When I built my Cleanex I used the Sonex recommend Princeton fuel probe and the MGL instruments. For fuel I have the older Velocity (Stratomaster) version known as the Flight 2.
Recently the Princeton unit would work but would blow the 3 amp fuse every now & again. I ordered a replacement from Red Avionics. It came with the probe (identical to the Princeton) and the control box (looks the same but has an extra wire).
I cannot get the MGL to read the new installation. Talking with Justin at Red Avionics confirmed that the green wire is used for capacitance and the yellow wire (a new addition that was not on the older Princeton unit) is for resistive type probes.
After going through the set up with him it appears the unit is seeing the probe but adding fuel changes nothing on the MGL. I’m at a loss. I’ve done all that I know to do. Justin at Red Avionics has been extremely helpful and said that he would contact MGL on Monday & see if he can find out why the two units are not communicating.
Is there any wisdom among the brotherhood here that might help me understand what I’m missing?
I was wanting to make Triple Tree later this week but I don’t want to fly a cross country without a working fuel gauge.
Did you replace the probe and the control box?
Yes. They came as a set as sold specifically for Sonex aircraft with MGL instruments.
Is there a dip switch setting that tells the control box which type probe is being used?
No. There are two wires that can be used for reading the fuel level. The yellow wire is for resistive probes (not used) and the green is for capacitance (same as on the older Princeton unit.) Justin confirmed that green was the correct wire for my instrument.
Does the MGL accept a DC voltage for fuel level?
See above.
Have you measured the voltage going to the MGL to see if it varies with fuel level?
The head unit for the probe is giving a “heartbeat” lamp indication that Justin says is proof the the probe is happy with the connection to the box. The MGL unit read the old probe perfectly and the wiring is exactly the same as before.
The old unit worked but it would sometimes blow the small 3 amp fuse. This is the fuse that powers the probe as the MGL unit is on a separate fuse that has never blown. The MGL has been flawless since I put it in.
Ok, I suppose the “control head” is programed to measure one or the other rather than it being selectable. I also thought you might want to measure the signal at the RDAC to verify that signal is or is not changing with fuel level.
That is a huge amount of current for that type of device.
I’ll take a stab in the dark. Red Avionics forgot to drill the hole in the base of the probe to allow fuel to enter between the electrodes
Thanks for pushing me into looking deeper. I now suspect the yellow wire is going to be the proper wire for the gauge connection after looking at other documentation. I should be able to measure voltage output on that line and I believe that is what the MGL is looking for. I do not have a meter to read capacitance.
I initially had the yellow wire connected but I’m not certain I had the probe box configured correctly so I may change it back & give it another shot tomorrow.
I do appreciate your willingness to dive into into this …
No worries. You did “sound” a little cranky though
Really, the only way the probe can fail is if the two electrodes get shorted or if the wires fall off. You can check that with an multi-meter (with the signal conditioning unit out of the circuit).
Me sounding cranky? Yeah … I apologize if my irritation was showing a bit.
I can check the probe on the new unit as it has a connector but the original unit doesn’t have a wiring connector as the small coax goes straight from the head unit to the probe.
I don’t believe the probe was ever the problem. I’ll know more more tomorrow. Either way I’ll strive to be less cranky …
Just to finish this out … after doing a bit of bench work on the old unit I reinstalled it today and it worked as good as ever and never failed a fuse.
I tried every possible combination with the new unit and have concluded that it is either faulty or nor compatible with my older Stratomaster Velocity Flight 2 instrument. The one I ordered was said to be a direct replacement but nothing I could do would make it work.
Installed the old unit and recalibrated it just as the instructions say and it was “easy-peasy, lemon squeezy” as they say. I also did not receive a promised call back form Red Avionics as promised (I do understand that folks sometimes get busy) nor did I receive a promised call back from another vendor that I reached out to for assistance.
I have considered installing a fuel gauge like in the Sonex Sport Acro:
