Gas gauge 74 Duster help

Discussion in 'Electrical and Ignition' started by Steve Agrella, May 10, 2018.

  1. Steve Agrella

    Steve Agrella FABO Gold Member FABO Gold Member

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    Gas guage reads 3/4 tank when totally full then it works normally for about half a tank of gas (sometimes this varies to only a few gallons) then it drops below E and doesn’t work
    Any suggestions would be greatly appreciated.
    I’m going to replace all of the guages with Auto Meter, want to shed some light before I do
     
  2. Frnknsteen

    Frnknsteen FABO Gold Member FABO Gold Member

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    Sounds like you have an issue with the resistance wiper on the sending unit. If I recall correctly, those sending units have a bimetal spring that the float arm wipes across. Might have some crud on it, so it isn't getting an accurate resistance as the fuel level drops. Might just need to pull it out and clean it..... might need to replace it.
     
  3. Steve Agrella

    Steve Agrella FABO Gold Member FABO Gold Member

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    Are the sending units readily available, if so do you have a source
     
  4. 67Dart273

    67Dart273 Well-Known Member

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    You can buy "repop" senders they are not accurate

    You "may" find someone to rebuild yours

    You likely won't find a NOS OEM sender

    There is a device called a "meter match" to calibrate an odd sender to your gauge

    TechnoVersions - MeterMatch for Analog Gauge Correction

    I bought one tried it on the bench. Have not run it in the car "it's apart" LOL
     
  5. Frnknsteen

    Frnknsteen FABO Gold Member FABO Gold Member

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    Rockauto has them for $50-$60. Don't know how accurate they are, but it just comes down to what the resistance is. You'd have to look into it, but on the 68-70, Ohm range was from about 73ohms at empty to about 10 ohms at full to operate the gauges properly. I went in there and looked at it and the ohms range it gave on the Rockauto site was 80ohms at empty, 10 ohms at full, so as long as it actually reads that,... it should work fine.
     
  6. 67Dart273

    67Dart273 Well-Known Member

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    No Sorry "it is not fine." It is, or was, a well known fact on here that "all" the repop senders are inaccurate. Part of the problem is that they don't follow the same resistance THROUGH THE RANGE as the original OEM, and part of the problem seems to be that the mechanical movement is simply not the same.
     
  7. Frnknsteen

    Frnknsteen FABO Gold Member FABO Gold Member

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    I didn't say it would fine. I said if the resistance is where it is supposed to be, it should be fine. Yes,... the site only tells you what it is supposed to be at full and empty, and yes, the resistance is not linear between full and empty. Gauge midpoint should be about 23-25 ohms.

    If he can't find one anywhere else, he could get one from there and test the resistance through the range, or at least at full, mid, and empty to see if it's going to work. If the resistance doesn't match up,..... send it back before installing to get his money back.

    80 ohms should just barely move the needle, if at all. I used 2 150ohm resistors wired in parallel to give me 73.8ohms. On my gauges, that JUST barely moved the needle. 23-25 ohms should put the needle in the middle position, and 10 ohms should put at, or slightly above the full line. I haven't tried to calibrate the gauges in a '74, but in my 68-70 B Body rallye dash, there are two windows on the back of the gauges that allow you to adjust the needle position at the top and bottom of their range. You calibrate by putting known 5 volts to the gauge with different resistors between the other post and ground source. I check the midpoint, but don't care if the needle is exactly accurate in that middle range. I need to know where it is running in relation to "Normal"

    I never insinuated that it was going to be just a "Drop-in" and it's going to work accurately, but with a little tweaking, it can usually be made to work. I just put a new temp sender in my truck (71 Chevy C10). Gauge was reading high with the new sender, but temp was where expected when checked with an IR thermometer. I added a 10 ohm sensor into the line to bring the needle back down to the midpoint and it's working fine.
     
  8. Frnknsteen

    Frnknsteen FABO Gold Member FABO Gold Member

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    I will say that MeterMatch looks kind cool, and would be handy if you are mixing and matching various components into a hot rod of some sort, but it still requires the sender to be working properly, and just adjusts resistance through offsets. For instance, for the people that want to put a Chevy motor into their Mopar (I know,.... yuck), but that system would allow it.

    Mopar needs 75-10 ohm range for their gauges to range from low to high. My 71 Chevy truck uses an ohm scale of roughly 700 - 30 ohm range to run the gauge from low to high. The Chevy sender would never move the needle in my Mopar past midpoint because even at its lowest ohm output, it would barely reach the 25ohm midpoint of the Mopar range. That MeterMatch would allow me to program in offsets to allow it to work, without having to find a Mopar sender to fit the Chev block. Handy little system for that!
     
  9. Steve Agrella

    Steve Agrella FABO Gold Member FABO Gold Member

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    How can I tell if it’s the gage or the sender that Is bad, I plan to replace the stock gauge with an Autometer gauge.
     
  10. Frnknsteen

    Frnknsteen FABO Gold Member FABO Gold Member

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    Judging from how you described it in the original post, I would say it is the sender. Here's why...

    Typically, the gauge either works or it doesn't. All it is is a bimetal strip that heats up and expands a given amount, depending on the resistance it sees. The less resistance, the more current it draws and the more it expands (Volts = Current X Resistance).

    The sender is a different issue. The sender is typically a wound bimetal spring connected on one end and open on the other. There is a set resistance through the length of that spring, but your wiper contacts it in different locations, depending on fuel level and position of the float. For instance,.... "Per the spec", completely empty with the float at the bottom and the contactor at the open end, there should be 80ohms across the contacts (empty). As you fill it with fluid, the float rises and the contactor wipes toward the connected end and resistance drops. Half tank should read approximately 25ohms of resistance. At full tank, the wiper should be as close to the connected end as it can go. Resistance should be about 10 ohms.

    From what you described, and assuming it used to work,.... I'm thinking there is corrosion on the contactors so ohms never drop anywhere near 10 ohms, which is why you never go above 3/4 tank. Then, as you burn up fuel and the float drops, at somewhere around the half tank point, there is a bunch more crud or corrosion that it has become an open circuit, causing the needle to drop to empty. My guess is if you were to pull that sender out. You would find some crud on the wiper and/or spring, adding to resistance in the upper half of the tank, and a whole lot of crud/corrosion on the area the contactor rides when the tanks is half full or less.
     
  11. 67Dart273

    67Dart273 Well-Known Member

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    You can easily test the gauge with substitute test resistors, which mimic the factory test instrument. All it was ..........was a 3 position switchbox with "full" "half" and "MT" resistors inside

    c-3826-jpg-jpg.jpg

    You can do basically the same thing with the sender..............You will have to determine your tank capacity, and use a decent ohmeter to measure the sender resistance when empty, 1/2, and full. The readings should be close to the tester pictured

    The other problem is wheter the autometer gauge actually "will work" with a factory sender even if the sender is "OEM accurate"
     
  12. joseph schrom

    joseph schrom Active Member

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    it dose sound like your sending unit my old 90 dodge truck had the same thing happen to the unit
     
  13. mopardude62

    mopardude62 FABO Gold Member FABO Gold Member

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    I agree with Del. The best thing to do is pull the original sender and send it off to have it rebuilt. I have used these guys in the past Home Last time I used them it was around $85 for a sending unit rebuild.
     
  14. joseph schrom

    joseph schrom Active Member

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    how's the camp's on the sending unit
     
  15. Steve Agrella

    Steve Agrella FABO Gold Member FABO Gold Member

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    Can anyone tell me where the sending unit is located in the tank on a 74 duster and how to get to it please
     
  16. 6pk2goDemon

    6pk2goDemon Mopar Mod Staff Member FABO Gold Member

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    It's in the front of the gas tank. Crawl under the car & you'll see it. Easiest to drop the tank IMO. Siphon the gas 1st....

    Also, keep your old lock ring as the new ones leak!

    IMG_0291 (Small).JPG
     
  17. Steve Agrella

    Steve Agrella FABO Gold Member FABO Gold Member

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    Awesome thanks
     
  18. 67Dart273

    67Dart273 Well-Known Member

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    I've posted this a skillion million bullion times. Go over to MyMopar and download (closest you can get) is the 73 shop manuals, for free. Those and others over there came from the efforts of the GUYS ON HERE
     
  19. Steve Agrella

    Steve Agrella FABO Gold Member FABO Gold Member

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    Just ordered the manuals off of flea bay should be here by Friday, thanks for the heads up just go ahead of myself a bit.
     
  20. joseph schrom

    joseph schrom Active Member

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    What I did was I took two Jack stands have one each side And unbolt
    the straps and unscrew the boot inside the trunk and then disconnect the fuel lines then let it come down slowly then you should see it
     
  21. Steve Agrella

    Steve Agrella FABO Gold Member FABO Gold Member

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    Thanks for posting this, appreciate all of the replies