My stock tach isn't accurate after 2500rpms so I replaced it with a sunpro mini tach but it won't read rpms. I returned one and picked up an exact replacement sunpro and the same thing doesn't read rpms. I'm purchased a pigtail to wire the green tach wire to the tach signal on the distributor, ground off battery post, white wire to headlight switch and red to ignition on fuse panel. Distrubutor is computer controlled i have the factory cc quadrajet. Big cap distributor, rebuilt 1000k miles ago. Tach is set on 8cyl but no rpms not even a flutter, do I need a filter for the tach to work?

 

Have you tried to replace the circuit board on the tach? I done this tach replacement and it does work. Straight forward and lots of videos on this project. took me 30 minutes and that's including taking off the cluster covers.