I have a 72 chevelle 402 SS, with 400 turbo trans. It has the tach dash, working clock and other gauges. i have replaced the printed circut overlay behind the cluster, but my tach only works properly, when i cut the lights on, and the amp gauge never mover, stays still. the amp gauges is new repo, but the tach is orginal. Anyone got any ideas where i can start looking for the fix? i have a MSD ready to run dist, and i have tried a ballist resistore, and without. i call the MSD tech rep, and he advised not to have resistor, since i do not run a msd box, so i tried that, still no working tach without lights on and no amp gauge working. any help will be appriciated.

Maybe a bad ground. With the lights on you end up with a path to ground for the tach.

Did you reattach the ground ring behind the light switch?

yes i reattached the ground wire and rechecked it. I had a MSD distributer, a ready to run one, with the blaster ss coil. I am just wondering since MSD has a different signal, that could be the cause.