1994 civic hatch.all of a sudden one morning,car warmed up and instead of idling around 800 rpm, the needle dropped down to 200 rpm.the motor was not affected by needle drop.the motor was still at 800 like nothing ever happened.me no understand!

 

Hook up an external tach to the tach plug in the engine bay to see whether it reads correctly. Alternatively, swap in a buddy's cluster to see whether its tach works fine in your car.

 

Thank you!