Here is a quick blurb from a jeep site that briefly describes the fuel sender/fuel gauge system. I belive the tractor should be set up similarly.
Dave
This link is pretty straightforward.
http://www.aa1car.com/library/fuel_gauge_diagnosis.htm
Here is the text from the jeep site
The fuel sending unit wires are located on top of the gas tank
where they are hard to get to without dropping the tank. The fuel sending unit should have a pink wire with voltage on the isolated center post. The other black wire on the sending unit with a tab style connector is a ground to the frame. Make sure it has good contact.
To be sure the problem is not the gauge, you can momentarily short the pink wire on the output of the sender to ground, and this should show up as FULL on your gauge. DO NOT hold it for very long in this position, just touch and release the wire. If the gauge does not move from EMPTY either the wiring has an open circuit (no voltage, or no connection to ground) or he gauge is bad. If it does move, the sending unit is bad.
The sending unit can be checked with an ohmmeter to measure the resistance between the round sender post (pink wire) and ground. It should be:
Resistance Reading
73 ohms Empty
23 ohms 1/2 tank
10 ohms Full
If the resistance falls in this ballpark (depending on how much gas you have in the tank), then the sending unit is fine. If it shows infinitely HIGH resistance, then the sending unit could be bad OR the wire from the tank to the gauge could be open.
The gauge can be tested with the resistance listed above. Run an appropriate resistor to the S terminal of the fuel gauge and to ground and check the readings.