Bench test procedure: The rear of the gauge should be marked "IGN" and "Sender". If not, just note which is which when you remove. Then hook up sender with a fused 12V to proper terminal on gauge and a wire from sender center tap to other terminal on gauge. Then there needs to be common ground path between sender mounting flange, gauge mounting flange and the neg of the battery/power supply. Hook it up with mostly test wiring, not exsisting wiring if you can. Be sure you have all good contacts (ground and 12V). I use a fused battery charger for my power supply on the bench. Anyway move the float up and down to see if gauge registers empty and full. Make sure all test connections are good. Ohm reading is about 30 ohm, on down to about zero. If it all works, it's wiring and/or ground paths. Corrosion builds at all the ground points, and they all have to be there to get back to the battery. As Bill has indicated, everything is isolated with rubber or pad material. Your grounds need to be from tank sender flange to frame, frame to body, body to engine, engine to batt neg. All that has rubber/insulator mounting to be overcome with CLEAN, SHINNY, METAL to METAL, toothed washer, contact for everything that draws power. After you decide what's wrong, you can use this sender (variable resistor, As said by Bill) to test other gauges that do not work. I.E temp.
