Fuel Gauges, both read wrong, but the same, when empty.

[Heloman]

I carefully ran each fuel tank dry on my C model during a long multi leg, flight last week. I’ve been suspicious of my fuel gages for a while. Funny thing is both stopped in the same wrong place when totally empty, a little under half a tank. I wonder if it could have the wrong senders installed? Needles don’t move off of full until someplace around half tanks.

 

 

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[carusoam]

Sounds like they need the same service....  clean and calibrate.

Or is it too late?  Did you run both tanks empty on the same, accidentally shortened flight?

Great time to consider the new style digital fuel level devices if you have a JPI or other nice engine monitor....

PP thoughts only, not a mechanic...

Good morning MS,

-a-

[mpg]

hey op...   do your senders have the wrong diameter cork floats on them??

too big, will float to full to the top, until the gas has gone way down,,,

too big, will float too high with a partial tank of gas,

and if they are too big in diameter,, when the have no gas to float on,,,  they will still show gas in the tank, even though there is No gas for them to float on...

[RLCarter]

Do either gauge read "Empty" with the master off? 

[N201MKTurbo]

You need to check how they are wired ar the inboard sensors. The sensors need to be in series. This requires the inboard sensor is not grounded and the wire from the outboard sensor is connected to the flange of the inboard sensor. The inboard sensor must be floating from ground by having TO3 transistor insulators under the screws.

from what you say I suspect you will find the wire from the outboard sensor on the post of the inboard sensors. If the insulators are there it would take the inboard out of the circuit and only indicate from the outboard sensor. Because of the dihedral of the wing it won't start to go down until you are half empty and when you are empty only half of the sensors are in the circuit so it shows half a tank.

The wire from the outer sensor should be connected to the inboard sensor by removing one of the flange screws and the insulator for that screw. Placing the ring terminal around the insulator and putting it back in the hole. This way the wire is connected to the sensor flange and isolated from the screw which is grounded.

Edited March 30, 2018 by N201MKTurbo

[neilpilot]

1 hour ago, N201MKTurbo said:

You need to check how they are wired ar the inboard sensors. The sensors need to be in series. This requires the inboard sensor is not grounded and the wire from the outboard sensor ....

OP has a M20C, with only one sender per tank.

[N201MKTurbo]

4 minutes ago, neilpilot said:

OP has a M20C, with only one sender per tank.

OK then. never mind. 

 

[Heloman]

Do either gauge read "Empty" with the master off? 

Yes, they drop all the way down with the master off.


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[Heloman]

Sounds like they need the same service....  clean and calibrate.
Or is it too late?  Did you run both tanks empty on the same, accidentally shortened flight?
Great time to consider the new style digital fuel level devices if you have a JPI or other nice engine monitor....
PP thoughts only, not a mechanic...
Good morning MS,
-a-

Funny! No, my flight went as planned. Unfortunately my old C is as tricked out as it’s going to get with me as the owner. It’s got a JPI700 and an FS450 fuel flow, it’s sort of the “ultimate cheap bastard C.” I want to upgrade my plane for one with more range , but I won’t leave this problem for someone ells to figure out.


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[carusoam]

Heloman,

0) go to step 8) first.... then back to 1)...

1) See if you can reach in the tank and exercise the floats through their entire range...

2) If they are not sticking or bumping into something, Expect the variable resistor device to not be working properly...

3) before chasing the variable resistor, Check the calibration of the gauge... there should be a procedure for the  adjustment.   (Mechanic may be required for the proper documentation)

4) Check the logs... See if anything has been touched in the last 50 years...

5) If Nothing has been touched, a small amount of resistance at each connection may be responsible for the funky readings...

6) Pulling the sensor for OH is not too challenging. You get to see the seal that may still be cork, if original... also get a look at a small piece of rubber hose link connecting the fuel tank to the fuel line.  Probably a collection of blue goo in the area, if nobody has been in there in decades....

7) There is a company that OHs the sensor... probably an easy search to find.... 

8) Use the normal high level of caution while working anywhere near fuel.  Fire extinguishers ready to be deployed. Put in extra effort to ground things even if not needed... 

 

The C has the most simple fuel gauge system... one float, one variable resistor, one gauge, one wire(?).... per side.

 

Memories of a PP, not a mechanic... did I miss anything?

Best regards,

-a-

[Marauder]

Funny! No, my flight went as planned. Unfortunately my old C is as tricked out as it’s going to get with me as the owner. It’s got a JPI700 and an FS450 fuel flow, it’s sort of the “ultimate cheap bastard C.” I want to upgrade my plane for one with more range , but I won’t leave this problem for someone ells to figure out.


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The factory original resistive gauges work fine with my factory gauges but I would get a fuel mismatch error on my JPI 900. 40+ year old floats are bound to be off at some point. Based on the fact the the gauges go to empty on power off, it probably are the floats and not the gauges.

If you have the tanks drained low enough, you can always pull the floats and send them out to the Lock Haven PA operation that will overhaul them. Pretty easy to assess once you pull the interior panels.

The sender on the left is the factory unit. The one on the right is the CiES unit I replaced it with.



Here is what the factory original looks like installed. They are between the front and rear seats.




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[cbarnes403]

Check the ground wire, clean and reinstall.