Traded for a 1985 F250 with 6.9L diesel. When I select the rear aux. fuel tank the gauge works. When I flip the switch to go to the front tank the tank selector switches and it runs off the front tank but the gauge does not work. I unplugged the connector from the tank and jumped it but the gauge still does nothing. I am not sure if the switch in the dash changes the tank and gauge or just the tank and the selector that the two tanks go to also changes the gauge. Bought a Chiltons manual but it does not show electrical drawing of how this is wired up. Need help.

 

Had same trouble on my 90 F-250. if the truck still runs when you switch to front tank then selector is working fine.
In my situation what happened was that floater was getting stuck with wires that go in the tank along with fuel pump so it was not moving. I droped tank and cleared wires a way from the floater and that fixed the problem, But I should of used a zip tie or something to hold them tight becasue now it happened again so I'm gonna have to drop tank one more time.