tach runs backwards if i rev the motor it will start reading the wright way. but then, i know by ear the motor is running bout 1800rpm the tach says 3600 what the help please cluster or a sensor?

 

most likely cluster, my truck does it also. somewhere on here there is a very good write up on replacing the cluster, which i plan on doing next year and at the same time replacing the broken door (and all good doors) in my HVAC box.

 

Also in that writeup there's a place that will rebuild them for about $200.