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Technical *TECH* Make a Gauge Tester/What-Sender-will-work-with-my-dumb-gauge Thingie

Discussion in 'Traditional Hot Rods' started by Dreddybear, Feb 6, 2014.

  1. thanks for posting this tech!
    kidcampbell71 likes this.
  2. Atwater Mike
    Joined: May 31, 2002
    Posts: 10,800

    Atwater Mike

    Very clever, Dreddybear.

    I had this idea floating around in the cobwebs of my mind for years, but the obscurity of all relative components dampened any real progress.

    Thanks for explaining what to most is a 'black art'.

    I'm gonna make one too now.
  3. GTS225
    Joined: Jul 2, 2006
    Posts: 1,203


    Dreddy; You did great with the project and write-up, but I feel compelled to shout out a warning.

    To those whom might want to do this for vintage (or stock) gauges. Make sure they aren't supposed to be 6vdc. You run 12vdc into a 6vdc gauge, and you'll probably let the magic smoke out, or at least, get incorrect readings.

    If this is the case for your gauges, just replace the 12 volt battery with a 6-volt battery.

    Carry on; Roger
  4. Flat Ernie
    Joined: Jun 5, 2002
    Posts: 8,406

    Flat Ernie
    Tech Editor

    A great reason to run a constant voltage regulator. Because not only will your alternator/generator put out 14.5 volts at speed, it will also put out whatever battery voltage happens to be when it's below it's cut-in speed, which could even be below 12V on a weak battery. With a CVR, you can get 12V (or 6V if you're running early gauges) no matter what the input voltage is (well, it won't plus up 11.5V to 12V...)

    Another great plug for CVRs. ;)
  5. Most gauges used 6V, we found the ford truck and vans were easiest to get to an they used until digital showed up. PULL A PART!
  6. bob35
    Joined: Aug 26, 2011
    Posts: 75

    from DFW, TX

    For those of you with mismatched fuel gauges and senders, have you seen this little gem?....,66534.html

    I used it on my car to solve a poor sweep problem on my gauge, but it's mainly for mismatches... acts as a "middle man"... you set the full sweep of the sender, then dial in the full sweep of the gauge needle, and it does the rest. It even adds a "low fuel" warning option... a negative signal that switches when your tank hits 15%. I used it to light up a red LED flex strip that I wrapped around my gauge (hidden behind the dash), so that the gauge glows red when it hits the 15% mark.

    For me, this thing was totally worth the price... solved a big problem I had been screwing around with for about a month to no avail.

  7. seb fontana
    Joined: Sep 1, 2005
    Posts: 7,443

    seb fontana
    from ct

    This thread isn't too old so I'm giving it a bump with some questions...I figure that a sending unit can be checked also, using a gas tank sender for the test, what would the test procedure be? As simple as hooking the meter to the sending unit and making the gas float go through its sweep? If the gauge is in the car just use [I figured this out] the wire to the sender? Is the highest resistance always the lowest gauge reading?
    Last edited: Dec 8, 2014
  8. AndersF
    Joined: Feb 16, 2013
    Posts: 812


    To test the tanksending unit its best to remove it from the car.
    Take the multimeter and Ohm test the sweep of the unit.
    One testwire on the terminal and one to the body.
  9. mwhistle
    Joined: Feb 19, 2007
    Posts: 314

    from sacramento

    Sorry, but where are you attaching your wire leads to test for the ohms readings? Thank you.
  10. wsdad
    Joined: Dec 31, 2005
    Posts: 1,258


    No. You have to remove at least one wire before taking a reading.

    When you measure resistance, you always have to disconnect whatever you're measuring. Otherwise the voltage coming out of the gauge will screw with the meter's readings.

    You can't just disconnect the battery either. You have to disconnect the gauge from the tester. Otherwise you'll be measuring the resistance of both the tester and the gauge.

    Here's how it works. (Caution: Really boring details ahead.)

    In order to figure out what the correct resistance is, your meter has to send a little bit of electricity through whatever it's measuring. The meter sends electricity down one lead. Then it goes through the resister. The resister won't let all of the electricity get through. The flow of electricity has been "resisted." The amount of electricity that is able to get through the resister and back up through the other meter lead tells the meter how much resistance there is. The meter says to itself, "Ok. I know the exact amount of electricity I sent out, but I'm only getting this much back. I know it takes this big of a resister to do that. So now I know how big the resister is." So it does a little calculation and displays a number on the screen for you.

    The gauge is also sending out 12 volts because it is also trying to "see" how big the resister is. The electricity from the gauge will either add to the electricity from the meter or take away from it. Either way, it screws up the amount of electricity the meter "sees" so it fools the meter into giving out a false reading.

    This only applies when you're trying to read resistance (Ohms). If you're trying to read voltage, you want to leave whatever you're measuring hooked up. If you're trying to read current (Amps), you have to disconnect one wire from whatever you're trying to measure, and remake the connection through your meter so that all the electricity has to go through your meter. But we're not trying to read voltage or amperage here so just remember to disconnect at least one wire from the tester before taking a reading with the meter.

    (Yawn.) Ok, back to the interesting posts.
    Last edited: Dec 9, 2014
  11. seb fontana
    Joined: Sep 1, 2005
    Posts: 7,443

    seb fontana
    from ct

    Thanks guys, cleared up what I needed to know..Now to get to Radio Shack down the street; maybe later as its freezing rain now..
  12. brady1929
    Joined: Sep 30, 2006
    Posts: 8,813


  13. seb fontana
    Joined: Sep 1, 2005
    Posts: 7,443

    seb fontana
    from ct

    Well, as usually happens with me....I found a "pot" that looked new I decided to take a time out and try the meter thing, even duplicating with HF meter...All it would do was show -1....So I get my "good" meter out, Radio Shack, and push buttons till I'm blue in the face [I got no clue where the instructions are] and I finally get a sweep, not in the range needed but a great success ..Woo-Wooo!
  14. 59Apachegail
    Joined: Apr 30, 2011
    Posts: 1,395

    from New York

    This is a great tech! Thanks OP! thanks 55 dude for bumping it! It works like a charm. ImageUploadedByH.A.M.B.1433197022.204422.jpg
    Last edited: Jun 1, 2015
    kidcampbell71 likes this.
  15. Grahamsc
    Joined: May 13, 2014
    Posts: 466

    from Colorado

    You just built a version of this Kent Moore instrument panel tester that we used to check fuel gauges at the dealership
    By the way most Gm up to about 1997-98 are 0-90 ohms
    The 3 potentiometers are in series.
    Ohms 0 to 100,0to 1000,0to 10,000
    That way you could come up with any amount of resistance from 0 to11,100 ohms.

    that's what Abe Lincolin told me.

    Attached Files:

    Last edited: May 28, 2015
    brEad and kidcampbell71 like this.
  16. qzjrd5
    Joined: Nov 23, 2004
    Posts: 1,333

    from Troy, MI

    A7159277-06BF-4AB4-BC56-C6A7A95D1A3D.jpeg A3E95D06-A29D-48AC-A1FF-FF940E9F6533.jpeg Thanks for this thread. Built Mine and checked my fuel gauge. Verified my sender is the reverse of what it needs to be. Shucks!
    kidcampbell71 likes this.
  17. [​IMG]
    This my attempt I made today, everything bought from eBay for pennies

    Sent from my iPad using H.A.M.B.
    kidcampbell71 likes this.
  18. johnod
    Joined: Aug 18, 2009
    Posts: 778


    Have you got a link for those plug in connectors on the side of the box?
  19. David Gersic
    Joined: Feb 15, 2015
    Posts: 2,499

    David Gersic
    from DeKalb, IL

  20. johnod
    Joined: Aug 18, 2009
    Posts: 778


    Thanks, now that I know what they're called that'll be a big help.
  21. cretin
    Joined: Oct 10, 2006
    Posts: 3,010


    I’ve been wanting to make one of these for a while now, and finally had a need for it, so I just put mine together. Can’t wait to use it! Thanks @Dreddybear !

    kidcampbell71 likes this.
    Joined: Feb 22, 2006
    Posts: 718


    Poton meter deluxe
  23. chiro
    Joined: Jun 23, 2008
    Posts: 1,065


    Cretin. Nice Telecaster knob on there.
    kidcampbell71 and cretin like this.
  24. Sporty45
    Joined: Jun 1, 2015
    Posts: 1,038

    from NH Boonies

    Great info! Thanks Dreddybear :cool:
  25. Airborne34
    Joined: Dec 4, 2007
    Posts: 572


    My gage pegs out at full when I hook it up. Adjusting the Ohms does nothing. I only have two posts on the back of the gauge. IGN/Sw. and GA I take this to be Ignition and Ground.

    the gage is from a 1932 Packard 900 and I’m using a 6v battery to test it out

    What would be the connections for this gage. Thanks 11CFE41C-016E-4D63-803D-0E2C6686E2A7.jpeg 96138539-3D96-475E-BCA7-B322FA0018AF.jpeg
  26. brading
    Joined: Sep 9, 2019
    Posts: 515


    Packard called the sender a Gasoline Gauge Tank Unit so GA is the one you connect the tester to.
    Last edited: Apr 4, 2021
  27. Airborne34
    Joined: Dec 4, 2007
    Posts: 572


    Thanks, I’ve tried several variations of the connections and it does not work according to the directions.

    Has anyone successfully tested a gauge with two posts like this. If so, how did you connect it.
  28. Sporty45
    Joined: Jun 1, 2015
    Posts: 1,038

    from NH Boonies

    I have one of these little testers and have tested the fuel gauge in my 47Olds. It is a 2 post gauge like yours. I will get some pictures later today for you. It does work well if the gauge is good.

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