Possible sender fault

TOY440 said

I noticed my fuel tank now reads full. I disconnect the feed wire from the sender (I only have one wire, some have 2 wires, like an earth, mine has one), and it still reads full, I earth out the wire, it reads empty but very very slowly creeps towards full.

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If I'm not mistaken the gauge reads full when the resistance to the gauge is low like 10 ohms and empty when the resistance is in the 72 ohm or above range. If you short the sender wire to ground the gauge should read full. If it is not connected or shorted it should read empty. If it is reading full when disconnected and not grounded I suspect you pinched the wire and it is grounded. Grounding the wire and you get empty but slowly raises to full might be you moved the wire away from the sort and when you let the wire be it moves back to the short.

Also it is important to be sure the tank is properly grounded or there is no path for the ground side of the sender to get back to the chassis and make a complete circuit.