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...

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