Why would my tach stop working when headlights are put on? It is on a 85 z28.

 

Sounds like a ground problem. Do u have grounds from the firewall to the engine then to frame?

 

Yes, it is a ground problem, but, it's not under the hood. It's because the ground to your instrument cluster has failed, so now your tach is getting it's ground through the dashboard illumination bulbs when the headlights are off. Switch the headlights on, and the dash bulbs are no longer a surrogate path to ground for the tach.

 

tkx for the info OrangeBird. U refreshed my memory about the tach. I had in indash tach in my '72 Buick GS (sold) and u reminded as to the ground