Hello I have a 1989 F250 7.3L diesel. Im having an issue with my lights and tachometer. When I turn my lights on my tachometer stops working and when I tun them off the tachometer works fine. If anyone has any Ideas please shoot them my way.

Thanks

 

welcome to FTE.
first thing i would do is check the instrument cluster gauge grounds.
you may also have a bad tach in the cluster.

 

Well last night I went to the store and when I turn the lights on the tach will bottom out to zero, However when I started to drive the truck the tach seemed to work fine.