Erroneous Fuel Flow Indication - Zenith Aircraft Builders and Flyers2024-03-28T08:57:22Zhttps://zenith.aero/forum/topics/erroneous-fuel-flow-indication?commentId=2606393%3AComment%3A539048&feed=yes&xn_auth=noGreat! Thanks for the closur…tag:zenith.aero,2019-11-04:2606393:Comment:6988332019-11-04T18:06:13.070ZLance W Millerhttps://zenith.aero/profile/LanceWMiller
<p>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.</p>
<p>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.</p> Yes, the fuel flow sensor has…tag:zenith.aero,2019-11-04:2606393:Comment:6987412019-11-04T17:47:15.658ZJohn Austinhttps://zenith.aero/profile/JohnLAustin
<p>Yes, the fuel flow sensor has been rock-solid reliable and accurate ever since! :>)</p>
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<p>John</p>
<p>Yes, the fuel flow sensor has been rock-solid reliable and accurate ever since! :>)</p>
<p></p>
<p>John</p> So, it's been a few years....…tag:zenith.aero,2019-11-04:2606393:Comment:6988302019-11-04T16:39:32.600ZLance W Millerhttps://zenith.aero/profile/LanceWMiller
<p>So, it's been a few years....so can we assume that the change to +12V did the trick?</p>
<p>So, it's been a few years....so can we assume that the change to +12V did the trick?</p> Well, it was cold and rainy y…tag:zenith.aero,2016-12-14:2606393:Comment:5398422016-12-14T19:27:45.706ZJohn Austinhttps://zenith.aero/profile/JohnLAustin
<p>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.</p>
<p>Hopefully this will fix the intermittent problem!</p>
<p>John</p>
<p>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.</p>
<p>Hopefully this will fix the intermittent problem!</p>
<p>John</p> Just came back from checking…tag:zenith.aero,2016-12-12:2606393:Comment:5395082016-12-12T20:53:09.791ZJohn Austinhttps://zenith.aero/profile/JohnLAustin
<p>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. <em>Then,</em> 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…</p>
<p>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. <em>Then,</em> 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!</p>
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<p>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.)</p>
<p></p>
<p>Thanks,</p>
<p>John</p>
John
I would wire the fuel…tag:zenith.aero,2016-12-12:2606393:Comment:5394992016-12-12T19:35:04.296ZTimothy D. McCallhttps://zenith.aero/profile/TimothyDMcCall
<p> </p>
<p>John</p>
<p>I would wire the fuel flow sensor to 12v (both power supply and 5.6k pull-up resistor) as a next step. </p>
<p>If that doesn't help start removing variables by disconnecting devices that could be the source of noise. </p>
<p> </p>
<p>Yes, when things go wrong a look at the last thing you did (added) would be near the top of the list of suspects. </p>
<p>You'll see grounding mention a lot. Be sure to ground sensors at the common engine ground and be sure they are…</p>
<p> </p>
<p>John</p>
<p>I would wire the fuel flow sensor to 12v (both power supply and 5.6k pull-up resistor) as a next step. </p>
<p>If that doesn't help start removing variables by disconnecting devices that could be the source of noise. </p>
<p> </p>
<p>Yes, when things go wrong a look at the last thing you did (added) would be near the top of the list of suspects. </p>
<p>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.</p>
<p> </p>
<p>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. </p>
<p> </p>
<p>Tim</p>
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<p> </p>
<p> </p> Tim,
Thanks for the specifi…tag:zenith.aero,2016-12-12:2606393:Comment:5395732016-12-12T14:03:26.230ZJohn Austinhttps://zenith.aero/profile/JohnLAustin
<p>Tim, </p>
<p></p>
<p>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…</p>
<p>Tim, </p>
<p></p>
<p>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! ;>)</p>
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<p>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 <em>worst</em>!</p>
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<p>Thanks again,</p>
<p>John</p>
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Ok, I'm here in case you ca…tag:zenith.aero,2016-12-12:2606393:Comment:5394192016-12-12T13:34:46.049ZTimothy D. McCallhttps://zenith.aero/profile/TimothyDMcCall
<p> </p>
<p>Ok, I'm here in case you can't reach Bob Nucholls. :)</p>
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<p> </p>
<p> </p>
<p>Ok, I'm here in case you can't reach Bob Nucholls. :)</p>
<p> </p>
<p> </p> John
If you haven't checked t…tag:zenith.aero,2016-12-11:2606393:Comment:5393812016-12-11T14:09:30.810ZTimothy D. McCallhttps://zenith.aero/profile/TimothyDMcCall
<p>John</p>
<p>If you haven't checked these:</p>
<p>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.</p>
<p> </p>
<p>Check voltage regulator ground for good connection. Most of these are grounded through the cast in mounting tabs.</p>
<p>You might try turning off your alternator/generator for a bit to see if the fuel…</p>
<p>John</p>
<p>If you haven't checked these:</p>
<p>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.</p>
<p> </p>
<p>Check voltage regulator ground for good connection. Most of these are grounded through the cast in mounting tabs.</p>
<p>You might try turning off your alternator/generator for a bit to see if the fuel flow rate seems normal.</p>
<p> </p>
<p>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.</p>
<p> </p>
<p>Tim </p>
<p> </p> Followup:
The next time I fl…tag:zenith.aero,2016-12-10:2606393:Comment:5390442016-12-10T19:02:30.705ZJohn Austinhttps://zenith.aero/profile/JohnLAustin
<p>Followup:</p>
<p></p>
<p>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.</p>
<p></p>
<p>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…</p>
<p>Followup:</p>
<p></p>
<p>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.</p>
<p></p>
<p>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?</p>
<p></p>
<p>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.</p>
<p>John</p>