I have a 1978 f250 that the op said the switch valve got gummed up so he hotwired just the front tank. He told me when I bought it that all it needed was the tank switch valve and I would be able to at least see the fuel level in the front tank. Well I bought a switch valve, plumbed the tubing, and hooked up the single wire, and nothing. The truck will run, but will not show fuel levels, and will not switch over to the reserve tank. I took apart the actual electronic switch on the heater controls and cleaned the contacts. I also verified that the dash switch was getting 12 volts. I'm at a loss, and new to Ford trucks. What am I missing?
Original tanks switch valve:

New set up:

 

Have you confirmed the fuel gauge itself is functioning correctly?

 

1975/79 F100/350:

D5TZ-9189-B .. Fuel Tank Selector Valve & Solenoid Assy.

D5TZ-9A050-A .. Switch-when activated, switches the tanks and the fuel gauge.

1967/74 is different. Has a manually operated selector valve on the floor, the switch is only used for the fuel gauge.

 

A Manual switch.....I need one! Must be for trucks with in-cab tanks huh?

 

Did you pull the wire plug off the top of the rear tank and check for volts there? follow that wire along the frame rail, i bet its been accedently cut or something. I think each tank has its own ground as well... if it hasnt been used for awhile the float may be stuck or gummed up as well - might hafta drop the rear tank .....

Just re-read your post - both tanks dont show level .... might be your gauge as montana said - id follow the wires along frame first ....

 

All 1967/74's with two fuel tanks used the manual selector valve. 1973/74: In-cab tank or not.

1967/72 with in-cab tank & optional 25 gallon auxillary tank / 1973/74 with in-cab tank, midship or aft axle tank / 1973/74 with mid-ship & aft-axle tanks.

D3TZ-9A050-B (replaced C7TZ-9A050-A & C8TZ-9A050-A).. Fuel Gauge Selector Switch-1967/74.
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1975/79's have a selector valve & solenoid assy (D5TZ-9189-B). The switch (D5TZ-9A050-A) activates both the tanks and the gauges.

But, you might see a manual selector valve on 1975/79's IF an aftermarket auxillary mid-ship or aft axle fuel tank was installed by a private owner, RV dealer, aftermarket supplier.

Some of these trucks came with only one aft-axle or one mid-ship tank from the factory.

Since FoMoCo did not offer dealer installed fuel tanks, they had to be added by aftermarket suppliers.

 

I know from experience that the gauge and the tank selector run on two different fuses. Make sure your fuse is good.

 

Finally just took it in. Ends up, the dash voltage regulator was shot. My mechanic installed a new one and now our gauge works again. Now I just need to clean the rear tanks and get it working.