On my 85 Z-28 when I crank the car the tach jumps up to 3000 RPM. As engine revs up tach starts moving from 3000 RPM.
On my 86 Iroc after I drive for 5 to 10 minutes in the past with lights on my oil pressure gauge,tach and tach light stops working. If cut off my dash lights the tach needle goes crazy sometimes pegging out on high side or it starts working as it should. Along with the oil pressure gauge as it should. In the last couple of months at times my tach and oil pressure gauges does not work even when I have all the lights off (driving during daylight hours). The ground wire coming thru firewall is connected to engine. I even ran a wire from ground side of battery to the ground where it comes thru firewall (engine side).
Both car did this when I purchased them.
The 85 Z is a 2 seated roadster from autoform # 145. Last four of vin #s are 1985. I know this and a dollar will get you a cup of coffee. That is if your a senior like me.
Any help on my problems will be greatly appreciate.