MGL Avionics User Forum

I've got the XTreme EMS w/ RDAC XF. I'm using the EI FT-60 for my fuel flow. Ever since I've installed everything my fuel flow has been all over the place. In cruise I'll see as much as a 3gph fluctuation. I've tried moving the FT-60 around and installing it in various ways:

-Before both fuel pumps
-In between the electric and mechanical pump
-After the mechanical pump (current setup, with a bubble trap / loop)
-5.6K pullup resistor powered from the 5V feed on the RDAC
-5.6K pullup resistor powered from 12V ship's power (current setup)
-ground to the RDAC ground terminal
-ground directly to engine (current setup)

I've even sent the FT-60 back to EI for testing. It tested perfectly. I'm stumped. I absolutely cannot get a good, solid reading. Any thoughts / suggestions?

Hi
I also experienced this problem recently.
In my installation fuel flow is connected to an Injector. Fuel used to show correct until recently. It jumps betwen 8 and 60 l/h
We checked connection between Injector and RDAC but the problem persists.
Where else can we look?
Thank you
Kind regards
Michael

That's the only place to check other than the fuel flow setup - make sure it's still set to look at injectors rather than turbine senders.

Other than that it can only be an electrical issue - the RDAC measures the amount of time the injector is open vs the amount of time it is closed - that's pretty much directly proportional to fuel flow in a constant pressure fuel flow system.

Also check your fuel pressure - if that is all over the place the injectors may vary the opening times to compensate - that depends very much on how your ECU operates of course.

For my install, I'm carbureted so reading an injector isn't in the picture. It's just reading the flow between the mechanical pump and the carb. Any other ideas?

The FT60 normally does not give much issues.

The RDAC simply counts the number of pulses over a 4 second period - that's then translated into flow rate using your k-factor. Nothing more going on. So if it's all over the place - either there is an electrical problem with the senders output (that needs an oscilloscope to see) or the sender is simply not seeing an even flow in one direction. One of the nastiest causes is fuel pressure on the stroke of the fuel pump inflating fuel hoses downstream of the flow sender - on the off stroke of the pump the fuel simply flows back through the sender. That causes the mechanical part of the flow sender (usually something that rotates with flow) to become very erratic. Oscilloscope can show that too - your pulse spacing will vary a lot and rapidly.

Thanks for the feedback.

We can assume that the FT-60's output is normal since I just sent it back to EI and had them check it out. They assure me that it is working absolutely normally as far as they are concerned.

I'll re-check my wiring. Right now I've got the FT-60 wired to the main bus. I'm also splitting this wire to feed the 5.6K pull-up resistor on the output wire. Ground is running on its own directly to the engine.

As far as the fuel lines go, there's about a 6" hose from the mechanical pump (engine is Lycoming O-320-B2B), through the FT-60, through a T fitting that has the pressure sender on it (typical cruise pressure ~3psi), then a bubble loop and down a foot or so to the carb. The FT-60 is secured to the engine mount so it should be pretty solid. If it's fluctuations from the pump is there a way to mitigate the pressure pulses?

How was your FT-60 mounted?
Could it have been affected by vibration?
Where is it in relationship to your fuel pump, gascolator, carb?
Is it tilted down enough to let trapped air travel back toward the gascolator or tank?

I've had it "free floating", hard mounted to the firewall and currently it is secured solid. No place has worked any better than any other. Right now it has a 6 inch hose on the inlet side and about a foot on the exit, both flexible hose. As far as vibrations go it should be about as isolated as it can get.

I am afraid the best way to debug something like this is an oscilloscope - anything works even the cheap USB scope dongles - you are looking at low frequency signals and big signals. Without that you are blind - a scope is perhaps the best tool for any avionics installer to own. A volt meter only gets you that far...

Using the scope is easy enough... having the knowledge to interpret the results not so much.