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I have an SVX tach, disassembled, on my desk right here. The way I understand it is that the tach takes ignition signals from the ECU and converts that into engine speed. Whether the IC in the tach converts this into a voltage for the motor that spins the needle, or just sends 1/6th of the pulses to the motor, I have not yet determined. I suspect something like the latter is more likely.
Anyway, I suspect the tach itself or its wiring in your case. You'd have other symptoms if it was something major.
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