I Have A 1966 Chevelle Malibu. I Installed A New Repodcution Knee Knocker Electroic Tach. Gray Wire To Insturment Lights, Brown Wire To Negative Side Of Coil. Black/pink Wire To 12 Volt Source. Tach Is Grounded When Mounted To Dash. I Have To Tap On The Tach To Get It To Work. This Is The Second New Tach That Has Done The Same Thing. Need Advise.

My guess is that you have a poor ground. Make sure it is bolted tight to the dash (bare metal) and use a star washer.

Reproduction = Junk..........Been there....just my $.02

Ken