W124 cruise control

[JHZR2]

My 91 300d cruise control works fine at low speeds, but doesn’t seem to work on the highway.

Around town doing 25-35 type speeds it sets, keeps consistent, accelerate and decelerate work, etc.

Driving on the highway at 65+ it will not actuate. I’ve tried with the pedal depressed, and with the pedal not pressed, while coasting. Neither work.

Travel of the cruise unit seems to be fine, not stiff. Linkages are all ok.

Any idea why this would be?

Are the control units similar to the w123?