My 1988 560 SEC's cruise control works but it seems to be
"surging" . It will kick in, but sort of goes back and forth between 2 or 3 miles per hour. It does this when I'm on a flat road at 65mph. Also, sometimes it just turns itself off.
Any suggestions? Thank you.

 

Probably the amp under the dash, above the brake pedal. Aluminum box, maybe 3 inches by 3 inches by 1 inch. Over time, the solder joints inside it fail. Take it out and bring it to somebody that knows how to solder electronic circuitry, and have them resolder all the joints. A new control is expensive - try to save the one you have.

 

Thank you, I will try this. Renorich.

 

I have joined this forum, just to say thanks!
I have a 1987 560SEC and my cruise control also surges, to the point where it is totally un-useable!
I read abut the soldered joints being a problem, so I dug in and removed the amplifier, pried open the tabs holding the printed circuit board in the box, and removed the board.
My hopes were immediately dashed, as the board is one of the nicest made boards I have ever seen. Each joint had the right colour, shape and shine, I could see nothing wrong, so I almost packed it back in it's box.
Then I thought I have nothing to lose. It cannot be used as it is, so I painstakingly re-soldered every small joint on the board. Just held a hot iron to each joint for maybe two seconds, until the tin started flowing.
I was in many doubts about the result, because the re-soldered joint did not look nearly as good as the original ones.
I fitted the board, and I have just come back from a test drive.
It works PERFECTLY!! I can't believe it. Easy, fast, perfect. the only small problem seems to be a VERY slight hunt, if the road surface slopes exactly enough to just about give the car the desired speed without power. Then there is a slight hunt, which I believe could well be the way the board would have controlled when new!
But thanks again. It was well worth the effort!!!