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Red Cube Problem

B

Bob'sRV6A

Well Known Member
I just noticed that with Skyview on, engine off, my Red Cube Fuel Flow Transducer reads 0.3-0.4 GPH on my EMS. Over time, the fuel remaining does in fact go down, too. The engine is off, there is no fuel flowing. Anyone have any idea as to what might be wrong? It had been working flawlessly for the last 120 hours.
Thanks,
Bob
 
Bob,
The red cube is a switch to ground, and pulses 60,000 times per gallon. SkyView counts pulses. A fixed high or low voltage won't cause any flow. So, for SkyView to read that, it's almost guaranteed to be spinning. Pretty hard to fake it.

Does it do this when you power up hours/days later? If not, my guess is that you do have fuel flowing somewhere, past a seal or fitting that is leaking. Do you have fuel pressure and is it dropping? Without the pumps on it will eventually stop.

If it's a carb'd engine, this could be the float leaking. If it's FI, the fuel servo. Do you have a purge valve? Primers? Those leaking could cause it also. Also depends where in the fuel system the flow sender is mounted.

Remember, the flow sender only cares about flow past itself. Any pressure differential from one end to the other will show flow even if fuel isn't "flowing". It's moving. With a hot engine, 0.2-0.3 GPH is probably low enough that it just vaporizes wherever it ends up and you never notice it.

--Ian Jordan
Dynon Avionics
 
Thanks Ian. I have an O360, carbed. Cube is between gascolator and engine fuel pump, forward of firewall. Aircraft had sat for a couple of days without startup, when I noticed the fuel flow while in the hangar practicing on my Skyview. No sign of fuel anywhere or fuel smell.
If fuel were flowing at .3 gph, It seems that my tank would be empty after two days. It is still full from last fillup.
 
I'd try disconnecting the Red Cube white wire and see if it stops. If so, then it's either bad, sending some pulses or it's really flowing something. I would think you'd notice .3 GPH missing from the fuel tank. If the EFIS still shows a fuel flow reading after removing the white wire it's the EFIS. Another possibility would be a shorting wire simulating a pulse.
 
Last edited:
Bob,
Was it after you pushed the mixture in or opened the fuel valve? Both could mean it wasn't leaking or flowing before, but was after that action. Alternately, does the fuel valve make it stop?

FasGlas' suggestion is a good one though. There is a chance that your flow wire has shorted to some other wire in the plane that has a digital signal on it. 0.3 GPH is basically 5Hz. If you have a backup battery on your SkyView, one thing to try is booting just the SkyView (button #1) and seeing if you have the flow even with everything else in the plane off.

It will be interesting to figure this one out!
 
Thanks FasGlas and Dynon. I'll try both your suggestions and report back tomorrow. The flow I saw was indicated before I touched anything. I only energized the main bus with an external power supply while the plane was in the hangar after not flying for two days. Nothing such as mixture, fuel valve, or throttle was touched.
When I came back from flying, I shut down the engine normally but before turning off master observed the same .3 gph flow.
Bob
 
I turned on the Skyview with backup battery and had zero fuel flow. When I then turned on master switch to BATT (and then on to ALT) I got 0.2 gph. I checked the wiring at the firewall and noticed that the wires coming out of the red cube were zip tied to the fat wire which runs from the master to the starter relay. I removed the tie, and moved the red cube wires away permanently. No change in the 0.2 gph.
I didn't cut the white wire, but instead closed up the cowl and went flying. When I came back and shut down, I had 0.0 flow. Go figure. I'll keep an eye on it and report back here if anything changes.
I had assumed some kind of induced current, and still think that may be the case, but rerouting the white wire would be a PITA.
Bob
 
Bob'sRV6A said:
Thanks Ian. I have an O360, carbed. Cube is between gascolator and engine fuel pump, forward of firewall. Aircraft had sat for a couple of days without startup, when I noticed the fuel flow while in the hangar practicing on my Skyview. No sign of fuel anywhere or fuel smell.
If fuel were flowing at .3 gph, It seems that my tank would be empty after two days. It is still full from last fillup.
Click to expand...

I have the same setup, o360, red cube between gascolator and eng. driven fuel pump and I have been getting gal/hr readings that are about 4 or 5 GPH too high, when I contacted EI tech support they suggested moving the unit downstream of the eng. driven pump... Reading your post makes me wonder if that will make any difference.
 
I also have this problem with the g3x, it turns out the my transponder antenna is too close to my red cube and causes interference. Try removing power from your transponder and see if you have the same issue. When I pull the transponder circuit breaker the FF goes to zero instantly, push it back in and it comes back. .2-.3 gph.
 
Antenna distance?

Jeremy,

What is the distance between your transponder antenna and the RedCube?

Thanks
 
My transponder blade antenna is below the gear attach weldment on the left side-which makes the red cube about 30"-36" away per the drawings. I'd need to go to the hangar to put a tape measure on it to be exact.
 
Thanks

I was considering placing my transponder antenna just aft of the firewall until I read these posts. I will start figuring out another location.
 
sahrens said:
I was considering placing my transponder antenna just aft of the firewall until I read these posts. I will start figuring out another location.
Click to expand...

I had mine next to the fuel vents. Transponder on one side ADSB on the other. No issues with the ADSB but had to move the transponder as stated above.
 
sahrens said:
I was considering placing my transponder antenna just aft of the firewall until I read these posts. I will start figuring out another location.
Click to expand...

Just an FYI, xpdr/ads-b antennas require about an 8" dia ground plane to work properly.
 
Transponder was the problem. I can change the 0.2 gph fuel flow reading to 0.0 by pulling the Transponder circuit breaker, and go back to the 0.2 gph by pushing the circuit breaker back in.
I imagine the white wire from the Red Cube routing into the EMS box might be too close to the Transponder coax--or Transponder?
 
Bob, as stated above, I fixed this same issue by moving my antenna; had nothing to do with the data wires. My antenna is now near the tail (my transponder is physically located behind the baggage bulkhead, so that made it easy).
 
Thanks Jeremy. But the cube is about three feet from the transponder antenna--do you think it can be influenced at this distance?
 
Bob'sRV6A said:
Thanks Jeremy. But the cube is about three feet from the transponder antenna--do you think it can be influenced at this distance?
Click to expand...

That is a good amount of distance....mine is now about 10' from the cube. My antenna cables and data cables were not near each other before or after the antenna move so in my case it was the emission from the actual antenna. Hope this info helps your trouble shooting. Can be frustrating!
 
If it's truly emission that's causing the problem, a grounded faraday cage around the red cube may help. For a ground test, try wrapping the red cube with aluminum foil and make sure the foil touches the firewall. If the problem goes away, then you can work on something a little more permanent for flight.
 
Kurt,
The red cube is aluminum. It by itself is a great shield for all the electronics inside. 5 mils of aluminum isn't going to help on top of the 200+ mils that is already there.

If RF is getting in, it's getting into the wires. Shielding these may help.
 
Thanks Dynon. Looks like I need to replace my Cube wiring with shielded wire. Until then, I can live with the slight fuel computer error. This must have been going on since first flight. My total remaining fuel amount has always read close to just about 1 gallon too high--good in my "ballpark" scheme of things.
 
Bob,
Not sure how much of an experimenter you want to be, but another option is to try a capacitor between the white wire and the black wire. Something in the .01uF range might just clean up the transponder pulses.

Re-locating, shielding, or putting a ferrite on the wires will probably all work well too. Sorry we don't have a more direct answer to how to fix it, this is a really unusual issue with the Dynon systems. I looked in our support system and couldn't find a single previous occurrence.
 
Dredging up an old thread...

I am finishing up an RV-9A project. The original builder installed the transponder antenna just inboard of the right fuel tank vent, and the red cube on a firewall-mounted bracket just above the antenna. Separation is less than 12 inches. With everything turned on, engine not running, the Skyview indicates 0.3 to 0.4 gph fuel flow rate. Pulling the transponder fuse drops it to zero. Clearly the same issue reported previously. Moving the transponder antenna would be a big project, moving the red cube only slightly less so. I have plenty of 3 conductor shielded wire so I plan to try that first. Question for the group: at which end of the cable should the shield connect to ground? Red cube or EMS?
 
Revisiting the old issue

Lars said:
Dredging up an old thread...

I am finishing up an RV-9A project. The original builder installed the transponder antenna just inboard of the right fuel tank vent, and the red cube on a firewall-mounted bracket just above the antenna. Separation is less than 12 inches. With everything turned on, engine not running, the Skyview indicates 0.3 to 0.4 gph fuel flow rate. Pulling the transponder fuse drops it to zero. Clearly the same issue reported previously. Moving the transponder antenna would be a big project, moving the red cube only slightly less so. I have plenty of 3 conductor shielded wire so I plan to try that first. Question for the group: at which end of the cable should the shield connect to ground? Red cube or EMS?
Click to expand...

Finally, the FF of 0.2-0.3 GPH error when parked, engine OFF, Comm's ON (i.e., Xpndr ON) ... has risen to the top of my list of TODO's.

The panel is ALL Dynon SkyView and AFP fuel injection FM-150 (filter, pump, servo) with Dynon "Red Cube" mounted between the engine mount (RV-6A). Elec power control is Vertical Power (VP-X Sport), integrated on the SkyView EFIS/ENG display). The Xpndr can be cycled ON/OFF with a softkey, while monitoring the FF. When OFF, 0.0 gph, ON, 0.3 gph, Andair fuel selector OFF, and there is no purge or return line. OFF is truly OFF. Clearly the Xpndr is a root cause of the FF reading in the RedCube, when zero fuel is flowing. The xpndr antenna (stub) is mounted adjacent to the right side fuel vent fitting, just behind the FW line.

After reading this and other threads the nexus appeared to be evident between the Xpndr and erroneous FF. The easiest fix is (will be) to relocate the Xpndr antenna, so I tested this first. Unscrewed the antenna, loosened the RG-400 coax and stretched it back to the wing root near the right inboard end of the flap. Mounted the antenna on a scrap of 0.025 alum and clamped it to the flap edge for grounding. Result 0.0 gph with everything ON. Then to be sure of the radius of the solution moved the antenna forward to the wing spar area (top side) and then to the LE of the wing root. Somewhere in the spare to LE the FF began to show erroneous flow again.

Conclusion - The xpndr antenna is causing erroneous Red Cube fuel flow, and was located too close to the Red Cube transducer. At least 36-40" (radius) away the antenna ceases to interfere, and beyond 48" it should be much less likely to be a factor. Now I have to extend the coax and route that thru the aircraft, drill a hole in the fuselage and hope it is perfect (ergo, doesn't cause RFI with my Comm antenna under the co-pilot seat pan).

The other option 'might' be to use shielded wire all the way to the Red Cube but that is a more difficult post-construction retrofit. Word of warning - do it that way during construction (add this to the gotcha-list.)

Last thought - the 0.1-0.3 gph fuel flow is not indicating actual fuel movement, and it would be easy to disregard this error but the EFIS doesn't 'know' it's not consuming fuel. Long 'buttonology/hangar-flying' sessions or other Master ON activities record this and result in consumption forecast errors. [Yes, the xpndr COULD be powered OFF to avoid this.] BUT - the real question is whether the 0.3 seen in the hangar is distorting the inflight consumption, too, and adding 0.3 gph to my entire cross-country trip? Why leave it that way and wonder?
 
Would be interesting if while in cruise all set up and bored to turn off the transponder and observe the FF to see if there is a change?
 
Future task ---

Larco said:
Would be interesting if while in cruise all set up and bored to turn off the transponder and observe the FF to see if there is a change?
Click to expand...

Yes, it will be. Seems that I'll get that chance soon. The 'fix' today was not the ultimate solution. In fact, the FF went from 0.3 gph to 0.5 after moving the xpndr antenna from the R front near the fuel vent to the L fuse under the pilot seat pan.

Another RV guy asked if the xpndr was tested in StdBy mode, power ON but NOT transmitting, so no RFI possible. No, I had not (yet) tried that, but did after I finished riveting the doubler and tieing up the wiring changes.

In fact, in StdBy mode the FF is 0.0 gph, as it should be. Confirming another test point but not explaining the underlying issues. The Dynon xpndr is usually in AUTO mode, which means it goes to 'GRND' mode until airdata is detected, and then goes to ALT. (Works nicely).

Ready to try turning OFF xpndr inflight to test that, but as the next 'fix' I will try the capacitor mentioned by Dynon above in this thread. (Wht and Blk wires) of the Red Cube ...

Frustrated but ready to get back in the air, this Condition Inspection is done.
 
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