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TH, My best guess is your sending unit located inside the tank is bad. Im unfamiliar with your tractor, but most of those sending units are a variable wire wound resistor with a slide wiper/contactor attached to the float, so as it goes up n down its at differing locations with a different resistance value which causes the gauge to deflect from Empty to Full depending on where the floats level is at and the accompanying resistance value at that point. Often when they go bad or become unstable and erratic its cuz theres a fault in the variable wire wound resistor or more likely the wiper/contactor isnt making good electrical contact at that particular location so the needle flops around. Usually if you dead ground the gauge on its terminal that leads to the tank sender (NOT the switched on hot one) she full deflects to FULL, while if its unattached she dont deflect and hovers near Empty. If it does that the gauge is probably good leaving a bad tank sender unit. There could be a bad or intermittent ground, but it kind of sounds more like a bad tank sender unit for now. NOTE The Gauge and Sender must be a match as to what range of variable resistance the Gauge was designed for. You need to replace the tank sender with the correct one to match that gauge and some generic off the shelf Auto Zone unit likely is NOT a match. Probably a bad tank sender unit is the problem !!!!! John T Nordhoff in Indiana, retired electrical engineer
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