I am trying to diognose the problem i am having with my cruise control. My cruise control seems to only work between 25mph and 40mph. after that it will jerk violently and after a few seconds it will quit. I thought it was the amp but the amp has been recently replaced. What else could be the problem?

thanks in advance

People have performed miracles by methodically lubricating all of the ball joint linkages (And there are lots of them. on my car, anyway) with transmission fluid. If your car is of the same era, I'd do this first, and then go from there. With the actuator linkage removed, throttle movement should be very smooth. The electronics package is designed to shut down if "unreasonable" movements are required, that's why you are shutting down. It's POSSIBLE that the actuator's wipers and/or PCB are corroded/dirty/worn, but this is the last place to look.

P.S. (assuming a car of the 80's era) you definitely have the correct encoding plug in the cruise control module, right?

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sorry i forgot to mention. it is a 91 300e.