I suspect your the replacement tach is not compatible with the original. Old school tachometers actually used pulse inputs from the ignition systems to calculate and display the RPM’s of the engine. I am pretty sure (not 100%) that the new ones since the 80’s are actually volt meters driven by the ECM/PCM. The same applies to the speedometer. The ECM/PCM calculates the speed and RPM of the vehicle and generates an analog voltage that is sent to the speedometer and tachometer.
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