1990 Ford F-350

Electrical problem
1990 Ford F350 V8 Two Wheel Drive Automatic 120000 miles

Ok. I just bought this truck. I have dual tanks. After I got it home I was playing around with it a little and I decided to switch the tanks to see how much fuel was in the rear tank. I flipped the switch and the truck stalled 2 seconds after. The rear tank was empty. So, I flipped the switch back to the front tank after I had shut the key off again. (The front tank had a half tank) Then I tried starting is again and it would not start. I'm figureing that the selector valve is now vapor locked or stuck somehow, the relay for the front tank had coencidentally went bad at that moment, or the fuel tank switch may have also coencidentally went bad at that point. I am not sure if there is two pump relays for this truck though. My 88' f-150 had dual tanks and had two relays; one for each tank. I couldn't get a chilton manual that could help me with this dilema. I am assuming that there is only one relay for the fuel pumps though. I think it is the green one. My 88' had a green relay and a brown relay. I would for sure like to know if there is more than one relay so that I can eleiminate that. I would also like to know where the selector valve is for the tanks. I pry should also add that when I use the rear tank I can hear the pump kick on for the second that it should to prime the line. When I try to use the front tank the pump doesent kick on but I can hear something click. Which is pry the relay. Thank you.
January 31, 2008.

Well now, to answer my own question. 1.) I have figured out that there is only one fuel pump relay for both pumps/tanks ( it is the green on on the driver side fender wall ), 2.) There is no external selector valve that switches the tanks. 3.) The fuel pumps themselves have a check valve on the outlet side of the pump that delivers fuel to the engine. When the pump is energized by selecting the tank the pressure in the pump builds up behind the check valve and forces it to open allowing fuel to be pumped to the engine.
THERFORE. I have one of two problems. The lesser of the two evils is that my selector switch is pry bad and needs replaced. Or the greater evil, my fuel pump needs replaced because the outlet check valve is stuck shut. Which means I have to figure out where I'm going to stick 9 gallons of gas in order to drop the tank. Since I don't hear the fuel pump kick on when I try to use my front tank I am hopeing its the tank selector switch. I will post my findings.

Jan 31, 2008.
It was the tank selector switch after all. I pulled the one out of my 88' f-150 and used it in my f-350. I flipped the switch to the front tank and turned the key on. Sure enough I heard the pump knick on and the truck started right up. The switch leg for the pumps power was not closing and the switch leg for the fuel gage was closing. Thats why the pump wouldn't work but the fuel gage still worked for the front tank. : D

Feb 1, 2008.