The speedometer cable and the cruise system are seperate systems. A bad speedo cable will likely cause your speedo needle to bounce at a certain speed and it will usually clear up at higher speeds. A bad speedo cable will not effect your crusie control as far as I know. A broken speedo cable would be an issue since the cruise system uses a speed pickup sensor at teh back of the instrument cluster at the speedo cable connection. It reads the rotation speed and uses the signal to maintain the desired speed. Unless the speedo cable is just doing something crazy it would not effect much. It would have to probably actually start rotating and then come to a stop and start again to cause any major jerking motion. I dont think that is really a possible situation. I think you have a bad rebuild

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Christopher Henkel
1990 190E 2.6 - Arctic white SOLD
1986 190E-16v - Blauswartze
1993 300CE - SOLD
2003 W208 CLK 320 Cabriolet - Magma Red