So the tach on my Terrano has been not working properly since I got it here 6 months ago. It would work but be a bit higher then it should, and then fly all the way to red line and then not work at all, all in a matter of 15 seconds. I wasn't concerned because its automatic and running fine.
I decided to try and fix it. I took out the tach, 2 screws up the top of the cluster and then 4 to remove the whole assembly. Once it is loose you will see the 4 screws that hold on the tach. One of the top ones is on a connection line that says GND next to it. This screw was not giving a good ground connection to the cluster. I loosed it and played with it while the truck was running and I could see the tach start working and then stop as I moved it. I ended up pushing it to one side and slowly tightening it.
My tach is now working normally.