Fuel Gauge Troubleshoot

Discussion in 'Maintenance Bay' started by ARFlyer, Dec 6, 2020.

  1. ARFlyer

    ARFlyer En-Route

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    My partner was preflighting our 172 this morning and found the Right Fuel Gauge is completely wonky. It will only read E when power is applied. I had him try gently bouncing the wings which did not help. The next trick is to defuel the tank partially to see if there is a dead spot in the sender at FULL.

    However if none of that works do any of y’all have any ideas?

    We’re replacing the senders with CEIS ones in Feb during the engine swap. They’re actually sitting on my work bench laughing at me currently. So we’d rather not have to go out and buy a Cessna one to only use it for two months.
     
  2. WannFly

    WannFly Final Approach

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    I believe it’s illegal to fly without an operative fuel gauge. Or others might chime in and say I am wrong
     
  3. Doc Holliday

    Doc Holliday Pattern Altitude

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    Find a maintenance manual and read up on how to troubleshoot it.

    From previous experience I would say you have a bad sender. A multimeter will confirm this.
     
  4. ARFlyer

    ARFlyer En-Route

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    That’s my thoughts as well. He took off the inspection plate for the sender and said it looks to be in bad shape. I’ll find my manuals and give it a shot. I think off the top of my head it needs to be around 100ohms or less. I’m wondering if the float has come off. As the needle moves to E when power is applied. So the circuit is at least good.


    91.205(b)(9) would agree with you.
     
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  5. ARFlyer

    ARFlyer En-Route

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    Left Unit: 26.8 ohms
    Right Unit: 111.0 ohms

    Yay me!!
     
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  6. Dan Thomas

    Dan Thomas Final Approach

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    You need to know that there were two different systems used in Cessnas. Stewart Warner was used for decades until Cessna switched to Rochester, maybe in the 1980s. The SW sender has a much higher and larger resistance range than the Rochester. Rochester might max out at 90 ohms or so (empty) while the SW can go to 240 or so at empty.

    If you measured the resistance at the sender without taking the wire off it first you didn't get a useful reading. Your meter was reading the resistance of the Wheatstone resistance bridge and meter inside the fuel gauge. The circuit looks like this:

    [​IMG]

    R2 is the fuel sender, a variable resistor. Point C is ground, D is the wire at the sender. Sticking an ohmmeter between that wire and ground (C and D) sends current though the gauge to point B, through Rx to ground. It also sends current through R3 to point A, which is the power connection to the gauge and which is connected via the breaker to the bus. The current will travel through the turn coordinator, the fuel gauge for the other tank, the oil pressure and temperature gauges (if they're electric), panel lighting, and whatever other stuff comes alive when you turn the master on. See why that wire needs to be disconnected first?
     
    Last edited: Dec 7, 2020
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  7. Tom-D

    Tom-D Taxi to Parking

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    which fuel gauges are we working on ? year of the 172 would help.
     
  8. ARFlyer

    ARFlyer En-Route

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    Thanks for the education! I redid the test like you mentioned and got the same results. They’re original Stewart senders. My mechanic said it’s shot. Once we started emptying the tank the gauge came alive but was all over the place. We stopped at a half of a tank to test it. It would read E, then F, then it settled on 1/4 tank before slowly dropping back to E. The ohms was all over the place but settled on 110-120ish. The left one is rock solid at 25ohms.

    1978 N Model

    I called Cessna and they said those senders were suppose to be thrown out years ago with a Service Kit to the Rochester system. I called around to several salvage places and found a used one for a reasonable price. So we’re just going to throw it in until we overhaul the fuel system with my CEIS senders later next year. They’re still sitting here laughing at me.
     
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  9. Tom-D

    Tom-D Taxi to Parking

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  10. steingar

    steingar Taxi to Parking

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    My airplane's fuel gauges are for entertainment purposes only. That said, I have a fuel stick and a watch.
     
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  11. Doc Holliday

    Doc Holliday Pattern Altitude

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    It's not that big of a deal to fix them to get them to work correctly. What other maintenance do you skimp on?
     
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  12. steingar

    steingar Taxi to Parking

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    None, but getting old fuel gauges to work isn't what I'd call "not that big a deal". Been on my radar for awhile, but COVID has rather put a damper on things. That squawk will resolved.
     
  13. Doc Holliday

    Doc Holliday Pattern Altitude

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    Actually it's not a difficult job to do. They are old (over 50 years) and that kind of system simply wears out with age.
     
  14. steingar

    steingar Taxi to Parking

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    What kind of nannybot are you? My first airplane didn't have working gas gauges either, I've never thought much of it. I wouldn't trust them even if they were working, what if one sticks in flight? I visual examination of the fuel with a calibrated stick, and time. It's what we're trained to do, or al least what we're supposed to be trained to do.

    By the way, my gauges are technically legal. They do read full when full (sorta) and empty when empty.

     
  15. Doc Holliday

    Doc Holliday Pattern Altitude

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    Some people are content with old worn out items on their aircraft, and many are content with not spending money and deferring maintenance, hence why the condition of a lot of the GA fleet today is in a sad state.

    Personally I would rather have a fully functioning machine that is up to date. But that's a personal opinion.

    Carry on.
     
  16. MauleSkinner

    MauleSkinner Final Approach

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    What does it read the rest of the time?
     
  17. Dan Thomas

    Dan Thomas Final Approach

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    Those old SW fuel senders are a wirewound rheostat. A tempered copper runner rides across a coil of resistance wire to vary the resistance. The copper slowly wears the wire until it breaks, and then funny things start to happen. The ground path for the current runs though the copper and then into the steel sender housing, which corrodes as moisture condenses in the roof of the tank. That adds even more error.

    Even in old, low-time airplanes they're worn out by the wind rocking the airplane and sloshing the fuel around a bit, moving that sender just a little and wearing the wire. The wind does a lot of damage to the airplane; it moves control surfaces a bit and wears cables and hinges and rod ends and whatever else gets shifted around for years at a time.

    McFarlane has new senders matched to the SW system. Changing to the Rocherster requires new senders AND new gauges. I did one once, and it's a pain. Getting the gauge cluster out is no fun. Changing the fragile gauges requires time and care. Reinstalling the cluster in some airplanes is the biggest pain, with long screws and spacers and tiny lockwashers and nuts, most of it nicely inaccessible. I think Cessna took the gauge cluster, propped it up in a jig, and built the airplane around it. The 210 was built around the power supply for the electroluminescent panel.
     
  18. Capt. Geoffrey Thorpe

    Capt. Geoffrey Thorpe Touchdown! Greaser! PoA Supporter

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    The important thing is to know are your fuel consumption rate, the fuel at the beginning of the flight, and how long you have been flying. That way, if the fuel gauges read zero because your fuel cap was not installed properly and you lost a dozen gallons to the wind, just ignore it because your calculations will keep you in the air.
     
  19. ARFlyer

    ARFlyer En-Route

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    I’m amazed at the simplicity of the job. It’s literally just five bolts, a gasket, and one screw for the wire. It’s almost easier then changing the oil! :D
     
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  20. mondtster

    mondtster En-Route

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    Remember how easy it is when the standard response is "they are always broken." It has always been a pet peeve of mine that people don't fix broken stuff, especially when the broken stuff is spelled out in the regs as being required.

    Maybe the thing to do is install the new senders you bought and add a gauge that will work with them? I've been pretty satisfied with the Aerospace Logic instruments in the airplanes I've used them in and they aren't terribly expensive.
     
  21. Skyrys62

    Skyrys62 En-Route

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    How does one easily tell what system they have installed.
    My plane falls into the 'could have either one' category according to the notes on McFarlane.
     
  22. steingar

    steingar Taxi to Parking

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    They read whatever they want. Like I said, for entertainment purposes only.
     
  23. MauleSkinner

    MauleSkinner Final Approach

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    So they aren’t legal.
     
  24. Dan Thomas

    Dan Thomas Final Approach

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    The "could have either one" thing might be because the airplane may have been modified to have the Rochester gauges.

    The Cessna stuff implies that SW was factory-installed in 172s up until 1985. Second page: https://support.cessna.com/custsupt/contacts/pubs/ourpdf.pdf?as_id=22158

    From McFarlane's catalog we see this:

    upload_2020-12-8_13-4-55.png
    You need to remove the sender to see what it looks like.
     
  25. ARFlyer

    ARFlyer En-Route

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    I am! We have a JPI 900 being built that will be installed during the engine swap along with the CEIS senders.

    Just Murphy’s Law prevailed and one of the old senders decided it was tired of living before everything was delivered. COVID pushed both the engine and JPI back from a next week delivery to a maybe February, if we’re lucky, delivery.
     
  26. Skyrys62

    Skyrys62 En-Route

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    Thanks Dan.. y'alls help and knowledge is always appreciated.

    I'm assuming that will take a couple of new gaskets to put back until the new part comes in?
    Also, are these gaskets treated with any goo of any kind, or just dry?
     
    Last edited: Dec 9, 2020
  27. Dan Thomas

    Dan Thomas Final Approach

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    The gaskets now tend to be a harder rubber rather than the old, thicker cork, and they won't seal well on an old tank. The old gaskets had a habit of deverloping seeage, so mechanics would just tighten the screws some more, which distorts the tank around the sender hole. I have used small water pump pliers and a small wooden block with a concave underside to level the metal off again so it will seal. If you use any sealant it should be Fuel Lube (EZ-Turn) and use only a little; you don't want a bunch squeezing out inside and dropping into the tank where it will foul sump drain valves and stuff. Then you have to be careful not to tighten the screws too much or that rubber gasket will actually squeeze out enough that it will leak around the screw holes. Use a bit of lube on the screw threads, too, since they go right into the tank and fuel will climb up them. Cessna used to have small washers and tiny cork gaskets on the screws, but they didn't really seal well. The fuel could still travel up the threads and seep out above the washer.

    There's an aluminum ground strap that's supposed to go against the tank under the gasket, held by one of the screws. That strap, if it has a wide margin around the screw hole, will reach all the way under the gasket and lift the gasket and let fuel out. Sometimes trimming it is necessary, and I used to bend its edge down just a little in one spot so it would make definite electrical contact with the tank. What that system really needs is two straps, one to the tank and the other to the top of the sender, since fuel residue or sealant on the screw threads means very poor grounding for the sender. Some airplanes had a U-shaped strip that went around the gasket and sender and was clamped against the sender under a screw head, with the other end against the main ground strap. Poor grounding means poor fuel gauge operation.
     
  28. Skyrys62

    Skyrys62 En-Route

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    Great info, thanks.

    Here is the documentation I was speaking of: https://www.mcfarlaneaviation.com/media/documents/doc-1189.pdf
    See note 2 on the last page. My serial is a later number than listed in the note, so it's in the 'could have' category.

    I'm wondering if there is a way to tell by looking at the top of the unit...such as different style connector or anything of the sort.

    Also looking for anyone that might have tips-tricks about keeping junk out of the tank when removing/cleaning the old gasket and surrounding area? Should the tank be empty?
    Don't want no jank in the tank.
     
  29. Dan Thomas

    Dan Thomas Final Approach

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    Not easy to tell. I have encountered very few Rochesters. Yes, keeping the crud out is fun, but it can be done. The bigger risk is fire, with that hole open and gasoline fumes drifting out all over the place. You want to make sure that the master switch is taped off and the sender wire terminal is taped up good too. Don't need any sparks if someone turns that master on. No electric tools. An empty tank can be dangerous too, as the fuel/air mix ratio can get into the combustible range. With fuel in it the mix inside the tank is too rich. It's the area around that open hole you want to be careful with. Gasoline fumes are heavier than air and will flow down and into the wing itself, too. Outside with a breeze is safer, but then you get bugs attracted to those tasty fumes and they fly into the tank.
     
  30. sourdough44

    sourdough44 Pattern Altitude

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    Yes, look into it. Regardless of any gauges, fuel totalizer, & flow, there’s always room for known fuel quantity & timing. The more gallons, the less exact your timing has to be.

    I was just looking at an accident report the other day. The pilot said his totalizer showed X gallons left, as he ran out of fuel.
     
  31. ARFlyer

    ARFlyer En-Route

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    @Doc Holliday @Dan Thomas

    So more troubleshooting. If you take the wire and touch it to ground the Gauge correctly shows F. Now if you turn off the master the gauge will slowly move towards E like normal. The transmitter ohms reading is normal with a few random fluctuations after we lightly sanded the crud off the screws and the plate. Now once we installed the transmitter back into the tank the Gauge only reads 3/4 when full. What's odd is the transmitter faces aft and not outboard like you think it would. The hole pattern ONLY allows it to face one direction. Also when ever you turn off the master the Gauge does not gradually move to E like the other one. It stays firmly at 3/4 of a tank.

    So does the gauge really read facing aft? And if so, whats causing the gauge not to read above 3/4 and not go to E when the master is off?
     
  32. Doug Reid

    Doug Reid Line Up and Wait

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    On my Cessna, the fuel gages read pretty close in flight, but did not read correctly in the hangar. The problem was caused by a poor ground connection from the sender to the fuselage skin. The fuselage was used as the ground point.
     
  33. Dan Thomas

    Dan Thomas Final Approach

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    That senders sits with the float arm facing aft. Facing outboard would put it so that it read empty while there was still fuel in the inboard section of the tank, due to wing dihedral, and it would probably shift more, since the fuel will move around more due to uncoordination in turns than to pitch changes or acceleration/deceleration.

    Sounds like the gauge is sticking. They do that when they get old. The gauge has tiny electromagnets in it, so the needle is mounted on tiny bearings that aren't sealed. They get dirt and dust in them. When people smoked in airplanes the instruments all got gummed up. That gauge might have to come out and get cleaned.

    The other factor is that the sender's resistance doesn't always go to zero when at full. Some will still show 30 ohms or so. There are stop tabs on the sender body to keep the float from touching the top or bottom of the tank so that it doesn't wear a hole in itself or the thin aluminum tank, and if someone has fooled with the top stop tab it might be stopping the float well down from the top of the tank. Or they've bent the float arm to achieve something.
     
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  34. Clip4

    Clip4 Final Approach

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    What do you use to determine you aren’t venting fuel overboard?
     
  35. Southpaw

    Southpaw Pre-takeoff checklist

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    My fuel sender has SW / USA /PAT PEND stamped on it.
     
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  36. Skyrys62

    Skyrys62 En-Route

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    Just as an update to this, mine also has S.W. stamped on the top.

    Now, to decide on replacement with S.W. or upgrade to new engine monitor :)

    PXL_20201229_234245656.jpg
     
  37. steingar

    steingar Taxi to Parking

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    My airplane doesn't vent fuel overboard. If yours does you should definitely have it repaired.
     
  38. Clip4

    Clip4 Final Approach

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    Remember that when the leak occurs in flight and you have chosen to ignore the fuel guages.
     
  39. Matthew Rogers

    Matthew Rogers Line Up and Wait

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    I am lucky that somehow my C150 doesn’t vent fuel overboard when I forget to put the cap on the tank, but I don’t want to test that twice.

    Let’s hope that your airplane does not spill fuel out of the cap when you too leave it off one day.
     
  40. steingar

    steingar Taxi to Parking

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    Hard to forget a fuel cap in a low wing airplane.