75 ford truck wiring
Here is the best I can find on that old of truck it covers all models 1975 ford full size trucks
If its a factory dual tank system there should be a switch on the dash to switch gage from one tank to the other.It will be located on the heater control panel. If its a add on dual tanks which a lot of them back then where there still should be a switch but its hard telling where the switch will be located.I have seen them in the glove box,ash tray, botXXXXX XXXXXp of dash, and by the manual valve in floor board. Here is a simple diagram of the fuel tank wiring system.
The way the system will work there will be 1 wire from each tank sending unit going to the switch then 1 wire will go from the switch to the gage. Common problems are bad grounds at tank unit or bad switch. I have also seen a lot of damaged wiring where it runs up the frame.