Dual Tank operation.

Discussion in 'Tech Discussion' started by mavtricks71, Jan 20, 2018.

  1. mavtricks71

    mavtricks71 Full Access Member

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    My 86 k10 has dual tanks...can somebody explain how the pollak valve works? Will the engine runs both tanks dry? I was assuming the Dash switch was only there to let you know how much fuel was in each tank.
     
  2. chengny

    chengny Full Access Member

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    The Cliff Notes version:

    On a 1986 (with carbureted engine) the Pollak valve is a motorized valve. To switch over tanks requires that you lean on the switch for the recommended 2 seconds. That will also change the tank level signal over to the active tank. The wiring for the gas gauge is mechanically coupled to the valve internals. After that, the plumbing (and tank level signal) will remain where you left it - until you push the dash button again and shift tanks.

    Will the engine runs both tanks dry?

    Yep, if fuel pump suction is allowed to remain on a particular tank long enough - it will strip the tank.


    As used with a TBI equipped vehicle, the tank select system actually has 3 functions instead of just the normal two performed by a system using a mechanical engine mounted fuel pump. In addition to switching tank feed/returns and level indication, the dash switch also energizes/switches the associated in-tank fuel pumps.

    Since the power supply to the fuel pump must continue - even after the plumbing and level indication have been changed over - the momentary switch/motorized actuator set up used from 81-86 was not an option (apparently the same wiring harness was to be used in model year 1987).

    A solenoid operated actuator operated by a rocker switch (the pre-81 design) was also not a viable option because solenoids only use an O/I type signal - they move in one direction when power is applied and only move back due to spring pressure when power is secured. The in-tank fuel pumps each needed their own power supply - that was switched on and stayed on - whenever it's associated tank was selected as the active tank.

    So, both pumps needed power even after the tanks were switched over. But a solenoid set-up was only on/off not left/right and a standard motorized actuator would burn up if power was left on for any considerable length of time.

    In the end, it appears that Pollak developed an actuator that was specifically for the 1987 model year. It is a motorized actuator with a rocker switch that killed the power only to the valve's motor windings (and yet continued to supply power to the pumps) after the valve had shuttled. That allowed GM to use the same wiring harness - and still supply power to the fuel pumps after the valve had shuttled .


    Here read this - it explains it much better than I am doing tonight:

    The information I have is the 87-91 carbureted & TBI trucks used a 4
    terminal dual pole dual throw rocker switch (D7089C - #15599081) and a motor
    type switch-over valve (not solenoid operated) that included a fuel level sender
    switching circuit and switch-over current control (#15548879). This system
    was very similar to the previous carbureted trucks with the exceptions of
    the rocker switch no longer is a momentary type switch and once the
    switch-over valve is in commanded position, the unnecessary current flow is
    ended by an switch and diode circuit within the switch-over valve itself.
    The left & right fuel level circuit switching is handled internally by the
    switch-over valve.

    TBI - post 86 :

    87_tk_selct.jpg
    81 - 86 wiring:
    Dual tanks.jpg

    01.jpg
     
    Last edited: Jan 20, 2018
  3. mavtricks71

    mavtricks71 Full Access Member

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    Ok thanks for information....Wich type of switch should be in my truck since it's a 1986 ....is it the one that says depress fully at the bottom?
     
  4. mavtricks71

    mavtricks71 Full Access Member

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    So if I get low on driver's side tank and I DONT flip switch to passenger side it will run passenger tank dry ?
     
  5. WFO

    WFO Full Access Member

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    No, it will only suck fuel out of the tank activated by the switch.
     
  6. chengny

    chengny Full Access Member

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    Yeah, the switch is GM part no. 15591149.

    It doesn't need to be that exact switch - although it will be a lot easier to install because it will fit the opening in the dash and accept the wiring harness plug. They are easy to find and not too much $$.

    Here is an NOS switch for under $15:


    https://www.ebay.com/itm/NEW-GM-AC-.../221964534708?_trksid=p2349526.m4383.l4275.c1

    1.jpg


    2.jpg


    s-l1600.jpg

    The spec sheet and installation instructions from Pollak:

    Pages from Pollak IS-41_b.jpg
     
  7. mavtricks71

    mavtricks71 Full Access Member

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    Thank you for all the information
     
  8. mavtricks71

    mavtricks71 Full Access Member

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    Will I only need to press and hold the button for 2 seconds to activate the left tank? And then to switch to passenger tank I simply push the switch upwards? Sorry about all the questions I've never had dual tanks.
     
  9. Dougnsalem

    Dougnsalem Full Access Member

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    Yup. BTW- do yourself a favor and don't run both tanks low. Last time I ran a tank empty, I flipped it over to the second tank. That tank ran out about 30 seconds later..... Lol.
     
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  10. mavtricks71

    mavtricks71 Full Access Member

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    Damn thanks for the heads up...I'm horrible at running tanks low.....it's procrastination due to spending money on fuel lol
     
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  11. Dougnsalem

    Dougnsalem Full Access Member

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    Exactly. Same here. At least if you do happen to run them both dry, don't admit to the gas station dude that you have dual tanks. They will seriously question your intelligence level.... Lol
     
  12. 86lsk10

    86lsk10 Junior Member

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    I’d like to re open this thread if I may. I’ve been doing a lot of reading of posts and this one. And I’m trying to mash all the info into my situation.

    I have an 86 k10 fleetside short bed. Dual tanks

    The truck has an Ls swap. I did not do the swap it was done prior to me buying the truck

    I haven’t even registered the truck yet. However today I see the driver side tank is leaking

    I’ve had the truck on the lift already. I know the motor is only sucking from driver side tank. The passenger has been disconnected.

    For the time being. Just so I can get the truck on the road. I’m going to swap the bad tank with a new poly one. Call it a day.


    My question however is with the dual setup. I would really like to get it all working again.

    It seems I would need the new pollak and I would obviously replace the aux tank and drop a 60 psi pump in it.

    The switch on the dash is broken. However I think I should replace it with one from an 87 because it was designed for efi. Am I correct?

    What my real question is. Is with the wiring. Do I have the capabilities? What should I look for. How does the pump not being used turn off. I’ve included some pics.

    I assume yellow wire from current worming tank is going to gauge? Is it a 1 wire setup?

    There is a cut pink wire over by the aux tank at the selector valve.

    [​IMG][​IMG]



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  13. chengny

    chengny Full Access Member

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    However I think I should replace it with one from an 87 because it was designed for efi. Am I correct?

    Yes, correct. The switch used with a mechanical fuel pump only needs to be closed long enough for the tank select valve to shift over. Hold it closed for like 2 seconds...and let it go. It's job is done until the next tank switch. That is why in 81 - 86 applications (carbureted fuel systems with mechanical pumps), a momentary type switch is specified. But from 1987 and on, the fuel pumps are electric and in -tank, so a rocker switch is necessary. Even though the tank switching process is over in a second or two, the pump needs power the entire time the associated tank is on line. The tank transfer valve has internal components that shut down the power after it has shuttled.


    Do I have the capabilities?

    As long as the original wiring for the NL2 (dual tank) option is still there; Then yes, you can utilize those leads to run in-tank pumps.

    In addition to powering the valve, those two leads (LT/GRN & DK/GRN) have sufficient capacity to also run a fuel pump. But, in order to do that , each of them will need to be spliced into and the branched line will have to be lead over to the associated pump.


    What should I look for.

    There should be two fairly heavy gauge wires (LT GRN & DK GRN) that were originally used to shuttle the Pollak valve from the LH to RH tank.

    When energized - by pressing the dash mounted switch towards LH - the DK GRN lead rotated the Pollak valve so that the plumbing was connected to the LH (production) tank. Conversely, if the dash switch was momentarily held in the RH position, power was applied to the tank select valve on the LT GRN lead, the valve rotated the other way and the plumbing was connected to the auxiliary (RH) tank. The contacts for the gas gauge sensing leads (PNK/WHT & PNK/BLK) are switched along with the valve rotation and connect whichever tank is on line to the common PNK wire that runs up to the instrument cluster.


    How does the pump not being used turn off?

    The design of the dash mounted switch is such that it only supplies power to one lead at a time - which lead is hot depends on whether the switch is in the LH or RH position.

    I assume yellow wire from current worming tank is going to gauge? Is it a 1 wire setup?

    IDK where that yellow lead came from - it is not original. There should be a pink lead running down along the LH frame rail that has an extension spliced onto it. The extension runs over to the Pollak valve. There are two branch lines that come out of the valve and run to the two tanks. When the valve is shuttled to switch the plumbing over, the fuel gauge sensing leads are shifted along with it.

    So, it's a one wire set up, until it connects to the valve body. After that it becomes a two wire circuit - three wire if you include the common lead that connects to the gauge.
     
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  14. roundhouse

    roundhouse Full Access Member

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    Good info

    I’m about ready to hook up dual tanks and I have anLS swap also
     
  15. 86lsk10

    86lsk10 Junior Member

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    This is great thanks. When I drop the tank I’ll check out that yellow wire. Not sure what it’s doing probably powering the pump they probably cut the sending wire behind the dash.

    Whoever swapped this motor did a hack job. I’m missing wires under the hood for the washer fluid tank and those hoses are cut too.

    I’m got myself a solid truck. No rut no rust. All tin. Nothing is perfect I guess work in progress.


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