plug the electrics of the float/pump (removed from tank) into the loom & turn on the ignition. move the float through it's full range & see if all the bars on the RID come on/go off as they should.
there is a 10 second delay before it will register BTW, so don't just wave it about.
if that doesn't work, try cleaning the contacts. there are 2 moving sprung contacts that connect to 2 separate tracks on the unit.
one track has many sections & is the fuel gauge, each section seems to correspond with a block on the RID. make sure they all work.
the other track has 2 sections & only switches on the low fuel lamp AFAICT.