The sender must match the gauge. If the truth be known, the sender is a variable resistor and the gauge an ohmmeter of sorts. You can research past posts on testing the sender with hot water with a real ohmmeter. Get different values of resistance for different water temps. Then you plug those resistance values into your gauge. You now know your sysyem works and what those little marks on your temp gauge really mean temperture wise. This of course assumes that the wiring and such is correct and working. BTW, and yes to the electronic techie guys, I know an ohmmeter is really an ampmeter.

Six volt guy living in a twelve volt world