This is a 1990 truck and according to the wiring diagrams for the 1990
> F-250, there is only one fuel pump relay and the power from that goes to
> the Intertial Switch, then from there to the Fuel tank Select Switch,
> then to the tanks. There is no whirring noise when the switch is set to
> the front tank, but there is a whirring noise when the switch is set to
> the rear tank. There appears to be only ONE fuel pump relay.
That would work (one relay). The switch probably just switches between the two electric pumps.
> What I am wondering (and no one seems to be able to tell me) is if there
> is some sort of a valve in the pump that is turned off that prevents
> fuel from being pumped into a tank that is not being drawn from. If
> that is the case, the fuel is just being pumped into the tank with the
> least resistance. I assume that since the front pump doesn't "whir"
> when power is applied to it, that this means it might be so corroded
> from not being used for several years (since the locking gas cap jammed
> up and I didn't want to screw with it - and just used the rear tank) and
> I assume therefore that this corroded pump is the culprit.
There is probably a check valve built into the fuel pump if it's anything like the '91 Explorer pump, which it probably is. It's just a one-way valve. It doesn't get "turned off" it simply allows fuel to pass one way only. Unless it's busted.
>