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Technical Wiring help for SW fuel gauge

Discussion in 'Traditional Hot Rods' started by fivefour, Sep 16, 2024.

  1. fivefour
    Joined: Feb 26, 2013
    Posts: 19

    fivefour
    Member

    I bought an old SW fuel gauge off eBay and I’m trying to wire it up to my 32 Ford. There’s no markings on the back except some wrote 40 on the side. I think the sender goes to the smaller terminal on top, how do I know which one the positive wire goes to from ignition on?
    thanks in advance, just a young mechanic learning to work on early fords
     

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  2. I suggest editing the thread title to something like:

    Wiring help for SW fuel gauge
     
    vtx1800 likes this.
  3. Hotwyr
    Joined: Apr 20, 2008
    Posts: 96

    Hotwyr
    Member

    You also need to make sure you have a compatible sending unit.
     
  4. Fordors
    Joined: Sep 22, 2016
    Posts: 5,730

    Fordors
    Member

    The top stud is the ground, per this instruction sheet. Also, you need the correct sending unit, SW uses 240 ohms empty and 33.5 full.
    IMG_1747.jpeg
    Stewart-Warner offers different sending units, right or left hand and also different depth tanks.
     
    scruff, Blade58, Budget36 and 5 others like this.
  5. Do you know if is 6 or 12 volt? Your hold down bracket will need insulating bushings in the stud holes too.
     
    partssaloon likes this.
  6. fivefour
    Joined: Feb 26, 2013
    Posts: 19

    fivefour
    Member

    Thank you! This is perfect
     
    vtx1800 likes this.
  7. fivefour
    Joined: Feb 26, 2013
    Posts: 19

    fivefour
    Member

    not sure if it’s 6 or 12 and that’s a great suggestion on insulating the bracket
     
  8. Kerrynzl
    Joined: Jun 20, 2010
    Posts: 3,202

    Kerrynzl
    Member

    Most fuel gauges are the "balanced" type [2 electromagnet coils inside] and can work successfully with either 6v or 12v.
    Once you know the correct wiring connections you can "spark it up" [bench test it] with an AC adaptor.

    For this purpose I use an old 12v 1a cordless phone charger AC adaptor.[with alligator clips]
    Then use a cheap potentiometer as the sender unit and you can dial it up /down on the gauge and use a multimeter to measure the resistance across the potentiometer.

    With these resistance values you can go searching for a tank sender unit.

    Even if you have a total brain-fart and connect it up wrong it will not damage the gauge [not at 12v 1a] The only major issues I've ever had were seized needles and broken wires [which is challenging for my eyesight to solder]
    I have never seen one with a meltdown inside.

    They are stupid simple in principle! With 2 electromagnets fighting each other [one has variable resistance via the sender unit]

    Here is a simple schematic [compliments of the interwebs] explaining how a Temp / Fuel gauge works. The only difference is the sender [and the resistance values]
    But there is not much that can go wrong with them.
    upload_2024-9-18_13-32-55.png

    Don't be scared of them [Even a fool like me can fix them]
     
    Last edited: Sep 17, 2024
    69fury, Bentrodder and Budget36 like this.
  9. I don't want to hijack this thread, but is it possible for people who know what they're doing to recalibrate fuel/temp gauges to match the resistance range of a sending unit it wasn't made for?
     
  10. Maybe; but Tanks makes a "black" box that will make things match.
     
    Tumblin' Dice likes this.
  11. Kerrynzl
    Joined: Jun 20, 2010
    Posts: 3,202

    Kerrynzl
    Member


    Firstly you try and buy the sender unit that is nearest match to the gauge. [This is half the battle won!]

    Now if math isn't your Forte you can use the "applied research and development approach" [aka: let's try it and see]

    With OEM gauges like 57 Chevys there is no increments in the sweep
    upload_2024-9-18_15-56-4.jpeg

    I like the needle to be at the 11 o'clock position with normal driving [cruising] but some gauges read too cold when at running temperature. and some gauges read too hot.
    So get a potentiometer with alligator clips and connect this in-series [between the sender and sender wire] and try and "tune" it to read normal.
    Read the resistance of the potentiometer and solder a resister of the same value into the sender wire.

    This ^^^^ usually works but sometimes you need to go the opposite direction.

    If this is the case, make sure the sender is connected and then connect the potentiometer parallel [from the sender to ground] and "tune" the gauge.
    Then measure the resistance of the potentiometer and make up a wire with a resistor and splice it into the sender wire [to ground]
    You can hide these anywhere along the sender wire

    This is also what you do if you change the thermostat in your engine and the gauge reads too hot or too cold.


    Edit: with fuel gauges which are also "Balance" type gauges ,GM and Ford gauges cannot be swapped over/mixed with the sender units
    GM fuel senders pull the needle down, and when disconnected read full. Ford's are the opposite.
     
    Last edited: Sep 18, 2024
    Thulsa Doom likes this.
  12. 1971BB427
    Joined: Mar 6, 2010
    Posts: 9,024

    1971BB427
    Member
    from Oregon

    If you don't know if it's 6 volt or 12 volt I sure wouldn't start by testing it on 12 volts! If it went up in smoke it's just wall art after the 12 volt test.
    Get the correct sending unit and test it all wired 6 volt on a bench to see if it works. Then if it's way off you can repeat the test with 12 volts to see if it's correct.
    But my question is why even bother if you don't have the correct sending unit? A sending unit for that gauge is going to be very expensive, and it might be cheaper to use a more common gas gauge and sending unit.
     
  13. This is great tech, thank you.
     
    Bentrodder likes this.
  14. @Kerrynzl , thanks for the sketch and explanation. Reminded me of an issue I had with old Chev pickup I drove. Just got it ready and started driving it. Read 1/4 tank, thought I'd fill it up. Only went to half-on the gauge. Mechanic friend told me of the 2 magnets in the gauge. Swapped it for one out of my parts stash, good to go. Never got in to the gauge to repair, but knowing me, I probably kept it to repair later.
     
  15. I'm sure I read somewhere that there is a way to make a SW fuel gauge 240-33 ohm work with a GM sender 0-90 ohm. I think it involves using a certain resistance resister in line. But, of course, now that I am looking for it, I can't find it. Anyone know??? Thanks, Gene.
     
  16. TrailerTrashToo
    Joined: Jun 20, 2018
    Posts: 1,329

    TrailerTrashToo
    Member

    Not going to work correctly.
    SW fuel gauge goes from high resistance to low resistance.
    GM sender goes from low resistance to high resistance.
    First, you need to add a 33 Ohm series resister to the circuit in order to prevent a overcurrent situation in the SW fuel gauge.
    Second, the SW fuel gauge will read backward.
    Third, the SW fuel gauge will only read about 43% of full scale.

    Other than that, if you understand all this, it is much better than not knowing how much fuel is in your tank.

    May I suggest that you Google: "fuel gauge 0-90 Ohm"

    Russ
     
    alphabet soup likes this.
  17. I did Google after I asked this. I found one on the Autozone web site and ordered it. Thanks for the info... VERY interestinnnk!!
     
    Last edited: Sep 19, 2024
    TrailerTrashToo likes this.
  18. fleetside66
    Joined: Nov 20, 2006
    Posts: 3,054

    fleetside66
    Member

  19. Have not... but I saw Tanks offers one too
     

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