My 1980 450SL has a surging cruise control. Approximately 2-3 mph at 40-75 mph, and the surging stops after about 8-10 minutes of driving. What should I be examining or where should I look? The vacuum system, the control or the linkage? Thanx in advance, Roger

The first thing I do on these is take off all of the throttle links one at a time and clean and grease them.

And adjust the play in the cable.

Thank you Benzmac and stevebfl for your response, I'll try to check the linkage for play and dirt this week. Roger

Incidentally, the MB-recommended lubricant for throttle linkage is automatic transmission fluid. Regular grease may attract too much dirt and eventually gum up the works.

Continued help on surging Cruise control
 

This weekend, I cleaned and lubricated the 3 throttle connections on the top of the engine. Are there more? As the speed surges, the speedo changes about 2 mph, the rpm 100 revs and the throttle pedal moves a bit. All changes are at the same time and rate. After 15 minutes, the surging stops and the 450SL cruises steadily. Am I looking at black box change?

The cable has a turnbuckle type adjustment in the middle. The cable should not be tight but just short of it. If it has anything to do with your problem there will be a half inch or more of play in the cable.