pages. PS - On my car, the actuator motor was worn out. (some armature windings open)

Good Luck!
Richard Wooldridge
'82 300TD/4.3L V6
'77 Jag XJ6
'77 280Z
'74 280C