Fuel gauge wasn't working on my MF 230 (1978). Ohmed out the fuel sending unit and it was open. Ohmed out the gauge and it read 120 ohms. Purchased fuel sending unit p/n 1079384M91; that is the fuel sending unit listed for my tractor. Installed new unit and fuel gauge now pegs showing tank full when tank is actually approximately 1/4 to 1/3 full. What's wrong?