I just brought home a 72 F-100 and I'm going through it fixing what I can find. Of course one of the first things is the gas gauge not working. Fortunately I can see my biggest problem, the wires on the auxillary tank kind of hacked together and need to be replaced. So it looks like I'm starting from scratch.

The tank behind the seat has been taken out so it's just the auxillary tank. However the wiring is still there for the tank behind the seat. Could I simply run the orange wire down to the auxillary tank and find a ground for the other connection to make my gas gauge work?

 

Dash full gauge not working properly (if at all): The fuel sending unit float is made from two pieces of copper soldered together.

Solder breaks down, gas seeps in, float settles to the bottom of the tank.

COAZ-9202-B .. Fuel Sending Unit Float / Available from Ford / Same: 1957/79 Passenger Cars/Trucks, 1961/79 Econolines & 1966/79 Bronco's.

Carpenter offers a one piece brass float, use it instead of the problematic FoMoCo original.

 

Touch the orange wire to ground and see if the gauge even moves(to FULL mark)? That will tell you if that part is working properly.

Then hook it to the aux tank and see if it reads anything.

There should be a switch on the dash to switch sending units between the two tanks(originally). Make sure you have it positioned correctly. Or is the aux tank not original to the truck?

 

If the auxillary tank is genuine Ford, the exposed fuel cap is located (1970/72 F100/350 Stylesides) below the scallop (bumpside) line, in front of the left rear wheel.

The switch is mounted to a bracket that is located under the dash, to the left of the steering column.

The bezel is marked: L. FUEL TANKS R. as the same switch & bezel were used on F500/750's with R/L cylindrical tanks.

1967/69 F100/350 Stylesides: The auxillary tank exposed fuel cap is located above the scallop line, in front of the left rear wheel.