88 Ford F250 rear gas tank will not work

You've not identified the truck model or year.. Some trucks had after market installations of auxiliary tanks. The Ford 150 with dual tanks had pumps in both tanks. Some suggest dual tanks were always quirky, problematic. However, dependent upon the year, the pumps might be located in the tanks OR the pump might be on the engine. Newer trucks with fuel injection actually had three pumps. The newest have a pressure pump in the front tank and a balance pump in the rear. Some switches affect only the guage. P haps one of the forums below can help when you supply more detail.

https://www.ford-trucks.com/forums/1110959-how-to-switch-dual-gas-tanks.html

https://www.cargurus.com/Cars/Discussion-c5249_ds468075

https://www.ford-trucks.com/forums/1401495-1985-f250-with-dual-fuel-tanks-pulling-from-wrong-tank.html

http://www.tanksinc.com/index.cfm/page/ptype=product/product_id=102/category_id=167/mode=prod/prd102.htm

I'm assuming this has dual tanks, do you know if it is not working on both tanks ?
Most people would check the circuit for the gauge at the tank. The sending unit grounds the gauge on the dash.

Sounds like the selonoid at the switching valve is not working.

wiring harness under dash.
wires coming off top of fuel tank.
check all fuses.
do you do a lot of off roading - you may have cut or pulled a wire out.
finally you may have:
1. a bad fuel tank sending unit - need to drop the tank to replace.
2. a bad fuel gage.
3. a bad instrument panel

A loose connection on the sending unit could cause the problem but also moisture in the tank in cold weather could also be a factor in which i would add fuel line antifreeze to a full tank of fuel..........Hope this helps you out.

volt meter, fuel gauge, oil pressure and coolant temp all share the same power and ground

the circuit you want to check is the yellow and white wire from the dash to the dual switch

Wes, try checking your float in your tank, or check the sending unit.

replace your forward in-tank fuel filter

Dual tanks sound like the float in the tank is stuck. Need to make sure its not binding up. (I did alot of those jobs)

2 things.
There is a sensor that goes from the dash to the fuel pump. If it is disconnected, dirty, or faulty, then thats an easy fix.
There is also a float in the gas tank itself. From what Ive been told after time, or going over big speed bumps, etc, that the float just simply doesnt float or register.
Mine actually shows I have a full tank or im empty at all times. No in between.