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Red Cube Failure Modes

Veetail88

Well Known Member
Folks,

I have an AFS 4500 and the standard red cube fuel sender that has been acting up. The GPH readings have been irregular and on my last flight, it dropped to zero and stayed there for a while, then began reading correctly again. Is this a common failure mode of the cube or am I likely to have a connection problem somewhere. I went through all the wiring yesterday and couldn't find any obvious problem.
 
Red Cube Failure

I had a red cube failure, when I inspected the cube I noted the wiring at the top had been heat affected and failed. The Red Cube was installed as per the plans RV14, very near an exhaust pipe. Albeit the exhaust pipe had a heat shield affixed trying to protect the cube, it didn't stop the heat transfer to the wiring. Recommend you check the Cube's wiring.

Alan
 
I'm seeing the exact same behavior with mine, just started a few weeks ago. It'll read great for an hour or so then go to zero for a bit, then comes back again. It can happen when cold before starting, or hours later in flight. Not reliable anymore, so thinking it has more failed than not.

My reading on other posts seems to indicate this is how they die, slowly :)
 
Seems to be a common problem with some Red Cubes. I use 3 in my plane, 2 in the cockpit and one behind the baffling. The one behind the baffling I've changed twice and it still does the same thing. Stops and starts during a flight. I've tried everything I can think of, and suggestions here, but it always does the same thing. It's mounted level, wires up, no teflon tape, behind the heat, moved the spark plug wires away from the Red Cube and wiring. I recently added a fire sleeve around but haven't tested that. The only other thought is to try vibration isolation ( rubber mount it ).
 
Are there any alternatives to a red cube for a fuel flow sensor?

The alternative is a Floscan but they can be pricey, and some non-aviation sensors. Red Cubes appear to be a very well built product, odd that they have problems.
 
I have been experiencing random Red Cube fuel flow failures in my RV-6 over the last 150 hrs. Fuel Flow drops to zero and then recovers to normal reading for no obvious reason. Overall, my Red Cube is reliable and accurate. TIS is about 600 hrs. My Red Cube is mounted on the forward lower side of my motor above the air filter housing. Have not yet resorted to manufacturer's (EI)recommended troubleshooting procedures.
 
I?ve posted on this before.

I installed the red cube in the RV-10 in the Van?s recommended postion - aft of the firewall in the tunnel. The boost pump does effect the reading when on, but over several years the accuracy has always been withing a half a gallon and the cube has never failed.

I?ve seen recent postings on RV-14 mounting locations and found they all tend to violate the red cube installation guidelines. I?m thinking heat and vibration are life limiting elements for the cube.

Just now working how to mount this in the new RV-8 project. I?m tempted to put it aft of the firewall but figured I?d try a spot between the engine fuel pump and the throttle body. It is hard to meet all the guidelines:
- straight fittings
- no mounting on the engine
- up slope
- away from heat

Here is the spot I found that meets the rules. I test fitted the 4 pipe exhaust and I have very good clearance so exhaust heat should not be a problem - but I may still put it inside some heat barrier material to shield against radiant heat.

Carl
193_D03_E7-_C081-4337-8_EB8-227_E5_C83_E5_E6.jpg
 
Folks,

... on my last flight, it dropped to zero and stayed there for a while, then began reading correctly again. Is this a common failure mode of the cube or am I likely to have a connection problem somewhere. I went through all the wiring yesterday and couldn't find any obvious problem.

I just had a hard failure of a red cube after less than 100 hours (my first one lasted over 900 hours). Location: suspended in fuel line between fuel servo and spider, per EI FP-5 instructions, page 20. (EI makes the red cube).

In my case, the fuel flow failure was not intermittent. I had a hard failure to isolate. Per Rob Hickman, long ago the AF-4500 plans were updated to so the red cube sees 10v (pin 29) vs 5v in the original design. I updated that, no improvement.

I verified the red cube red wire had 10v, and the red cube ground had good continuity to ground. Then, with EFIS (and ship power) turned on, I disconnected the red cube white wire from the AF-4500 harness and went tap-tap-tap to ground with the wire that leads from the red cube (white wire) to the AF-4500. This simulates the fuel flow pulse train (pull down) that the red cube should produce. If this produces fuel flow indication on the AF-4500, then you know the wiring from that tap-tap-tap point to the AF-4500 is fine.

In my case, the tap-tap-tap produced an indication of fuel flow, so we knew the red cube was bad. Replacement red cube solved the problem. Advanced Flight Systems replaced the <100 hr red cube under warranty with EI.
Source of red cube failure is unknown.
 
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