question about a tach sitting on 800rpm

[rerfert]

My boat tach sits on the 1400 rpm needle mark and doesn't seem to respond to anything...I found another tach (same Teleflex brand) and the seller says it works good but the needle is sitting on 800 rpm?
My other tachs(Sun brand...in a sand rail,and a test run stand) both go back to zero when the engine quits or I shut the key off.
Do the Marine Tach's hold the rpm reading of the motor when the key is turned off?
OR is there a problem with that tach as well?
Thanks in advance.

[SmokinLowriderSS]

My old tach from '78 didn't hold idle rpm, my new one (from 2004) does. Zeros when you turn the key on again. It is designed to do this so likely nothing wrong with yours. Mine were both marine tachs, brand I don't recall.