just posting this for the record, maybe it will be helpful to somebody.

I was having a hell of a time with my cruise control; after fixing the surging problem (thanks to other posts on this board) I got it to the point where it would run fine but then all of a sudden shut off (at any random time, though running over a bumpy patch of freeway was almost sure to kill it).

I checked my brake lights and yes, they were functioning and of the proper type. No fix. I tried resoldering the connections on the CC amp circuit board. No fix. I took apart, cleaned, and lubed the CC actuator (which is a rather simple and robust mechanism; no wonder it is rarely a problem). No fix.

Oops, worse than no fix -- my cruise control stopped working!

I removed the actuator again and took it apart to see if I'd messed anything up. Looked put together properly, and it worked fine when I tested it with a 12v battery. The only thing I'd messed with besides the actuator itself was the plug where I disconnected it.

AHA! Turned out one of the wires on the female end of the connector had come loose from its socket. I resoldered it, put everything back together, and bam... CC works, and it doesn't cut out anymore! I guess that wire has been loose for a while, causing the intermittent shutoffs (especially on bumpy highways!).

Moral of the story (after all that I guess I have to have one): You can check all the expensive and complicated stuff, but sometimes it's just a simple bit of wiring that isn't doing its job...

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

That's what I was hoping was the problem with mine (will accel/decel with the actuator, but won't stay "on" cruise at all). Went through all the wiring, tried two cruise amps (one has a conformal coating that is impossible to solder through) and nothing, still won't come on and stay on, but will accel with the operating lever (once you let go of the lever, it slows down.)

Will probably never find the problem with mine, glad you found yours!

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1984 300D Turbo - 4-speed manual conversion, mid-level resto

1983 300D - parts car

1979 300TD Auto - Parts car.

1985 300D Auto - Wrecked/Parts.


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"If you don't know where you are going, any road will get you there". Lewis Carrol

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)

Tom, try a search for"cruise control "or" performance products" with my name in the right box...
I have posted pics of the information they used to send out.... sounds like what you have described...