is this guy going to read this info? lol

 

yes sir i definitley have been reading. i just didnt sign in. the info you guys have giyen me is great. i still havent fixed my problem yet im kinda not sure still what to do. my selector switch seems to be working ok. would that cause both tanks to cross fill into each other ??

 

like i said before i dont think it is a switch, probably just both of your FDM'S are both on causing a pressure problem. try hooking up a gauge and finding out what the fuel pressure is.

 

That's not how a selector switch fails.

 

Then explain how a selector switch fails. Is it not possible for both pumps to be on at the same time due to a failure of a common item? Is it impossible for the switch to short between the tow separate pumps contacts within the switch. Chevy uses a sector valve body mounted under the truck. As I understand it from this thread Ford does not. Is this correct?
DT

 

easy, easy. Ymeski was stating that the issue:"fuel returning to the unused tank" is NOT how the dash switch would fail. Normally that issue is the check valve in the tank it's returning to. i.e. my rear tank fills the front tank when I'm using it, has been for over a year, the front tank does NOT fill the back tank so, the front tank's check valve is bad.

I've unplugged my selector switch while it was running and it killed the engine. The switch does supply power for the fuel pump relays and switches the power between the two, however, while it is possible for the switch to activate both pumps the more common problem is not powering either pump.

 

OK let's see if I get this right.
When I select the rear tank the check valve for the rear tank is overridden to allow fuel to return into the rear tank and the front tanks check valve automatically closes (a normally closed or fail closed valve) so fuel only goes into the rear tank.
Is this correct?
DT

 

erm.. not quite. On my 94' w/dual tanks if I'm running on the front tank, the selector switch powers the front tank pump and sending unit, when I switch to the rear it shuts off the front tank pump and turns on the rear tank pump and sending unit. The check valve is in the tank, it's basically just a one way valve, except when it's broken. Normally, what ever tank your using the unused fuel will return to but, it will take the path of least resistance so, if the check valve in one of the tanks is bad, the fuel will always return to that tank, which is what's happening on my truck, the fuel always and only returns to the front tank cuz' the check valve in that tank is pooched. It's not pumping the fuel directly from one tank to the other, it's just returning to the wrong tank.

Hope this helps. On some other model years, the switching is accomplished a different way.

 

My guess is also the selector valve. The electronics could be corroded causing front take to be on at all times, and in a guess it could become louder because that pump to work harder cuz there's no where for the fuel to go. Check switch and filter first though because in the small chance its the switch its a cheap fix and it never hurts to replace a filter on these old trucks.

 

I know what a check valve is and how the principal by which they operate. BUT, if you are going to have a check valve allow flow AGAINST it's intended design direction of flow what controls that function.
So.
My question is what controls the check valves? Something has to be telling one to open and the other to close otherwise fuel wouldn't be able to return into either tank if both are closed.
If there is only a dash switch that controls only the pumps and no selector valve to control what tank the fuel returns to what controls the check valves?
DT