Cruise control pulsing, why?

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Nov 15, 2014
Des Moines, Iowa
1985 regal, v6 car with factory cruise. It works, kinda. Sometimes it pulses. It seems to directly coincide with the bouncing of the speedo needle. I can slap the dash make the needle jump and the cruise will momentarily drop out. It acts fine sometimes but usually after a while will end up pulsing like your pushing the gas and letting off quickly. The cruise light never goes off so the system is active the whole time, I'm sure I have the vacuum lines hooked up correctly, it wouldn't work at all otherwise, but I know it takes it's reading from the speedo cable... Will a bouncing needle/bad cable cause this?

I'm cornfused.
Will a bouncing needle/bad cable cause this?
Yes, the speed sensor used by the Cruise Control (and PCM) is mounted in the speedometer head, so any bouncing affects the speed input to the cruise control unit.

You may be able to clear up the problem by lubing the speedometer cable.
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