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?"

John

N750A

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Well, it was cold and rainy yesterday, so I decided to go ahead and switch the pull-up resistor to +12V.  I flew today and the indicated fuel flow was perfectly normal. (Of course, it was normal the last two flights before switching the resistor, too!)  Anyway, as Tim advised, it's probably better to have a higher voltage pull-up, anyway.

Hopefully this will fix the intermittent problem!

John

So, it's been a few years....so can we assume that the change to +12V did the trick?

Yes, the fuel flow sensor has been rock-solid reliable and accurate ever since!  :>)

John

Great!  Thanks for the closure on that.  I'm about to start on my 650 wing kit and wanted to address this area of concern early rather than after I've already closed it up.   Been thinking on alternatives to the side-tank mounted fuel sending unit path.

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