Hi, Welcome to Just Answer, I am Clayton. I can help you with this.
What is the last 6 of the vin number?,
The tach is most likely driven off of the ECM, but I will look and make sure your truck is wired like this.
I have a wiring diagram I can look at so you can ohm out the wires from the tach to the ECM if it is wired this way.
VIN #838265 Freightliner FLD 120
Could you send wiring diagram so i can follow sequence