I bought a repop dash tach for my 1969 roadrunner. The thing zero's when you turn the ignition on but when you turn the lights on the needle drops below zero as if the tach looses power. I check the voltage at the tach and it stays at 12 volts with lights on or off. The engine is in the shop so I can't see if it works or not. Is this normal for the needle to drop on these tachs? Thanks Jimmy

factory tachs such as they are very inaccurate.