Fuel - Switch to rear tank doesn´t work

Hi all! It looks that the switch to the rear tank doesn´t work properly at my truck. When I switch from front to rear it shows the rear tank as 100% filled but is still using the front tank as the truck continues running and fuel level of front tank gets down. The rear tank is shown in the cockpit without any movement.

Any idea where to look at?

As I´m planning for a longer drive it would be helpful to have both tanks available.

Thanks in advance.

It seems like the fuel tank selector valve isn't working properly.

As shown in the lower left corner of the schematic below the switch in the cab has two sets of contacts - one for the fuel tank selector valve and one for the sending unit.  The sending unit contacts are working as the gauge shows the correct level for the tanks - right?  But when you go to the rear tank it still pulls from the front tank.

If you look closely you'll see that the powered-off state for the solenoid in the valve is the rear tank.  In other words, any time there's no power to the valve it should go to the rear tank.  So your valve is stuck in the front tank mode, even when the power is off.  That means it is a mechanical problem in the valve and not an electrical problem.

I would first try tapping on the valve with a plastic or wooden hammer/mallet when the power is off to see if it will free up and move.  And you could lay under the truck with your hand on the valve and have someone flip the switch back and forth with the power on and see if there is any click or thunk from the valve that would indicate it is trying to move.

But if tapping on it doesn't help then you either need to replace it or remove and try to free it up, maybe by using air pressure or lubricants.  However, I've not done that so don't know if it is even feasible.

Good luck!  

Gary Lewis wrote

It seems like the fuel tank selector valve isn't working properly.

As shown in the lower left corner of the schematic below the switch in the cab has two sets of contacts - one for the fuel tank selector valve and one for the sending unit.  The sending unit contacts are working as the gauge shows the correct level for the tanks - right?  But when you go to the rear tank it still pulls from the front tank.

If you look closely you'll see that the powered-off state for the solenoid in the valve is the rear tank.  In other words, any time there's no power to the valve it should go to the rear tank.  So your valve is stuck in the front tank mode, even when the power is off.  That means it is a mechanical problem in the valve and not an electrical problem.

I would first try tapping on the valve with a plastic or wooden hammer/mallet when the power is off to see if it will free up and move.  And you could lay under the truck with your hand on the valve and have someone flip the switch back and forth with the power on and see if there is any click or thunk from the valve that would indicate it is trying to move.

But if tapping on it doesn't help then you either need to replace it or remove and try to free it up, maybe by using air pressure or lubricants.  However, I've not done that so don't know if it is even feasible.

Good luck!  

Gary, my '86 5.0L uses a 6-pin selector switch and a strictly mechanical selector valve on the frame rail. I believe the correct diagram is on page 104 of the EVTM (below).

https://www.garysgaragemahal.com/uploads/6/5/8/7/65879365/4652683_orig.jpg

As you know, the mechanical selector valves are switched by pressure from whichever in-tank fuel pump is running but they can get sticky over time. I opened mine up and freed up the mechanism with WD-40. There are Youtube videos showing this. One shows resealing it with RTV but that is not necessary. Just don't lose the little o-ring when you open it. The selector switch on the dash can be rebuilt per this thread:

https://forum.garysgaragemahal.com/Rebuilding-a-6-pin-fuel-tank-selector-switch-td110027.html

You may also want to verify that both of your in-tank pumps are working. And there is a situation where one tank will fill the other if the selector valve is not working right but I don't remember the details off hand. Maybe someone else can chime in.

Thanks guys! Today I started to hit the valve softly as step one.
While my son moved the tank switch up and down by my command. When he did the noise changed. Unfortunatly I could not feel any strenger vibration or movement in the valve. I gonna watch it on my next drive hoping this quick fix worked out. If not I will take out the valve and open it. Enjoy your Sunday!

Steve - Thanks for the correction.

Thomas - Good luck!