Fuel tank selector valve

Subject: trk#2 on my signature:
HELP! LOL, Ok, I suck at reading wiring schematics. I've been working on this truck for a while, I purchased it not running, I installed new fuel tanks with new fuel pumps & sending units & I only put fuel in the front tank, after finally getting it running and drivable I thought I was getting horrible mileage but come to find out I was getting fuel returned to my rear tank. I figured out that my plungers in the selector valve were basically stuck half way open/closed. I took it apart, cleaned it up, freed everything up, plungers manually move freely now, reassembled & reinstalled. Now here's the problem I'm chasing, the dash switch isn't switching the valve, I had power to the red & the Yllw w/wht trcr wires at the valve harness but when I flip the Frnt/Rear switch nothing changes at the harness as far as which wires/pins get power. I'm not sure which pins should be hot when the switch is in a given position. Advice welcome. Thanks

According to this picture red is rear and brn/wt is front.
One or the other should be hot depending on the position of the selector switch.



Sender signal should switch yel/wt (power from gauge) between yel/lt bl (rear) and dk bl/yel (front) senders when the selector valve motor shifts.

So does the harness have to be plugged in at the fuel pump/sending unit at the tanks to complete the circuit to get voltage signals at the harness plug at the selector valve? I wanted to check to see if my switch in the dash is working properly but seems like you have to remove half the freaking dash to get the ac control head out to get to the switch. Is there any where I can test the in dash switch for proper functioning besides right at the switch, seems impossible to get to. Thanks

Power at red should shift to power at brown/white with the switch in the front tank position.
Edit: You should also then see power at the front pump (brown/white) connector to the fuel tank.

Gauge sender switching is done internally to the valve, so pulsing ~5 volts at the yellow/white will stay the same.

Unfortunately it seems the selector switch in your dash is not working.  

testing it from the valve end to see if the switch is working is the easiest at this point but check fuses and relays first. the switch cannot give voltage that it doesn't get itself.
if getting to the switch proves necessary then yes there are a couple things to remove that make it easier and with 40 year old plastic very important to keep from breaking stuff. Trim bezel to start with. ash tray, floor heat duct, and it's not mandatory but this is much easier with radio out.

Power at the red wire of the selector valve proves the fusible link, the pump relay and the inertia switch.
It also proves power is getting through the selector switch at the climate control.
But if moving the switch doesn't kill power on red and deliver power on brown/white the switch isn't functioning (and the motor in the selector valve can not shift both the ports and the gauge sender signal)

So I removed the ac control unit from the dash, what a pain in the butt, anyway, I have no power to/at the selector switch in the dash, I checked the connectors for continuity & it seems to be working as it supposed to I guess, I'm trying to figure out where the power to the switch comes from. Thanks

The fuel pump relay is triggered by the EEC.
This provides power to the selector switch through the inertia switch.
It will be powered for only a couple of seconds each time you turn the key to 'run'

The engine has to be turning (crank or running) for the relay to hold closed.

You should get power at the pink/black wire of the switch connector.
Look at the diagram above. If you need explanation I'll be glad to help, if I can.

How would I bench test the switch valve as far as which pins I would apply power to and which ones id ground or jump so I can chk to make sure its not burned up, I want to verify that it'll run & the plungers open & close before I reinstall the fuel tank, it looked ok when I had it apart. Thank you SO MUCH for your help.

Apply 12V to the pin connected to red and ground to the pin connected to brown/white: You should have continuity between yellow/white gauge pin and yellow with light blue (rear sender) pin.
The port for the rear tank should connect to the port going to the high pressure pump on the frame rail.

Apply 12V to the pin for brown white and ground to the pin for red: You should hear the motor shift and now should have continuity between yellow/white sender pin and dark blue/yellow (side tank) sender pin.
The side tank port should now be connected to the port going to the high pressure pump.

Note that the switch valve grounds through the respective fuel pump(s) ***and back through the selector switch to G701 to the right of the radio, inside the dash.
I'm going to include the complete diagnosis section in a new post***

You say that you've replaced both in-tank pumps, but you should check that the connector for the side tank has a good connection to ground.

If the black wire going to splice 161 (shown above) is broken or the connector to the fuel tank for wire 57 is bad, the valve won't shift and the pump won't run, even if the selector valve proves good.

***Edit***

Note: Electric Fuel Pump Control (dual tanks) 5.0 EFI Engine -and- Fuel Tank Units Don't Switch (dual tanks) starting in the center column of pg. 108.

Note: in the diagram (above) Connector 154 between the tank selector switch and splice 155 of the ground wire going to G 701.
This needs to be connected and Component Location is shown as being on the left hand fender apron.

Thanks SO MUCH for everyones input and direction, I got'r back together today and it's working like she's supposed to. Unfortunately I don't think I needed to remove the HVAC control head, putting that back together tomorrow morning. On to the next issue.

Well done!  

I'm sorry to have suggested the problem lay with the switch if it wasn't at fault.  

Glad you've got the switch valve working again. As Gary said Good Job!

So what was causing the valve to only receive power on the red wire?
How did you figure it out?

I didn't have the harness plugged into the fuel tank, I guess it needed to be plugged into complete the circuit, as soon as I plugged it in everything started working as it is supposed to. I also did not realize that the power at the switch was only momentary when the key was turned on and the engine was being cranked, once I checked the switch while I was turning the key on and cranked  it over then I could see that I was getting power to the switch.

Right.
But look above at the ground path for the valve and switch...
That's why I suggest that connector 154 on the inner fender must be disconnected going back to ground 701.

The system shouldn't have to ground through the in-tank pump windings to shift the valve and sender contacts.
The switch itself is what changes power from red to brown/white, and it wasn't doing that for you before.

this sounds more like an incidental change. like you disturbed a corroded contact. yes, it is smart to go back through and clean any and all grounds as rust and corrosion are the enemies here. it does not always have to be visible to the naked eye. this happens a lot with older vehicles and more so on ones that have had a bit of idle time out of service.

this sounds more like an incidental change. like you disturbed a corroded contact. yes, it is smart to go back through and clean any and all grounds as rust and corrosion are the enemies here. it does not always have to be visible to the naked eye. this happens a lot with older vehicles and more so on ones that have had a bit of idle time out of service.