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

Thanks for the Info Hatzie.
I tried 4 used relays and 1 brand new relay, all had same result... 12.4V back probing Terminal A and 8.9-9.1V on Terminal E, grounded direct to battery.

I'm not great at finding info in these forums, I feel like I've searched every possible combination of word search... would like to know any suggestions for a next step.

P.S. Some Clarification:
91 V3500 454: While testing the A terminal (key on) 12.3+/- volts. Testing the E terminal I get 12.3V+/- at initial key on and while cranking, but falls back to 9Volts +/- when not cranking/not triggering fuel pump....
Interestingly enough I did the same test on my 91 V2500 Suburban 350 it gives similar readings except when testing E terminal key on when not cranking/not triggering fuel pump it then gives a 9 MILI VOLT reading.
Any Idea why the V3500 gives 9Volt reading while the same test on the V2500 gives different reading of 9 Mili Volt???

Also I checked firewall and Engine Ground both seem perfect.

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

Either the relay is not passing volts through the switch contacts or the terminal in the E position of the plug is corroded or loose or both.

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

not sure if my question is good for in here... but here it is...:lol: I am looking for info on how the tank vents were/are setup on dual tanks related to the venting lines & the charcoal canister? i am guessing there is a "diagram" somewhere or a hose routing for this?
thank you for any info provided....:metal:
Hatzie, i have mentioned it before, but feel it needs to be mentioned again that your time & effort on this thread is VERY appreciated :metal:

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

There are three crossover lines from the LH tank to the RH frame rail. Supply, Return, and Vent.

On my 76 I ran a single vent line from the stock vapor can location under the LH battery across the cross-member under the radiator and up the RH frame rail to a Tee. The stock vapor line should be the same for the single and dual tanks.

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

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 ?

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

Depends what the instructions say.

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

dont got no destructions

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

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?

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

How are you measuring voltage for the fuel pump?

Does your fuel gage work?

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

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

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