I have a VDO speedometer that was recently refurbished by Palo Alto Speedometers, who also supplied a matching cable. The needle is steady and moves smoothly when the cable is attached to a variable-speed electric drill. But when installed in the car the needle bounces around. It seems to be effected by road bumps, but even on relatively smooth surfaces it still wanders.
If the speedometer and cable are fine in a bench test, I’m assuming it has to do with how the cable is attached to the wheel. Or maybe not. Any ideas?