info on 90 fuel light plz

hi all i have taken my fuel sender out of my 87 defender. there are 3 wires that go to it and i have taken the one off that operates the fuel light and earthed it down and the light comes on. but wen i connect it back on sender it dont work. i cant see any break in the connection and cant really see how it operates to come on. they do work on thease dont they or is the sender made only for tank contents. any ideas if it is the sender.

There is an 'empty' contact on the fuel level sensor, unscrew the sender and have a look at the contacts. Being that old they may be corroded. A bit of a clean may sort them or you may need to replace the sender