[pnaleahy]

The Smiths fuel sender relies on (a) electrical connection to the coil by attachment of the cable on the outside of the sender unit, and (b) an earth connection to the WIPERS which are attached to the float rod through a pivot hole in the body of the sender thus reflecting the movement of the float onto the wipers.
The earthig issue which stops these units from operating is that the body of the sender unit eventually oxidises and the pivot hole through which the float rod operates the wipers also oxidises thus preventing the earthed body from making contact with the wipers. If the wipers and not earthed due to the oxidised surface and often worn pivot hole, the sender will not transmit to the guage.
One solution to this problem if a new sender unit is unavailabe, is to positively earth the float rod. This can be achieved by using a small flexible braided copper wire which is clamped to the float rod at one end, say two inches below the sender, and the other end soldered to a brass ring with a tag, and formed to sit under the sender unit. The flexible braided wire needs to be light and loose enough to not impede the float rod. This solution has rectified this issue on my 405.