1973-1991 Dual fuel tank systems theory of operation

[SunSoaked]

Originally Posted by bigdaddy4x4

i have 77 gmc crewcab with dual tanks did a ls swap and used the 87 efi tanks with 96 vortec fuel pumps and pollack switch over valve my ? is does the dual tank small fuel gauge i got to replace the orginial oem gas gauge need to be wired different for the small fuel gauge to work like it should ?

Depends what the instructions say.

[bigdaddy4x4]

Originally Posted by SunSoaked

Depends what the instructions say.

dont got no destructions

[cdub2010]

My 1991 R3500 truck stopped running, no fuel to injectors, since I know the right side pump was dead, I dropped the drivers tank and found the ground was pinched and destroyed, the tank was leaking and sender was doused in tar. Best to replace and start new. I had found a broken switching valve so that was replaced as well. Connected everything it still wouldnt start.

Checked with a test light no voltage at the drivers pump connection. I have voltage at the nl2 connection, and I have traced voltage to the pass. side pump connection, but pass. side shows voltage in both right and left switched positions.

my wires on the dash switch are all white and 1 black, this truck has been a disaster when it comes to wiring, total clown show.

What to look for? ground problem? wire burnt or rubbed thru?

[MiniD]

How are you measuring voltage for the fuel pump?

Does your fuel gage work?

[MiniD]

BTW... this is the schematic that helped me the most:

[cdub2010]

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.

[PowerdbyChevy79]

Some good info here...subscribed!

[rodstored-72]

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!

[rodstored-72]

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

[hatzie]

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

[rodstored-72]

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!!

[MiniD]

Originally Posted by rodstored-72

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!!

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.

[hatzie]

Originally Posted by MiniD

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.

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



This is a cutaway view of the valve in question.

It is NOT A SOLENOID valve.

[MiniD]

Originally Posted by hatzie

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



This is a cutaway view of the valve in question.

It is NOT A SOLENOID valve.

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.

[rodstored-72]

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.

[MiniD]

Originally Posted by rodstored-72

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.

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>

[Dleslie212]

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

[MiniD]

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.

[Dleslie212]

Originally Posted by MiniD

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.

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

[Dleslie212]

Originally Posted by MiniD

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.

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

[hatzie]

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.

[Dleslie212]

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?

[SunSoaked]

Originally Posted by Dleslie212

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?

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

[Dleslie212]

Originally Posted by SunSoaked

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

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

[hatzie]

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