I don't understand the workings of the cruise control too well, but having taken apart the actuator I now have just enough understanding to throw out completely random guesses that just might be right...

There is a variable resistor on the small circuit board in the actuator. It uses a couple contacts on the lever arm gear to send back a varying signal depending on which position the lever arm is in. It is possible that because of wear or dirt the variable resistance doesn't work properly, confusing the CC amp -- you could accelerate with it manually, but because it doesn't know what position it is in the amp can't control it.

Have you tried taking apart the actuator? It's easy; just unplug it (follow the wire; should end at the left fender by the tach pickup), disconnect the throttle linkage, and undo the bolts that hold it onto the rubber mounts, and you can take it out.

From there all you gotta do it take off the arm (use a 13mm wrench on that little nut) and unscrew the 4 phillips screws and the top will come off. There isn't much to the innards, it's simple enough to put back together when you're done.

Make sure the inside is clean, re-grease the gears, and make sure the contacts on the quarter-gear are making proper contact with the circuit board. You might want to test the resistance if you have the proper equipment (hopefully someone more electronics-minded here can help you with that).

Good luck!

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'81 300TD Wagon 355k (Miss Diesel)
'83 300SD 180k
'84 500SEL 190k (Parting it out)