Float fuel senders

Hi folks, I'm having issues calibrating my fuel tanks with float sensors and a G3X touch. I've determined that the resistance option (as opposed to voltage) is the correct option, but when I generate the calibration curve, the difference between empty and full is only about 34 millivolts. Garmin says it should be volts. When I save and re-boot the G3X, it shows my tanks are empty. My understanding of the Stewart Warner senders is that empty should give 240 ohms, and full around 30 ohms. I'm going to drain the tanks and check that I'm seeing those numbers (I think I can push them up thru the tank drain hole).

Anything else I should check?

Verify the sender ground. Do this using a simple multi meter to measure the resistance between the sender base and aircraft ground.

If good, then verify your wiring to the sender center post (as in connected but not grounded).

Carl

Thanks

I checked and both sender units are adequately grounded.

Also had trouble

Interested in your solution. I also had trouble. I would also ignore the “ground” calibration, as this really is just for tail-draggers and not used useful in flight.

What was your problem?

Yes, I only used the "in flight" mode.
What problems were you having, and what was the solution?

Av8torTom said:

Hi folks, I'm having issues calibrating my fuel tanks with float sensors and a G3X touch. I've determined that the resistance option (as opposed to voltage) is the correct option, but when I generate the calibration curve, the difference between empty and full is only about 34 millivolts. Garmin says it should be volts. When I save and re-boot the G3X, it shows my tanks are empty. My understanding of the Stewart Warner senders is that empty should give 240 ohms, and full around 30 ohms. I'm going to drain the tanks and check that I'm seeing those numbers (I think I can push them up thru the tank drain hole).

Anything else I should check?

Click to expand...

First put an ohmmeter between the senders outer plate, NOT a screw, and the airframe, not the wing. Should be<.1 Next guess is that the sender may be stuck and is not moving. Might be able to snake one of those cheap borescopes through the baffle holes to get a look, but may not be possible. I bet it will fit through the hole in the drain flange though. Can probsbly also get a wire in there to move the float and observe.

Av8torTom said:

Hi folks, I'm having issues calibrating my fuel tanks with float sensors and a G3X touch. I've determined that the resistance option (as opposed to voltage) is the correct option, but when I generate the calibration curve, the difference between empty and full is only about 34 millivolts. Garmin says it should be volts. When I save and re-boot the G3X, it shows my tanks are empty. My understanding of the Stewart Warner senders is that empty should give 240 ohms, and full around 30 ohms. I'm going to drain the tanks and check that I'm seeing those numbers (I think I can push them up thru the tank drain hole).

Anything else I should check?

Click to expand...

If you are using SW 385 senders, why are you using volts? Assuming you installed them with the standard 5 screws into nutplates, and the screws are in good contact with the sender base plate, you should have a good ground - but you can confirm this by connecting an ohmmeter directly to the sender and cycling the sender (easier done with the unit removed...but a fill and drain does it too).

Assuming you have a GEA 24 as part of your G3X system, tank 1 sender should be connected directly to P244 pin 6 and tank 2 sender connected to pin 9. No other connections or resistors are needed. Note that the GEA 24 is expecting 0-500Ω input, so any inline resistors will cause the system to not sense any change. If you have an inline resistor, this makes sense that the system is showing empty (highest resistance).

Voltage calibration is intended for composite airframes and uses both an inline resistor and pull-up resistor. With an RV, this is not needed, but rather the single wire connection works best (and simplest).

Note the drawing calls for a shielded wire, however, my experience shows a good milspec 20 GA wire does the job just fine. If you do choose to use a single conductor shielded wire (available from ACS in Tefzel), you could connect the shield from a ring terminal on one of the five sender screws and then to GEA pins 7 and 10, respectively, to ensure a good ground, but that isn't needed as long as the sender is well grounded to the tank, which it is bolted to.

rongawer said:

If you are using SW 385 senders, why are you using volts?

Click to expand...

He stated that he DID select the resistance option. However, internal to the G3X, the circuitryu is set up as a voltage divider and the actual voltage for any given float position is used in the software for determining position. Therefore, the calibration page is showing voltage values. Same thing for trim.

Garmin recommends using shielded wire.

Some people, including me, crimped the connectors to that shield, without realizing that the shield should be cut back so the single center wire can be the contact point.

If the shield is contacting anything, that will result in the unchanging voltage reading.

Happened to me. Also happened to someone else on this site last week

lr172 said:

He stated that he DID select the resistance option. However, internal to the G3X, the circuitryu is set up as a voltage divider and the actual voltage for any given float position is used in the software for determining position. Therefore, the calibration page is showing voltage values. Same thing for trim.

Click to expand...

Larry, there are two wiring methods described in the G3X IM, one uses a voltage input (which uses a Pull-Up resistor from a voltage pin) where the GEA expects a 0-5V input, the other installation method is direct resistance measurement where the GEA expects a 0-500Ω input. I am not referring to the internal method the GEA uses. I am referring to the resistance installation method defined in the Sensor drawing section of the IM (attached to my previous post).

I've also seen it suggested on VAF that an inline series resistor be used with a fuel sender; this is neither needed or appropriate with a 385 series fuel sender connecting to the G3X as the GEA is ready to accept the 20-240Ω the sender provides directly.

I think it is unlikely that both senders are failed "high" (indicating empty), so I am suggesting that the resistance from the sender to the GEA pin is higher than expected and to verify that the resistance of the wire from the sender to the GEA pin is <1Ω.

Until told otherwise, I continue to suspect that if the G3X is indicating empty, the resistance presented to the GEA is higher than 500Ω.

Thank you

For all the helpful suggestions and insights.

I just purchased a new float fuel sender from AutoZone. I'm going to connect it outside the fuel tank to the fuel level wire and ground it to the fuselage, then due a G3X calibration moving the float by hand in 10 increments (to simulate adding 2 gallons at a time) and see what kind of curve I can generate. Obviously it won't be a usable curve, but it will be a quick and easy way to determine if my issue is with the sender itself, or if I should be looking elsewhere.

I had a similar experience when calibrating my senders (G3X touch), very small voltage differences. In the end I found my left and right wires to the GEA24 needed swapped. Disturbance/movement while filling enough to get some change. GEA24 wiring guide shows Channel "1" and "2" on GEA24 but wasn't clear which displays left or right in GDU. Channel 1 is "Left" tank display. I now have .3v-.5v difference for ~5 gallons. Manual was a little confusing there.

Fuel sender calibration

I have a similar problem with my G3X and my right tank calibration. My left tank is properly calibrated. Never had a problem with it. My right tank says it’s calibrated via the fuel gauge. I fly and it reads no fuel not calibrated and when I land it goes back to showing I have fuel. Been this way since I built the plane.

My solution: had the fuel floats replaced. No change. Then I replaced the GEA 24 unit. The floats were grounded and the wire had continuity. Still had same problem. So I reasoned the last item that could be causing the problem was the plug and the sub d pins. So I cut the old ones off and crimped new ones on. (Did this because I couldn’t get the pins out of the plug by using that cheap flimsy extractor tool).

I still have the same problem! Going to try one more idea of recalibrating the right tank and stopping the calibration at 18 gallons when the sender doesn’t read any more changes rather than keep adding fuel level inputs. If that doesn’t work I was thinking about buying a pair of those expensive powered floats.

If someone else has an idea I am open for suggestions. Not much else to think of?

Ditto

I had trouble like Alan above. When calibration done on resistive setting with vans Stewart Warner floats the calibration curves were measly and closely spaced. In flight it did not register any fuel change over an hour on either tank dispute using both tanks at 11 gph.

Do you have the yellow "Message" notification at the bottom of your screens? I had major difficulty when calibrating my tanks, too. Went thru all the procedures discussed in this thread. Katie at Garmin came thru in a BIG way! I was getting EIS errors reported. Turns out I had the PMags on an incorrect setting. Somehow, this locked me out of calibrating my floats. Makes no sense, but that fixed my issue.

"Katie at Garmin suggested RPM1 could be an issue with a setting of Mag, PLead. I set that to none for now and error message went away. Can now calibrate fuel again." "

I decided to go at three gallon increments. Following is what I got:

Gallons Volts
0 2.213
3 1.648
6 1.303
9 1.006
12 0.816
15 0.547
18 0.294
21 0.294

You can see the float has reached its upper limit somewhere between 15-18 gallons.

Hope this helps!

Update

I picked up a new sender from AutoZone and wired it to the G3X outside the tank, then advanced the float by hand in steps that would emulate adding 2 gallons at a time and I got a perfect curve. Guess I need to replace my senders.

I had the same issues as you this past fall getting my 7 ready to go. I filled and drained and the tanks several times with no success, i.e. the voltage was not changing. I knew my floats were free and not hanging up internally in the tank from the bench test then it hit me... the floats are very weak and the windings on the senders somewhat coarse. So I emptied the tanks again and at each increment I tapped gently on the sender covers and bingo I got the voltage curve needed. They are still a little sticky but I think as I fly (@50 hours now) they will "wear" in and with the vibration they seem to be working fine and I really rely more on the FF anyway.

My set up is also G3X and standard SW floats... FYI they are "full" until you get to about 16 gal a side due the dihedral then they start moving down. My .02.