cheak the earth on the sender and the 2 conections are clean and tight
on the sender unit  ?
or empty the tank remove locking ring and remove sender using a meter
look at the resistance slowley from empty to full and full to empty
if the meter goes to 0 then the track inside the sender has gone so its time
for
a new one (oe) or scrap yard or mini rally (take your meter to test)