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Mornin John, First be sure the sender has battery voltage on its terminal that DOES NOT go to the tank sending unit when its turned on. You gotta have power to the gauge before it can work. Then, if you take the wire that goes to the tank sending unit (the other gauge terminal), either at the sending unit itself or right at the gauge or anywhere else, and ground it out, the needle should deflect. If it has power going to it and grounding out the wire hooked to the sensor (the other gauge terminal)wont move the needle, its a bad gauge. NOTE Dont leave it grounded very long, just briefly, the sender has around 30 ohms resistance so a direct ground over currentS the gauge. Often, the trouble is in the sending unit or its missing a good ground instead of a bad gauge. Another thing to check is that the sending unit assembly itself is grounded. It either is grounded to the tanks frame via its sheet metal attachment screws, and/or theres an additional lil ground wire terminal on it (NOT the gauge wire) that ataches to frame or tank case ground. A bad or open or lose ground wire or if the tank has lost a good ground to the tractors frame could cause a gauge to not operate. Thats why if you direct ground the wire to the sendign unit (assumign the gauge is good) the needle will move even if the sender is bad or theres no sender at all. How a gauge works is power is applied to its terminal causign current to pass through the gauge (deflects the needle) out its other (non power) terminal, to the sending unit, through its variable resistor, then to ground. So a missign ground or a bad sensor (variable resistor or contactor bad) makes them NOT work even if the gauge itself may be okay. Ground out the wire leading to the sending unit, and assumign the gauge has power to it and the wire you ground out is good and attached to the gauge, the needle should full deflect unless the gauge is bad. Be sure and check for bad or open or missign grounds at the sending unit and that the unit is actually well grounded. Some have the extra ground wire (other than sending unit gauge wire), others just rely on the tank being well grounded as its attached to it via the mounting screws. Hope this helps, let us know. Ol John T Nordhoff in Indiana God Bless the USA, please pray for our troops
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