fuel indications

[Planegary]

Hi again all  I am coming up on my annual and was gonna try to see if i could find what makes the fuel gauges less accurate than I would like . At this time finances prevent any upgrades so I am looking for inexpensive fixes. Does anybody have the range that the fuel sending unit is supposed to work within from full to empty? if I check the wiring what plugs in back of the indicator should i be plugged into.I have the original garwin pack where I can pull each gauge out individually to test that gauges wiring. I am hoping to find maybe some poor connections or grounds since the gauges work just not like I would like to see (they neither read full) thanks in advance for the help it is all appreciated

Edited June 1 by Planegary
doubled by accident

[N201MKTurbo]

If you take the gauge apart, there is a wire wound adjustment sector with two wipers. Mark their original positions with a pencil mark. Put some contract cleaner on the pivot and the wipers and work them back and forth then return them back to their original positions.

If you have resistance decade box or a good pot that makes the correct range, you can adjust the calibration. This is assuming the senders are in good shape.

[Planegary]

Thanks I will do that today hope it helps

[Planegary]

Did the cleaning. One of the gauges work much more accurately. The other was no change so I got A 50% success rate out of it 

[Pinecone]

The wire wound pots can get worn or dirty.   Dirty you can clean as described.

Worn needs to be rebuilt.  

FYI, the senders run a range of 0 - 30 ohms resistance, empty to full.  Some models have 2 senders per tank.  Inner and outer.  They are wired in series.  Battery to gauge to inboard sender to outboard sender to ground.

In those planes the gauge reads full at 60 ohms total resistance.

[M20F-1968]

If you have the original senders, you will want to replace them with the Rochester senders.  I have some rebuilt ones if you are interested.

John Breda

[Ragsf15e]

16 hours ago, Planegary said:

Did the cleaning. One of the gauges work much more accurately. The other was no change so I got A 50% success rate out of it 

Also, the senders can get stuck at a certain point.  It is possible to gently move them and see if that changes the indication.  Depending on the tank setup, some are more difficult/impossible to reach…

[MikeOH]

55 minutes ago, Pinecone said:

FYI, the senders run a range of 0 - 30 ohms resistance, empty to full.  Some models have 2 senders per tank.  Inner and outer.  They are wired in series.  Battery to gauge to inboard sender to outboard sender to ground.

I'm curious about this as I've never had mine apart or measured resistance.  The factory gauges in my F appear to be pretty basic D'Arsonval meter movements; current driven.  With master off, no current, needle to the left side reading zero.  So, I would think low resistance (0 ohm) would result in more current such that full tanks would be 0 ohms, and 30 ohms would be empty; what am I missing?

[Planegary]

Thanks John,  right now i have to determine is it the senders the gauges or the wiring or maybe the ground or contact point for the sending unit  these questions should be answered this week during the annual. The gauges are working and indicating just not as well as i would like and i would prefer them to be more accurate.The one I cleaned the other day has been a LOT better so right now only one to focus on

[Pinecone]

On 6/2/2024 at 1:31 PM, MikeOH said:

I'm curious about this as I've never had mine apart or measured resistance.  The factory gauges in my F appear to be pretty basic D'Arsonval meter movements; current driven.  With master off, no current, needle to the left side reading zero.  So, I would think low resistance (0 ohm) would result in more current such that full tanks would be 0 ohms, and 30 ohms would be empty; what am I missing?

I may be remembering backwards.

But I am pretty sure about the 0 - 30 ohms.

But master off, no current to move the needle, so the spring takes over.

[jetdriven]

Those senders get gunked up with corrosion and stuff and we’ve had really good luck sending them to Keystone and for about 100 bucks apiece they will come back like new, I just did this with my plane and then we calibrated it with the 275 engine monitor and it’s deadass all the time

[Igor_U]

7 minutes ago, jetdriven said:

Those senders get gunked up with corrosion and stuff and we’ve had really good luck sending them to Keystone and for about 100 bucks apiece they will come back like new, I just did this with my plane and then we calibrated it with the 275 engine monitor and it’s deadass all the time

Byron,

Would that be Keystone instruments in Lock Haven? http://keystoneinstruments.com/index.html

Thank you.

[jetdriven]

Yep 

[TaildraggerPilot]

Per FAA standards of any particular aircraft type certificate, the fuel gauges only need to be accurate when they read empty…..

[jetdriven]

1 hour ago, Taildraggerpilot said:

Per FAA standards of any particular aircraft type certificate, the fuel gauges only need to be accurate when they read empty…..

I’ve heard that, but it doesn’t seem to make any sense. I thought they  had to indicate the fuel quantity in the tank. Not just when it’s empty or just when it’s full either.

[jetdriven]

These guys here just plain old say it’s false.https://americanflyers.com/the-myth-of-the-inaccurate-fuel-gauge/

[MikeOH]

1 hour ago, Taildraggerpilot said:

Per FAA standards of any particular aircraft type certificate, the fuel gauges only need to be accurate when they read empty…..

I've heard that many times; my belief is it is a myth as I've never seen any citation to support the claim.

Do you have one?