Trouble shooting C172 Fuel gauges.

Discussion in 'Maintenance Bay' started by Southpaw, Oct 2, 2020.

  1. Southpaw

    Southpaw Pre-takeoff checklist

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    Been trying to get on Cessna 172 Club.
    A rather frustrating experience . I'm sure at one time I signed up but have forgotten where I wrote down the User name and password. Then it says email sent . Don't know who they sent it to but not me. :(
    I can access the site through a internet search of topic , and read topics but not search.

    My 1959 C172 fuel gauges are Not working properly . Fellow who delivered airplane mentioned they fluxuated. Now both read E no matter the amounts of fuel in tanks.
    Looking for a web site to better understand how they work. And how to trouble shoot .
    Thanks
     
  2. EdFred

    EdFred Taxi to Parking

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    Three things it could be to start. Sending unit bad, wiring bad, guages bad. Pull sending unit(s) and test, that's easy enough with a multimeter. And that would be my guess as to where the problem is....unless they are sticking. Also easy to test without removing - but it is a 2 person job.
     
  3. Dan Thomas

    Dan Thomas Final Approach

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    That sending unit is the problem at least 80% of the time. In old airplanes you might also have corroded or loose connections between the sender and gauge. Only once did I ever find a gauge burned out.
    Your schematic:
    upload_2020-10-3_10-59-33.png
     
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  4. Southpaw

    Southpaw Pre-takeoff checklist

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    All new wiring and panel in 2015 ,all fuses replaced with breakers . Wings with Horton STOL kit from another 172 in Dec 2018.
    Have a multimeters and other test tools.
    Thank you for the input Dan and EdFred.
    Suspect senders . If one removes lead from transmitter the gauge should read empty or full ?
    If no change, have to track wireing for shorts or open
     
  5. Dan Thomas

    Dan Thomas Final Approach

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    Full will read something like 15 ohms. Empty closer to 200. Empty if disconnected. These things are not precise. The have a brass arm in them, actuated by a float on a lever, and the brass runs across resistance wire wound around a piece of micarta. The brass wears through the wire and things start falling apart. If the airplane is tied down outside, the wind rocks it and the fuel will move a bit and the float moves the arm and the wear continues even when the airplane never flies. Moisture in the tank causes corrosion in the sender's riveted connections and increases resistance. There's a thin aluminum ground strap between the wing root rib and the sender that is often not doing its job, too.

    Make sure the gauge assembly is getting power and that its case is well-grounded before tackling the senders. The recent work might have disturbed something.

    McFarlane has aftermarket senders.
     
    Last edited: Oct 3, 2020
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  6. Tantalum

    Tantalum Final Approach

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    Fuel gauges are some sort of dark magic enigmatic aviation instrument..
     
  7. Dan Thomas

    Dan Thomas Final Approach

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    No magic. Just money. Lots of money just to get an idea as to whether there's any fuel in there at all.

    Actually, they're more than just a rheostat and a meter. There's a resistance bridge inside the gauge itself, with one leg of the bridge being the fuel sender's rheostat. They do it that way so that varying system voltage won't mess up the gauge reading. And it's more accurate.

    [​IMG]

    R2 is the sender. V is the gauge. R1, RX And R3 are fixed resistors.
     
    Last edited: Oct 4, 2020
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  8. Tantalum

    Tantalum Final Approach

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    It's amazing how complicated something can get that should otherwise be simple. My dad just replaced the fuel tank on his sailboat and had a world of a time trying to get the fuel gauge to work right. It turned out it was exactly an issue with getting a shunt and resistance bridge to work well with each other

    He's an engineer by trade (and hobby lol) so he definitely geeked out on it a bit as well
     
  9. Greg Bockelman

    Greg Bockelman Touchdown! Greaser!

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    If R2 is the sender, it wouldn’t be the fixed resistor, would it? Rx is the other fixed resistor?
     
  10. mondtster

    mondtster En-Route

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    I don’t understand your question. R2 is not fixed in that drawing.
     
  11. Greg Bockelman

    Greg Bockelman Touchdown! Greaser!

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    That’s my point. Dan said in his post that R2 is a fixed resistor. It looks to me that it is a symbol for a variable resistor.

    Of course, my electronic education is over 40 years old. I acknowledge that my memory is shaky at best.
     
  12. mondtster

    mondtster En-Route

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    I see now. I didn’t notice that Dan said it was fixed.

    While many things have changed in the last 40 years of electronics, that hasn’t, so your memory is correct.
     
  13. Stewartb

    Stewartb Final Approach

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    It’s unlikely that both sides failed simultaneously. I’d start looking for a bad ground.
     
  14. Dan Thomas

    Dan Thomas Final Approach

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    Yup. I typoed that. Fixed it. Thanks for catching that. Nothing wrong with your memory. Just my cognitive skills.
     
  15. Dan Thomas

    Dan Thomas Final Approach

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    That's why I suggested checking power and ground at the instrument case first. Both gauges are in the same case.
     
  16. arnoha

    arnoha Cleared for Takeoff

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    Although it was a bit expensive, I finally gave up on the garbage senders in my 172 and installed CiES senders. They're amazing. Very precise, very reliable, very repeatable. They're also cheaper, at only $435 each side.

    The expense comes from install. They're active, so they need power run to the wing. They also prefer to output a digital signal, which isn't compatible with the stock gauge. I've heard they can be adapted to the stock gauge, but I didn't choose to do that. Instead, I installed a digital gauge.

    I went through three stock senders in three years before giving up on the things. After install, the new senders have been working flawlessly for a few years.
     
  17. Greg Bockelman

    Greg Bockelman Touchdown! Greaser!

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    Glad to know!
     
  18. Dan Thomas

    Dan Thomas Final Approach

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    Interesting. Hadn't heard of that one. I bet it will last well. No potentiometers or rheostats to wear out. They don't give details other than calling it "magneto-resistive," which to me sounds like it's a variable inductor or a Hall-effect affair. Potentiometers of any sort are a royal pain when they get a little worn.
     
  19. arnoha

    arnoha Cleared for Takeoff

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    The early pot used in the A model referenced in this thread isn't too bad. It's a wire-wound model that doesn't fail at too great a rate, but still not awesome. However, the unit spec'ed for the S model I have is a carbon film unit that fails at great rate, especially for aircraft kept outside. (Wire-wound models fail at a slightly greater rate outside, as well.) The reason is the elements, but not the ones you'd think. It's not sun, it's not rain, it's not cold, it's not heat. It's wind. Even small breezes move the plane when tied down, causing sloshing in the tank and small wiper movements. It's especially bad for aircraft like mine, which, because of club rules, is automatically fueled to about the same level after every flight. The same little spot that corresponds to filled to the tabs gets wiped incessantly all day, every day. The wire wound models can mostly put up with this abuse, failing slowly. The film ones get opens right at these locations. Not open end-to-end, as the edges of the film continue to conduct, but just under the wiper. This also screws with the calibration. And Cessna is charging over $1000 each for them! There was one guy doing repairs on these units, but he passed away, unfortunately.

    The CiES units are amazing. They're mechanical porn. They're as impressive in person, or rather more so, than they are in the pictures. Especially if you have the stock unit in your other hand: it's just night and day. The CiES unit is completely sealed, anodized aluminum construction, unlike the stock unit that has an open frame and the bits are all washed in gasoline. The float arm is nothing more than float, the pivot, and a permanent magnet. There's basically nothing to go wrong on the wet side. The sensor is buried in aluminum housing from the outside, then completely potted with only a three wire pigtail sticking out. (And, yeah, that third wire is what causes all the extra expense to get one installed, since it's the power wire.) The outside is basically also impervious to anything. The packages comes with a very nice gasket, which Cessna charges extra for.

    This is the fuel gauge I use: http://www.aerospacelogic.com/index.php?dispatch=products.view&product_id=249 I've heard the CiES can also be adapted to stock, but in my case, the stock unit had failed in my plane (the needle fell off the gauge! how the hell does that happen!), that was a moot point. It can also feed any of the engine monitors.