I have completed the swap FROM my manual dual fuel tank selector next to the seat TO the dash mounted electric one on my 1975 F250.

I used all NOS parts except for the fuel solenoid valve(got that part used)
Everything works properly on the main tank setting, draws fuel and the gauge drops as fuel is consumed also no 12v present on the fuel solenoid valve wire.

On the Aux setting,fuel gauge swap happens; I get 12V as required on the single fuel solenoid valve wire but it seems as though the valve does not switch to aux tank and still draws fuel from the main tank.
I proved this out by selecting AUX tank and:
A: disconnecting the AUX tank fuel flow and the truck keep running
B: my clamping off the main tank fuel flow and the truck runs out.
So I know its not switching over even though I can see the the voltage when on AUX setting.

Could my used valve be bad?
What is the failure mode for these valves?
I would think that they would fail in an naturally "open" flow through position and be able to draw fuel from one(main) tank as a "fail-safe" mode.
Any other way to test my existing solenoid valve that i'm missing?

Thanks for any help.
Tbruz

 

1st check your ground make sure that's good. I just wrecked out a 78 s/c and removed that valve. You did things right but you may have to remove the valve to test properly. What I found on the bench test was :
#1 good ground
no 12v, fuel from main tank [default mode] you can blow air through this line
12v applied, aux tank opens/main tank closes you should hear and audible click and be able to blow air through this line
I had to apply power a few times while tapping the valve with a hammer and that freed the valve of rust and it works now
try that, is it dosnt work the valve is bad

 

I agree that it might not necessarily fail, but may be stuck. I had one stuck half closed so that it would draw from both tanks. Imagine my surprise when I emptied one tank and switched to what I "thought" was the other full tank.

 

Ground is good as I added star washers between solenoid body and mount locations. Ground checks out with a meter as well so I can rule bad ground out.
I hadn't thought of stuck; did not hear any clicking as I change from Main to Aux will go tap body and see if anything happens.
Ready to go aftermarket with Duralast FSV4 for $44 if all else fails.
Thanks
Tbruz