yet another fuel gauge thread - reads full all the time

Disconnect wire at the sending unit and see if the gauge still reads full. If it does your sending unit wire is shorted to ground somewhere. Not a common problem but its possible. My guess would be an issue with the sending unit. The new sending units are garbage. I gave up and just keep track of my mileage. I had mine working perfect and then it started doing what yours is doing. Reads chock full for a long time then drops to around a quarter.