Hello all, I recently bought my '77 F-250 Camper Special, and as some of you may know it comes with dual fuel tanks. I have a few questions about these tanks that I'm hoping to get answered. The first is does the switch by the air conditioning controls actually change which tank is feeding gas to the engine or is there another thing I must do? The second is how big are the stock fuel tanks that came with these trucks, both the main and auxiliary ones? The third is have there been known issues with the fuel gauge showing the wrong amount of gas in each tank? I look forward to connecting with you all!

Thanks,
Josh

 

You should have a 16 gallon side tank and an 18 gallon rear tank, the switch on the dash controls the switching valve under the frame where all the fuel line connect, the fuel gauge is also controlled by that valve, when the valve slides to change tanks it has contacts in there that control the fuel sender. there is always lots of problems with the sending units, they loosen up and seem to work only when they want to, also corrosion on the connectors at the tank causes lose of ground. most of the work is done by the selector valve, the switch is pretty reliable. the valve is in a bad spot and you get road debris that ruins the valve, LMC has some of those parts, it's easier to pull the bed to replace the valve. good luck!!

 

Welcome to FTE, There are a lot of good, knowledgeable people here.
The aft axle tank holds 19 gallons whether it has evaporative emissions control or not. The mid-ship tank holds 20.2 gallons if the truck has evaporative emissions and 22½ gallons if it doesn't have the system.
If your truck has evaporative emission system, there should be a charcoal canister in the engine compartment, mounted on the inside of the passenger side frame rail, close to the radiator.
In '77 the switch by the A/C panel switches both the gauge and the tank. The switch changes the gauge directly, it doesn't work through contacts in the valve. On the '73 and '74 models with dual tanks, the switch only changed the gauge. There was a manual valve on the floor by the driver's door that had to be switched to actually change tanks. That changed in'76.
Incorrect/non-functioning fuel gauges are quite a common problem. One of the more common reasons a gauge indicates the wrong fuel level is the metal float in the tank cracks allowing fuel to seep in and making it read incorrectly. Eventually, once the float fills with fuel, the gauge reads "E" all the time. Dennis Carpenter sells a float that's made from a synthetic material and will never get fuel logged.
Dennis Carpenter Ford Car, F-100 Pickup Truck, 8N, 9N Tractor and Cushman Scooter Restoration Parts