The cruise control has a speed pick-up sensor located under the dash that is driven by a short cable from the speedo. I'm guessing the sensor is a small generator that outputs a higher voltage as the speed increases. Or, the output signal could be pulses produced by a cam and micro switch or gear and prox switch or some other arrangement. I doubt if the signal is pulses from an optical encoder (glass disc with etched lines rotating between a photocell sender and reciever). If the short cable breaks, you are correct, only the cruise is effected. jp