Both fuel pumps would not come on

Hello I have a 1994 f-150 with a 4.9L and dual tanks. I shut off the truck and it would not start. Both fuel pumps would not come on. The fuel pumps will sometimes both work then when I shut off the truck and then they both wont hum when theyre priming. Ive changed the maxifuse the relay and the inertia switch. I can hear the relay clicking on when I first turn the key on. The only thing I havent changed is the tank selector switch but I took it out and made sure it had a secure fit. And when its working I can switch to either tank with no problem, then both will die. Any help you can offer would be greatly appreciated. Thanks again. Joe