SonexBuilders.net

I purchased a Princeton fuel probe to replace my BeLite probe (see other thread). I bought the Sonex version from the factory.

It came with confusing directions and a diagram that shows three wires from the interface box. The probe I got has four wires. Red and black (easy - power) and orange and yellow.

Two questions for the crowd

Does anyone know what the yellow and orange wires are for?

Has anyone used one of these with the BeLite fuel gauge?

Thanks and blue skies


Robbie Culver
Sonex 1517
Chicagoland
N1517S

Never use ethanol infused gasoline with it. It will kill your probe. The Belite gauge uses 0- 5 volts to display. It should be pretty easy to figure out which wire (orange or yellow) provides the reference voltage.

Two questions for the crowd

Does anyone know what the yellow and orange wires are for?

Robbie,

Most sensors like this are simple strain gages - you need two wires to power it, and the signal comes out of the other pair. Sometimes a manufacturer will simplify this and use the negative wire of the power in to be the same ground as the signal out...
As a customer, I'd be disappointed that the manufacturer did not provide any paperwork or manual on how to identify which wire does what.
If lieu of this missing information, try to measure the output of your new transducer with a milli-volt meter to identify which one is ground or positive. You need to change the pressure on it (slightly - don't blow into it very hard - these are sensitive as we're discovering, and could be over-ranged/damaged) You should be able to verify which way it is driving to understand polarity in case of a tiny electrical offset (that might be slightly negative volts) and figure out which wire is positive/negative.

Depending upon how 'smart' the gauge your wiring into is, it might not even matter - it may well accept a progressively more negative calibration value as you increase or decrease fuel level for the gauge to still indicate as you desire...
Hope this makes enough sense to get you in the neighborhood of working as desired.

My princeton sender looks like the one on the left of this picture and only has 3 wires not counting the one that is connected to the probe. Doesn't sound quite like what you describe, so I'm not sure what your fourth wire is unless it connects to the probe.



The red and black supply power to the microcontroller and circuitry inside that capacitively measures the fuel level. The yellow wire emulates a resistor to the RDAC or whatever gauge you have that is compatible with a resistive sender.

The calibration process is completed on the EFIS. Here are the instructions provided on Sonex's website:
https://www.sonexaircraft.com/documents ... 102708.pdf

Thanks folks!

The great team at Princeton sent me updated instructions. The other wires are resistive and capacitive. Now I know what to use.


Robbie Culver
Sonex 1517
Chicagoland
N1517S

I forgot to add the text that mattered:

There are two output wires. The green wire is for all MGL equipment and is a resistive type. The green wire will not work with resistive analog gauges. The yellow wire is for GRT, Dynon, Garmin, and 0-5V analog guages. Only use one of the output wires



Robbie Culver
Sonex 1517
Chicagoland
N1517S

A quick update -

I have not solved the problem yet, but Todd from Princeton called me last evening well after business hours. He took time to review the problem with me, explain what I should see for voltage output, and what I should try next.

I am going to try re-calibrating the fuel probe this evening and see if I can get it to show +5VDC out when the tank is full.

Todd's call was service above and beyond what I expected - fantastic customer service!

Make absolutely certain the fuel you are using is not ethanol laced. Ethanol absorbs water and you will never get consistent readings with your Princeton probe. Here in Michigan, ALL fuel at the pump for automobiles is 10% ethanol by law, and there is no longer a requirement to post this on the pump as it was a few years ago. You can purchase a cheap ethanol tester at the usual aviation supply stores pretty cheap. $6.00 from ACS. http://www.aircraftspruce.com/pages/ps/ ... Tester.php

Thanks - using 100LL

I was able to successfully calibrate my Princeton fuel probe this evening and the Belite gauge read full. This time the calibration remained after I cycled power.

I should be able to do a test flight tomorrow and verify it is working in flight.


Robbie Culver
Sonex 1517
Chicagoland
N1517S