[wstaab]

I checked it at the connector which was disconnected from the tach.
The red/white wire at the connector show 0 volts dc on a digital meter with the instrument lights on. When you check continuity to the main ground lug in the fuse panel the red/white wire at the connector shows is is grounded. When the instrument lights are on the bulb in the tach is illuminated. If the engine is running the tach reads "0" when the instrument lights are on. Turn the instrument lights off and the tach functions normally. Next step is to disconnect the red/white wire from the connector and verify tach functions normally with the instrument lights on or off.