You might have a fault in the wiring or the gauge.
My sender wire was chafed where it went through the firewall and I had to remake it.
There should be ignition live to the "+" of the gauge in the dash, and the "-" of the gauge runs to the tab on the sender. There's also a brown wire running from sender body flange to the bus body that completes the ground circuit.
If the sender wire gets connected (eg shorted) to ground, the gauge reads full.
You can test the gauge, the sender and the wiring between by disconnecting them and testing for contiuity, shorts to ground etc.
Or gauge operation tested by taking the sender wire off the sender tab: gauge should read empty; touch the sender wire on ground: gauge should read full.
HTH