cruise control/ '90 300E
 

Yesterday I set the curise control @ 3000 rpm on a flat section of road with no headwind. I could feel a pulsating motion as I drove. The rpm's did not change or fluctuate, only the "fuel economy" gauge showed any change. When I turn off the cruise control there is no pulsating action and the car drives smooth and steady. Could this be a vacuum issue? Any assistance is appreciated
Tex

could be 2 issues, hopefully the cable at the throttle body is a little loose and needs to have some slack taken up, if not it's most likely the cruise control brain that is located under the dash just above where your left knee is when sitting in the drivers seat, not a hard change out but a little time consuming

Does your speedometer needle flutter at that speed - with or without the cruise control engaged?

Duke

I'd definitely be looking at your speedometer cable, small changes in the speedometer needle can cause the pusling feeling that you describe.

cruise control/ '90 300E
 

There is no speedometer flutter. Steady as a rock.

Okay, I've have some cruise control surge at highway speed, and the speedometer has had intermittent flutter with or without cruise control engaged. With the cruise control engaged it surged when the needle started to flutter. There was a definite correlation.

Last year when I was undernearth replacing the clutch slave cylinder I removed the speedo cable from the transmission, and it turned smoothly, so I reinstalled it. Both the cable and speedometer head were replaced on warranty circa 1991.

For some unknown reason, the problem has not reoccurred, but I barely used the cruise control in the last year.

Suggest you look at the cruise control service info on the braingears site. Be sure to include "technical modifications" and "program repairs". I recall from skimming it that there was some information about adjusting the link from the actuator to the throttle linkage as a cure for surge problems.

Duke