It’s pretty easy. The cluster itself is modular so the tach portion pulls right out.
Check out this vid. At the end of the video I’m pulling the cluster apart to remove the tach. The beginning part talks about calibrating the tach but you can use this to just test to see if the tach actually works. The tach works off three pins. 12v+, ground and a signal wire. That wire is the Neg side of the coil.
But if the tach from the Saleen cluster worked, it points to a bad tach in this cluster or maybe an issue with the ribbon on the backside of the cluster. Flip the cluster over and find the three pins for the tach and where they connect to the ribbon. One of them is labeled S for signal. Trace that back to where the body harness plugs connect to the ribbon board and see if the connection end is in good shape. Often times those get bent up or break and don’t make a good connection.
Unfortunately just saying the tach is bad doesn’t tell us if it’s the actual tach, or a connection issue on the cluster wiring.
This vid is more for calibrating but it shows how you can test function and how to disassemble the cluster
View: https://youtu.be/UkHoSIdTpbc