Tango Mike
Well Known Member
My newly purchased G3X-equipped RV-7 has a fuel indication issue.
The fuel calculator function works as expected. With full tanks confirmed prior to a series of flights, fuel used and fuel remaining always equal 42 gal. So far so good.
The problem is that while the total of the L and R fuel gauges equals 42 immediately after the tanks are filled, as fuel is burned they don't decrease proportionately. A recent comparison: the fuel calculator indicated 14.8 used and 27.2 remaining but the gauges indicated 16/20. Sticking the tanks proved the gauge totals were bogus. The difference hasn't always been 8.8 gal, but the gauge total has never agreed with the calculator.
Examination of the fuel tank calibration page in the configuration mode proved that the builder had performed the procedure by incremental additions of 5 gal and calibrating the voltages for each data point. What I didn't know is if he had done it correctly, so I decided to do it myself just to be sure.
That turned out to be a great decision for the left tank, which shows full when full and decreases proportionately as fuel is used.
But the right tank refuses to cooperate. Here are the data points (gallons/voltage): 0/1.987; 5/1.898; 10/1.896; 15/1.895; 20/1.894. Compared to left-tank voltages of 1.947/1.284/0.887/0.335/0.296, it appears that the right-tank sending unit is either physically not rising with the fuel level or has failed internally.
I recorded the data points entered by the builder before I replaced them with my own, and his right tank values were very similar to the ones I obtained on the left tank, which indicates that the sending unit was working at one time.
I called Garmin to ask if there might be something in the G3X that could cause this, but the answer involved a discussion of how the wiring might have been done. That doesn't seem to be a likely cause when the builder's calibration appears reasonable on both tanks.
Which leads me to the most probable cause, a malfunctioning sending unit. The tech rep suggested that I remove the lead from the sending unit, connect a 5k rheostat to the center contact and ground either of the outside terminals on the rheostat. I wouldn't be able to compare absolute voltage values without matching the rheostat to the sending unit, but it would verify that the G3X end of the system is responding to changes in resistance at the tank end.
I've heard RVers talk about physically manipulating the float arm with the sending unit in the tank, but I don't know if that's a valid test or even possible. I could remove the unit with the center lead attached, ground the mounting flange to the aircraft, and manipulate the float with the G3X in the calibrate mode while a helper notes the voltage reading. But if I go that far, I'm tempted to replace the unit rather than reinstall it even if it checks out.
I'd appreciate opinions on whether I'm on the fight track with this and any suggestions about how to proceed from here.
One note: I've heard RVers say that you can't remove and replace a sending unit with the tank installed on the aircraft. I'm not buying that as a hard and fast statement, because last month I removed and replaced a sending unit in an RV-6. It wasn't particularly fun, but pulling a tank off to deal with a bad sending unit doesn't seem to be a good option if the alternative exists.
Thank you in advance for any assistance.
Tosh
The fuel calculator function works as expected. With full tanks confirmed prior to a series of flights, fuel used and fuel remaining always equal 42 gal. So far so good.
The problem is that while the total of the L and R fuel gauges equals 42 immediately after the tanks are filled, as fuel is burned they don't decrease proportionately. A recent comparison: the fuel calculator indicated 14.8 used and 27.2 remaining but the gauges indicated 16/20. Sticking the tanks proved the gauge totals were bogus. The difference hasn't always been 8.8 gal, but the gauge total has never agreed with the calculator.
Examination of the fuel tank calibration page in the configuration mode proved that the builder had performed the procedure by incremental additions of 5 gal and calibrating the voltages for each data point. What I didn't know is if he had done it correctly, so I decided to do it myself just to be sure.
That turned out to be a great decision for the left tank, which shows full when full and decreases proportionately as fuel is used.
But the right tank refuses to cooperate. Here are the data points (gallons/voltage): 0/1.987; 5/1.898; 10/1.896; 15/1.895; 20/1.894. Compared to left-tank voltages of 1.947/1.284/0.887/0.335/0.296, it appears that the right-tank sending unit is either physically not rising with the fuel level or has failed internally.
I recorded the data points entered by the builder before I replaced them with my own, and his right tank values were very similar to the ones I obtained on the left tank, which indicates that the sending unit was working at one time.
I called Garmin to ask if there might be something in the G3X that could cause this, but the answer involved a discussion of how the wiring might have been done. That doesn't seem to be a likely cause when the builder's calibration appears reasonable on both tanks.
Which leads me to the most probable cause, a malfunctioning sending unit. The tech rep suggested that I remove the lead from the sending unit, connect a 5k rheostat to the center contact and ground either of the outside terminals on the rheostat. I wouldn't be able to compare absolute voltage values without matching the rheostat to the sending unit, but it would verify that the G3X end of the system is responding to changes in resistance at the tank end.
I've heard RVers talk about physically manipulating the float arm with the sending unit in the tank, but I don't know if that's a valid test or even possible. I could remove the unit with the center lead attached, ground the mounting flange to the aircraft, and manipulate the float with the G3X in the calibrate mode while a helper notes the voltage reading. But if I go that far, I'm tempted to replace the unit rather than reinstall it even if it checks out.
I'd appreciate opinions on whether I'm on the fight track with this and any suggestions about how to proceed from here.
One note: I've heard RVers say that you can't remove and replace a sending unit with the tank installed on the aircraft. I'm not buying that as a hard and fast statement, because last month I removed and replaced a sending unit in an RV-6. It wasn't particularly fun, but pulling a tank off to deal with a bad sending unit doesn't seem to be a good option if the alternative exists.
Thank you in advance for any assistance.
Tosh
