My STOL 750 has an Electronics International FT-60 "Red Cube" fuel flow sensor.  The signal wire goes to my MGL RDAC XF interface board, then the RDAC is connected to my MGL XTreme EMS engine monitor. The red cube signal wire that goes to the fuel flow input on the RDAC has a 5.6K ohm pull-up resistor that is connected to +5VDC on the RDAC board.

This has worked great for about 2 years and 120 flight hours.  The red cube has been uncannily accurate - my fuel totals usually agree within a few tenth's of a gallon of my top-offs!  However, recently I noticed during a flight that my fuel flow was about double what it should be. All engine parameters were completely normal, so I was sure that the indicated fuel flow was in error.

On the return flight, initially, the fuel flow seemed normal. But, after about ? 15-20 minutes ?, the fuel flow went quickly back up to roughly double again and stayed that way for the remainder of the 1 hour flight.

The next flight, the fuel flow was indicating roughly double again for the entire flight.  I verified my actual fuel flow was normal by topping-off and the amount required was consistent with my normal fuel burn and pre-top-off fuel tank levels and not what was being indicated by the fuel flow. Also, there was no evidence of fuel leakage down-stream of the red cube, but even if there was a leak, it should have been reflected in the re-fuel top-off quantities, which were normal.

I talked to Electronics International support and the tech was at a loss to explain it. He said usually failure would be towards decreased fuel flow, not increased! He could only theorize that a trapped bubble in the fuel could possibly do this, but this seems unlikely since it has persisted over three flights and he said I had a good orientation of my sensor so as not to trap bubbles. The bottom line was send the sensor in for diagnostics if I found nothing else.

I was wondering if this could be that the 5.6K Ohm pull-up resistor (that I installed at the initial installation of the sensor) is failing/failed? Do resistors fail in a manner to allow increased current?  If so, wouldn't this pull the voltage on the signal up and be consistent with a falsely elevated signal?  

Just wondered if there were any electronics-knowledgeable members out there who could tell me if this is plausible or not.  If all else fails (pun intended!), I think I'm going to cut out the old resistor and wire-in a new pull-up resistor to see if it has any effect before pulling the red cube out of the fuel plumbing.  I suppose the RDAC could be at fault, too, so I have a support inquiry into MGL. However, I don't want to even think about pulling the RDAC and having to hook it back up again! Ugh!

Any insights or, has someone "been there, done that?"



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I would say its more likely for the resistor to increase in value if it changes at all, which would cause your current to decrease. If the resistor is soldered in and easy to get to, I would touch a hot iron to the solder joints to make sure the connection is still good. Changing the resistor would probably be the quickest and easiest thing to try first. Since your problem does seem intermittent though, I don't think the resistor is going to be your problem. Just guessing of course.


Today, I inspected the wiring, wiggled the attaching wires to the RDAC, attempted to tighten the RDAC terminal screws (they were tight!) and then took a 0.7 hr flight and the fuel flow worked perfectly the entire flight!  So, maybe I have a bad solder joint on the resistor (it's under heat shrink so I can't easily resolder it) or maybe the wire has an internal break and moving it restored connectivity, who knows?

Since it is not a critical to safety of flight item, I'm going to let it be for now and see if it recurs.  I've got replacement resistors on-order, anyway, so if it acts up again, I'm going to replace the resistor and wiring downstream of the resistor and see how it goes!




I checked the specs on your fuel flow transducer to see how it works.  Since it seems to be an "open collector" type device and the way that it measures fuel flow is by creating pulses relative to fuel flow. An open-collector circuit works like a switch, a very fast switch connecting the output line to ground which creates a signal like: 5 volt - ground - 5 volt - ground - 5 volt -  ground, which we see as pulses.   These pulse/time are calculated in your MGL panel and displayed as fuel flow rate. 


As well as limiting current which would destroy the "open collector (transistor)" the job of the resistor is to "pull" the output line back up to 5 volts after the open collector turns off and removes the ground.


So, if you are seeing an increase of fuel flow measured by your electronics and no actual increase in fuel flow there are pulses being created by something other that fuel flowing through an impeller, etc.  Usually this is caused by "noise" or ground problems (the device could also be faulty).  Noise in the circuit can cause the open collector circuit in the device to falsely trigger creating extra pulses.  Poor grounds can create an unstable circuit as well as cause the circuit to lose it's ability to defeat noise.  

Of course we've only mentioned the fuel flow sensor. Noise and grounds may be affecting only the MGL panel and there may be nothing wrong with the sensor.

 Food for thought:  This may be a case of having something else going bad and creating a lot of electrical noise.  Your fuel flow sensor may now be trying to tell you there is something else starting to go, (voltage regulator, alternator, battery, etc.)  







I had a basic idea of how the sensor worked, but your detailed explanation makes it very clear!  Thanks!

Matt at MGL suggested checking for interference from other devices, etc.  The RDAC's  +5V  terminal that the resistor is connected to also is used to power the fuel level sensors, so I guess there could be some interaction there.  

I had "tunnel vision" on the resistor when checking the wires - sounds like I need to take a second look at the ground for sure!

If this recurs and nothing else is found, I may switch the resistor over to the +12V on the avionic's regulated power supply (the sensor is OK for +12V) so that it would not be sharing power with the fuel level senders.

Like I said, it mysteriously worked OK again after "fiddling" with the wiring!  Hopefully, I'll get to fly again today and see how it goes. 




The next time I flew, the fuel flow was too high again <sigh>.  I re-inspected the wiring and actually disconnected and reconnected the sensor's wires where they attach to the RDAC.  I was also able to access the resistor and it checked-out within spec.

I flew again and it was still indicating way too high.  One thing I noticed is that the magnitude of error seems to get worse with increased engine rpm.  I recently had installed a Hall-Effect tach sensor that also inputs into the RDAC.  It is powered and grounded forward of the firewall, but the (unshielded) signal wire is connected to the RDAC. I'm wondering if that could be the source of extra pulses?

Fortunately, I installed the sensor with an in-line plug so it will be simple to disconnect it and see if that has any effect on the fuel flow indication.  I have a back-up rpm signal on the right mag, so I can temporarily use that for rpm.


John, run it past Bob Nuckolls on the Matronics electriconics form



If you haven't checked these:

Be sure to check the large electrolytic filter capacitor that is connected to the output of your voltage regulator.  It's job is to "clean and smooth" the DC output of your generator/charging system. Check battery connections.


Check voltage regulator ground for good connection.  Most of these are grounded through the cast in mounting tabs.

You might try turning off your alternator/generator for a bit to see if the fuel flow rate seems normal.


I would also connect both  power and pull-up resistor of your fuel flow sensor to 12v supply.  This would make any noise in the system have to overcome a much higher level of DC to have any effect.  Connecting the pull-up to 12volt will have a proportional increase of current in the device collector circuit (2ma) but this should be well within limits.





I retract my entry, I believe Zenith .Aero has it's own in house electronics professional. I will refer to Tim.



Ok, I'm here in case you can't reach Bob Nucholls.    :)




Thanks for the specific checks and suggestions!  Do you think the recently installed Hall Effect tach sensor could be the problem?  The signal wire from it connects to the RDAC.  There are 6 pulses per rev (3 flywheel magnets x 2 poles each), so at cruise rpm, the number of pulses being fed into the RDAC are about 16,000 to 18,000/min.  I wondered if these pulses could be somehow creating interference in the RDAC? (And those are +12V pulses vs the +5v fuel flow sender pulses!) I said "in the RDAC" because at least the wiring from the two sensors is widely separated and doesn't share a common bundle anywhere. I think my next diagnostic step is to simply disconnect the tach sensor and see if that restores the fuel flow to normal.  If no joy there (or if it does work!) then I think your idea of +12V to the fuel flow sensor rather than +5V sounds good if no other problems in grounds, etc., are found.  In fact, I had already made up a new lead with the 5.6K Ohm resistor to get +12V from my regulated power supply instead of the +5V of the RDAC's power supply - great minds think alike! Ha!  ;>)

Honestly, I think the fuel flow operated OK after the sensor was installed and as mentioned earlier, there have been times even since it has been erroneous that it appeared to work OK - so it was at least initially intermittent - intermittent faults are the worst!

Thanks again,




I would wire the fuel flow sensor to 12v (both power supply and 5.6k pull-up resistor) as a next step. 

If that doesn't help start removing variables by disconnecting devices that could be the source of noise. 


Yes, when things go wrong a look at the last thing you did (added) would be near the top of the list of suspects. 

You'll see grounding mention a lot.  Be sure to ground sensors at the common engine ground and be sure they are grounded at one end only.  A circuit with two ground points makes a fairly good antenna for both radiating and picking up noise, EMF, etc.


Tach pulse rate: That is a fairly high pulse rate but I suspect that the RDAC has enough filtering and isolation within to manage it. 






Just came back from checking things and a couple of test flights.  Since it was so easy, I disconnected the tach sensor and took a flight and the fuel flow worked perfectly!  I really felt smug that I had, at least, diagnosed the cause. Then, I reconnected the tach sensor and took another flight and the fuel flow worked perfectly again!  Of course, I've had one flight previously where it appeared to be working OK despite it not working correctly on the prior flight or the following flight! ha!

So, I'll probably leave it as-is and see what happens. If it acts up again, I'm definitely going to try the +12V on the pull-up resistor. (The fuel flow sensor's power wire is already +12V.)




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