a few months ago i switched my gauge cluster to the 145mph, and now whenever the car is running with the lights off, the tach works like normal.. but if i turn the headlights on the tach doesn't work and turns backwards to around 7000rpm and then stops..

doesn't anyone know what causes this and how to fix it?

 

Sounds like a problem with the ground wire.

 

the ground wire for the tach or the headlights?

 

Well, since I searched, and this was the only thread that seemed to match my problems - I too would like to know. Does this sound like a ground for the tach, or headlights? I had an alarm that was installed poorly in my car, and I figured that was the cause for this problem. In my case when I turn the headlights or parking lights (anything that turns on the interior guage lighting) on, the oil guage drops to 0 psi, and the tach freezes at around 1000 rpm. I removed the alarm carefully, and completely - and I still have this issue. Any ideas?

 

Well the ground is not with the headlights, and I guess I could have thought harder and figured that out, seeing as the problem occurs with the headlights off, as soon as I turn my parking lights on. I'm going to pull the dash apart soon, and start tracing wires. Wish me luck?