Re: 1973-1991 Dual fuel tank systems theory of operation
 

My fuel gauge was inconsistent. I was using a simple test light.

Fixed the issue today. The problem was the ground wire on the tank selector switch. Found it was hacked up and just twisted together. Had to remove the dash to get a good look at it and fix it properly. Truck fired right up.

Re: 1973-1991 Dual fuel tank systems theory of operation
 

Some good info here...subscribed!

Re: 1973-1991 Dual fuel tank systems theory of operation
 

I think this is relevent to the thread... here it goes.... is there a particular "proceedure" when utilizing (switching tanks)? does truck need to be running? can you switch "on the fly" while driving? does the proceedure differ if the setup is carbed/fuel injection(swapped to LS engines)?
thanks!

Re: 1973-1991 Dual fuel tank systems theory of operation
 

agree.. lotsa good info thanks hatzie for your sharing of the knowledge. ;)

Re: 1973-1991 Dual fuel tank systems theory of operation
 

You should be able to switch it on the fly even with the diesel engines.

Re: 1973-1991 Dual fuel tank systems theory of operation
 

ok, another question.... how does the tank selector valve work specifically related to the power to it and how does it move to open/close either tank when switched at the dash switch? what turns switch "off" when needed after tank selected. finally how can "a guy" check/verify the selector valve is "switching fully" when activated by the dash switch??

thanks!!

Re: 1973-1991 Dual fuel tank systems theory of operation
 

It is a simple solenoid that is not spring driven. It is moved by reversing the polarity of the 12v and ground to the tan and brown wires. This is done at the selector switch in the dash. It slides a manifold back and forth that lines up ports to route the fuel delivery<and return if EFI>.

The there is a switch that is also activated, though I'm not sure how it works in relation to the movement of the solenoid. That switch directs what sending unit is supply a ground to your 12v from the pink fuel gage line.

If you're looking for more information on the valve itself, I'd suggest youtube and visual explanations.

Re: 1973-1991 Dual fuel tank systems theory of operation
 

You obviously didn't read the original sections on how these valves work.

http://johnnynightstick.s3.amazonaws...%20cutaway.jpg

This is a cutaway view of the valve in question.

It is NOT A SOLENOID valve.

Re: 1973-1991 Dual fuel tank systems theory of operation
 

hatzie, thank you for the reply, so do the contacts start/stop power to the motor and how can "a guy" check to verify the motor switched completely from one side to the other (not partially switched or in between the ports)?
the system in question is a LS swapped '86 c10 with '87 tanks, '87 sending units & EP381 F.P.s. the 12v signal is coming from the signal at the stock '13 ECM and harness.by chance does the motor require a timed amount of power to move completely? my F.P is low (50-55psi) and trying to verify the valve is not causing the lack of fuel.... ??

again appreciate your time and knowledge.. you are always very helpful on the board.

Re: 1973-1991 Dual fuel tank systems theory of operation
 

The valve motor shuts off when it reaches the end of travel.
The in-tank fuel pump runs from the valve motor wiring in stock dress.
Notice the diodes inline with the motor.
http://johnnynightstick.s3.amazonaws...Electrical.jpg

Re: 1973-1991 Dual fuel tank systems theory of operation
 

OK... a motor instead of a solenoid. That doesn't really matter in this context. It's a polarity reversal moving a plunger back and forth.

Re: 1973-1991 Dual fuel tank systems theory of operation
 

Don't use the standard tank selector valve with an LS setup. The selector valves are only rated up to 65 psi. You can use it for the return but the supply needs a different solution. I went with a straight T fitting from the tanks (each tank feed needs a check valve prior to the T).

The tank selector switch (EFI - which locks in position) will also toggle the power to your fuel pumps or fuel pump relays. This can decide which tank to pump from. You can still have the stock switching valve work for the return line routing (much lower pressure) and that way original wiring still works for the fuel guage.

<edit>since you said "FPs", I assumed you are referring to in tank fuel pumps. You can use the stock fuel tank switch if you're using a fuel pump up-stream</edit>

Re: 1973-1991 Dual fuel tank systems theory of operation
 

Is itnpossible to wire these in such a way that return goes to the incorrect tank? I've verified my return lines aren't crossed, but when I fill both tanks, the tank that I DONT currently have selected starts spewing fuel from the fill port. I just completely rewired the truck, so wiring is the only thing I can think of

Re: 1973-1991 Dual fuel tank systems theory of operation
 

It is not possible to wire it so the returns go to the "wrong" tank. The tank switch switches both the supply and return with the same signal. It is possible to wire it so that the wrong tank is selected but the supply should be coupled to the return.

Of course, there are ways to screw that up but they involve re-plumbing your fuel lines. Just re-wiring can't do that.

Re: 1973-1991 Dual fuel tank systems theory of operation
 

That's what I was thinking. I can easily just cross the return lines, but it bugs me not knowing why that's happening

Re: 1973-1991 Dual fuel tank systems theory of operation
 

That's what I was thinking. I can easily just cross the return lines, but it bugs me not knowing why that's happening

Re: 1973-1991 Dual fuel tank systems theory of operation
 

I went ahead and replaced the six port valve today, just to make sure it wasn't a problem with the valve itself. As I was connecting the six lines, I double checked the paperwork/diagram and the routing of the lines to make sure everything was in the correct spot.

Right now, both my tanks are fairly full. I filled them a few weeks ago, drove maybe 20 miles, and since then has been just idling in the driveway a total of maybe 20 minutes - so both the tanks are still pretty full. I don't have a fuel gauge hooked up just yet.

When running on one tank, if given long enough, the opposite tank starts spewing gas, as if the returns were backwards but they're not. If I switch tanks when the spewing begins, it will stop spewing after a second, and after awhile start spewing from the opposite tank

The strange part is that, before I swapped the valve today, the old valve and lines had been in place for almost three years with no problems, even with both tanks completely full. Nothing ha changed between now and then except for a complete rewire of the truck, selector switch included. Tanks, senders, straps, fuel lines and regulator were all replaced at the same time about three years ago.

Anyone have any ideas?

Re: 1973-1991 Dual fuel tank systems theory of operation
 

Do have a charcoal canister and is it connected to the vent line?

Re: 1973-1991 Dual fuel tank systems theory of operation
 

No charcoal canister. The vents on the two tanks are teed together by the selector valve, with a single line running up to a breather under the hood. But no canister

Re: 1973-1991 Dual fuel tank systems theory of operation
 

The charcoal canister isn't a big deal to plumb and it isn't a performance hit.
It's only three connections.

1. a vacuum line from the valve to a ported vacuum source,
2. a tee to the PCV valve for the purge port on the canister
3. a connection to the vents on the tanks

Re: 1973-1991 Dual fuel tank systems theory of operation
 

How would a charcoal canister keep gas from spewing out of the tanks?

Re: 1973-1991 Dual fuel tank systems theory of operation
 

I spent a bit of time after work today troubleshooting. I started the truck up running on the left tank, and removed the gas cap. After a few minutes, I can see the gas level rising up through the fill port, until eventually it starts spilling out the fill port on the left tank.

Once that happened, I removed the power wire from the selector switch that feeds the left tank, so only the right tank was hooked to the switch, and started up again. Same result

I disconnected the two vents where they tee together and plugged them off to rule out fuel somehow crossing from one tank to the other via the breather hose, and same result.

Finally, just for ****s and giggles, I swapped the return lines at the fuel selector valve, and same result.

I have no idea wtf is happening.

Re: 1973-1991 Dual fuel tank systems theory of operation
 

Look at post #146. It sure seems like your return lines are crossed. That seems like the only way you could get fuel transfer from one tank to the other…

Re: 1973-1991 Dual fuel tank systems theory of operation
 

I swapped them last night just to try it out, with thr exact same results

Re: 1973-1991 Dual fuel tank systems theory of operation
 

I would completely disconnect one of the tanks feed and return line and then power up the other side. This should help you narrow it down.

Re: 1973-1991 Dual fuel tank systems theory of operation
 

Great info in here! It is nice to know I can ditch the earlier problematic switch with the simple 2-position switch from the newer trucks. I'm hoping to get my second tank operational sometime this year. I have a feeling I should pick up the switch and a new valve at minimum, then go from there. The entire 10 years I have owned the truck I am not even sure what is in the left tank if anything lol.

Re: 1973-1991 Dual fuel tank systems theory of operation
 

Did you try this?