Fuel gauge wiring 1940 Hudson

My basket case came with a restored tank and an new sending unit. I will not be using original wiring harness and will be going 12v after market cheap hot rod wiring kit so will use a voltage reducer for original
gauge. Bottom terminal had one wire top terminal had 3 wires going to it. Is there a simple way to explain how fuel gauge system should be wired?


  • charles4dcharles4d Posts: 576Expert Adviser
    edited January 2019
    I think the 3 wire carry electric  to other  gauges so it would be easier  to wire the system you need a wiring  diagram to see how the old system works to understand  how it works good luck  I m still working  on my 1940 wiring my self if I can help let me know my system is a 6 volt
  • 29sptphaeton29sptphaeton Posts: 319Member
    You will need a runtz for your 6 volt gauge to work with 12 volts. It attaches between the incoming live wire and the gauge. They’re available from Speedway Auto. You need one for each gauge with the aftermarket harness instead of feeding from one gauge. For the lights you just replace the bulbs with 12 volt bulbs
  • Thank you both!! Just what I needed to know!!
  • PAULARGETYPEPAULARGETYPE Posts: 1,755Senior Contributor
  • hudrodguyhudrodguy Posts: 18Member
    Hi there
    I am working on the same problem ... 1940 hudson with 12 volt upgrade ...
    Do you know the ohms of the original guage ?  Can it be found with a tester ?
    Are you stating that 12 volts would work without dropping the voltage to the guage ?
    Any help would be appreciated ...

  • hudrodguyhudrodguy Posts: 18Member
    Also ...
    New tank and sending unit ... Stainless steel tank
    New Sending unit is vdo 226 007 ... 0 ohms empty. 90 ohms full
  • bob wardbob ward Posts: 1,207Senior Contributor
    edited November 2020
    A trap for the unwary. Applying battery current (6V or 12V) directly to the fuel gauge will fry the gauge very quickly.
    Assuming the innards of a 40 fuel gauge are the same as those in the the 34 to 39 Mechanical Procedure Manual, on 12V your gauge will theoretically read empty at 300Ω and full at 60Ω. This derives from the fuel gauge test procedure in the manual where it says the gauge should read empty with a current of 0.04A and full with a reading of 0.2A. V=iR and all that.
    The gauge is a make and break bimetal strip device as was the sender. 
  • 29sptphaeton29sptphaeton Posts: 319Member
    I converted my 46 to 12 volts with a new harness and All I had to do was buy a runtz from speedway. The runtz reduces the the 12 volts to 6 volts and my gauge works. It’s a little coil thing that mounts between the wire and the gas gauge. 
  • 40indianssgmailcom40indianssgmailcom Posts: 246Senior Contributor
    I am a little confused by bob ward's comment that cannot connect battery current directly to gauge. IIRC the wiring goes to post on gauge then from other post to sender in tank.  Grounding is through frame and body.  I bench tested my gauge and sender with 6 volt input to gauge and connection to sender with a temporary ground from gauge body to sender body.  Moving the float up and down resulted in gauge moving full to empty and to full. This was with a 12 volt battery with a $5 eBay electronic voltage reducer to 6 volts.  If you don't have voltage in how is the system to function?  This is for a 47 pickup
  • bob wardbob ward Posts: 1,207Senior Contributor
    I knew what I meant :)  Always connect the gauge to the battery via the sender. Testing the gauge to see if the needle moves by directly applying 6/12V to each side of the gauge will cause problems. If the needle does move it quickly will stop doing so.
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