Sonex fuel level probe and MGL calibration

[Murray Parr]

Thanks Bill,
I checked the ground and it's fine. I must have something not set right or perhaps the probe could be faulty. Princeton don't respond to emails so I am not sure what the empty calibration number should be. I am thinking the 2760 I'm getting could equate to full and that could be why it doesn't change when adding more fuel.

Has anyone had this problem and found a solution?

[sonex892.]

Hi Murray. The sender I use to the RDAC previously was used with an MGL Flight 2. Pretty sure its only 0-5 v and not resistive. If you have a universal sender with both reistive and 0-5 v options try using the yellow wire 0-5v to the RDAC with the pullup resistor OFF.
Also make sure the RDAC grounds are common to the probe and princeton sender.

[XenosN42]

Hi Murray,

Each Princeton fuel probe will have a slightly different electrical resistance when there is no fuel in the system. So when you compare yours to others don't expect the exact same values. With that disclaimer out of the way, here are the values from my Princeton fuel probe attached to a MGL iEFIS Explorer. OneX fuel tank.

Gallons Resistance (Ohms)
0.1 2715
2.7 1750
5.6 1360
8.3 983
11.2 658
14.1 326

The important thing to note is that the resistance will decrease as you add more fuel to the tank. Since you are not seeing that trend I suggest you check the following.

1) I found that the connector between the probe and the interface module didn't always lock together. If this connection isn't tight you'll have an open connection. I held the connection tightly together and used tape to make sure it didn't disconnect.

2) There are green and yellow wires exiting the interface module. You should be using the green wire. Tape off and ignore the yellow wire.

3) Make sure that the ground from the interface module shares a common ground with the RDAC's ground wire.

4) Fuel level 'pull up' dip switch, found on the RDAC. I didn't make a note of the position of that switch. However, because I didn't I would assume I didn't change it from the factory default. The next time I go to my hanger I look at it. However, if you're still having trouble with calibration it wouldn't hurt to turn off the system, change the DIP switch and see if the resistance moves.

5) If all of that fails check the resistance by by-passing the RDAC & EFIS. Use a multi meter connected to the wire which would attach to the RDAC. Attach the multi meter ground to your common ground.

Good luck.

[bvolcko38]

Mike,
These are the same instructions I used to calibrate mine, but I was silent because I can't find the documentation/instructions anywhere. Where did you find them?

[Scott Todd]

I don't remember any instructions. I just went to the calibration page in my MGL Xtreme and it was pretty straight forward. Add a little known quantity and press calibrate. Add some more, repeat. I've only used 100LL but its been real accurate over the first 40 hours.

[XenosN42]

Mike,
These are the same instructions I used to calibrate mine, but I was silent because I can't find the documentation/instructions anywhere. Where did you find them?

Hey Bill,

You'll need to cross reference two documents.

Most important: http://www.princeton-electronics.com/uploads/9/8/7/9/98793968/sonex_universal_fuel_level_probe_v2.pdf

The manual for your RDAC. This is a link to MGL's newest. If you use an older model you'll need to Google it. https://www.michiganavionics.com/download/rdac-xf-xf-map-installation-manual/ See page 3 for the description of the fuel level pull up DIP switches.

Hope that helps.

[Murray Parr]

I think I found the smoking gun. After connecting a temporary ground from the electronics module to the engine ground, I was able to calibrate the unit (although the readings fluctuated by around 50 ohms). All seemed good until I finished calibrating and restarted the IEFIS. After this it didn't matter where I put this ground the unit never worked again so I got thinking of why it worked for a short period and figured I must have bumped a wire somewhere. Using Micheal's advice I started with the Coax connector and was able to get it working again (although quite arradically now) by forcing the connector together tightly. After dissasembling the Coax connector, I couldn't find much wrong with it except for the tiny center pins aren't that tight when joined back together so I will now use new Molex connectors for this (due to not having parts or tools for RG316 coax cables) and see if that fixes the problem. Hopefully the different type of connector doesn't mess with the unit too much. Probably will have to calibrate again if this works.

Thanks to all who chimed in here.

[flyingbear]

I have worked with my MGL Discovery Lite for well over a year trying to get it working. I had to have both the main unit and the RDAC repaired even though it had never flown....the ASI/Barometer unit had decayed and broken just sitting there. I do not know what was repaired in the RDAC. I paid an aviation electronics guy a hundreds of dollars and got nowhere.
After weeks and months I got all but my fuel quantity working ....and mine does the same as Murrays...won't keep readings when power is turned off. I have 220 hours on my Aerovee and have yet to have FUEL quantity readings. MGL says they do not know anything about the Sonex Princeton probe.

Why cannot Sonex just write up a page on HOW to set the probe up and how to set the dip switches on MGL units.
My instructions are for 1,2 or 5 set point probes but nowhere does it say what MINE is nor how the different types are set up.
This is the most absurd thing I have ever seen. Well Over a year, 220 hours flown and still it does not work.

Any ideas??????????????????????????

[T41pilot]

Don't know if this will help but the eprom in my diy fuel probe circuit did not like a shutdown which involved a gradual voltage decay instead of an instantaneous off. I don't use a battery in my ultralight and just use ships power when running to operate the electronics and lights. When I shutdown the engine the voltage would gradually bleed down to zero due to the large capacitor on my voltage regulator. My fuel level the next day would indicate very low or zero. Tying my fuel probe circuit to the engine monitor circuit which was on a switch and shutting that off before killing the engine solved the problem. Make sure your setup is tied to something that gets shutdown before the engine does. My probe in the Sonex is tied to the avionics buss and seems to be functioning correctly. I however am running A GRT unit, not MGL.

[Murray Parr]

I have worked with my MGL Discovery Lite for well over a year trying to get it working. I had to have both the main unit and the RDAC repaired even though it had never flown....the ASI/Barometer unit had decayed and broken just sitting there. I do not know what was repaired in the RDAC. I paid an aviation electronics guy a hundreds of dollars and got nowhere.
After weeks and months I got all but my fuel quantity working ....and mine does the same as Murrays...won't keep readings when power is turned off. I have 220 hours on my Aerovee and have yet to have FUEL quantity readings. MGL says they do not know anything about the Sonex Princeton probe.

Why cannot Sonex just write up a page on HOW to set the probe up and how to set the dip switches on MGL units.
My instructions are for 1,2 or 5 set point probes but nowhere does it say what MINE is nor how the different types are set up.
This is the most absurd thing I have ever seen. Well Over a year, 220 hours flown and still it does not work.

Any ideas??????????????????????????

I still have the same problem with mine and I am expecting to do the first flight soon (probably with no fuel level indication).

For most of this year I have tried everything that the princeton dude has sugested (can't imaguine how many times I have had to drain the tank and start over) and it always works fine if I never shut the power off but as soon as I do that it goes back to the zero calibration value no mater how much fuel is in there and reads zero until I put more fuel in it than the new zero setting is. So frustrating, I am thinking of throwing it in the trash and buying a different brand. In the mean time, I installed a cheap camera and light to look at the fuel level, however I think the light is too bright and it is hard to easily see the level.

Ranier from MGL has assured me that the calibration settings are saving to the IEFIS correctly so I don't think it's a problem with the RDAC or IEFIS.

I know others have had this same problem as I have seen it briefly mentioned in a few older posts and I was hoping they would chime in with how they resolved this.