2.3L ('83-'97) Tack not working right

[John501]

My ranger is a 1990 2.3 manual, today I swap in a new radiator and thermostat. After I fired it up, I noticed my tack was idling at 500rpm, which the engine was not. So I took off, and it seems the tack is unresponsive, the highest it will go is 1300ish even why I have the engine around 3000rpm. It tried to work right once and shot up to the right rpm, then just flew back to 1000. I don't know what is wrong or how the guages works, if someone can help me figure this out? Also since I've got the truck when I turn it off, the tack never went to zero, without me taping the cluster.

 

[tomw]

The tach itself may need some work. Mechanisms can get sticky, and have too much friction for the coil(magnetic?) to overcome and make the needle move. My theory(untested ignorance) is that the number of pulses per second is used to generate a field, which works in opposition to the instrument field, overcoming spring tension to zero the needle. If there is dirt or solidified lube, or the bearings for the needle shaft are sticky, it can jam and not move.
Take it apart, not fun with not much percent success, or get a replacement??
tom