If you are open to trying a few other things. Using the old fuel sender and an ohm meter see what the readings are when at the empty, half full and full. Using the old fuel sender and leaving OUT of the tractor, but connect to the fuel gauge move the float up and down and see what happens. What I am wondering is there something in the tank that is keeping the float from going down.
Does anyone have a tech. manual for the 4310 and could you share what the ohm readings high and low for the fuel sender are?
For what it is worth here is how it works.
Sender assembly consists of a movable float mounted to a variable resistor (potentiometer). Resistance of potentiometer on gauge sensing circuit controls the fuel level shown on gauge. As float moves up or down with changes in fuel level, electrical resistance of potentiometer changes accordingly. When tank is full, resistance is low and gauge needle moves to full position. When tank is empty, resistance is high, and gauge needle moves to empty position. Potentiometer resistance between these two positions varies in direct proportion to changes in fuel level. This allows gauge to accurately display quantity of fuel in tank regardless of its level.