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I have a 2002 Civic Ex. I've search but haven't found an answer that quite fits my problem.

On the highway the other day I put the car in CC and was quite shocked when the speedometer kept bouncing up and down, even though the speed of the car itself was constant. I've seen others here with the same problem, but this only happens when I use CC, not when driving without it.

Is it an issue with the VSS, or more connected to the CC?

 
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I should add that there are no warning lights, and the car doesn't seem to have any other problems.

 
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Looks like the VSS signal is shared by the PCM, Gauges, and the Cruise control.

Would be interesting to see if the datalist from the PCM reflects what you see on the speedo when this happens.
This info might lead to a decision about what to do next.

Got a friend with an identical car? Swap the cruise (control unit or actuator, depends on which model--coupe and sedan are different) and see what happens.