Tach wiring question

[russhow73]

Sun Pro Mini installed
Black-ground
Green- neg coil
Red- ign
White- spliced into oil gauge light wire(after market)

Tach works fine, but when I pull the light switch the tach goes to zero. The light on the oil gauge still works fine and I have no light on the tach.

Help

 

[Broncobowsher]

Fix the ground.

The tach shares the ground with the bulb. With a bad ground the light wire is in search of a ground. It is finding it through the lights. Adding power to the lights breaks the ground (puts power to the ground of the tach). Since the tach no longer has a ground (power to power, power to light, power to ground) it is like a bird on a power line. No sense of the voltage and shuts down.

Want a simple confirmation? With it running and lights off and the tach working break the splice for the tach light.