Odd tach problem
On a 72 with SS gauges, I have an intermittently working tach. Sometimes it behaves normally. Sometimes not at all. But often at idle the tach will display about negative 200 RPM (the needle rest below zero. Increasing engine speed will increase tach, but always reads lower than it should. When I shut off the ignition, the gauge will jump up to the O mark and stay there. That is the needle will properly point while the engine is off. All other instruments requiring a good ground seem to be okay.
I just replaced the printed circuit board if that might have anything to do with it. BEFORE the replacement I had the same intermittent operation, but had never seen the negative rpm behavior.
Thanks for any help!