Avionics expert needed.

johnM · Post by **johnM** » Fri Jul 24, 2009 6:43 pm

Hi

I have a problem with an MGL EFIS installation and after about 40 hours of fiddling I am completely baffled.
I am in need of hiring someone in the South East who can troubleshoot when things don't work.

I am not an electronics expert and have spent so much time on this that I am probably missing the obvious.

I know that there are those who repair airframes and engines but are there any chaps who do the same with non-certified avionics?

mike hurn · Post by **mike hurn** » Fri Jul 24, 2009 6:52 pm

whats the problem ? what efis voyager/oddesy ? and what plane are you fitting it to ?

I have a voyager fitted to CTSW 912s which i fitted

Rgs Mike

johnM · Post by **johnM** » Fri Jul 24, 2009 7:11 pm

The installation is an Ultra XL on a Sportcruiser.

I have fluctuating readings, particularly the oil pressure. I have tried swapping sender, the RDAC, using a separate 12v supply, adding additional earths etc etc etc. The system worked fine for the first 25 hours. I am now finding that on long flights the water figs are fluctuating and also the compass. I am not an avionics expert and having spent 40-50 hours swapping items, measuring resistances etc i feel that i am too close to the problem with too little knowledge and I need to hire someone.

I am now so frustrated that I want to bin the whole thing but re-doing the entire panel with steam instruments will put me out of the air for ages and cost a fortune in money, weight and time.

the UK distributor has been very helpful but is not a technical expert . I have also asked an avionics company if they will troubleshoot but as the kit is not certified they say that they have no experienc

Simon Clifton · Post by **Simon Clifton** » Fri Jul 24, 2009 8:51 pm

John

Simon C here, fellow SC builder (Dynon though). We have one MGL instrument, and I rate the UK distie highly for service. I happen to have a degree in electronics, but not much direct experience in avionics. I'm also a bit far away being in the Midlands.

Maybe I can help with some basic questions, even if you have already covered them.

You said it was working fine for the first 25 hours. Are you 100% sure - if so then the next question is 'what has changed'?

Assuming that the engine itself is okay (it probably is), then it is either the sensor, the RDAC, or the Ultra XL. You say you have swapped the sensors and RDAC - what exactly did you swap, and what difference if any did it make?

If you can rule out the sensor, RDAC and Ultra, then you are left with wiring being the problem. I would have to say that in my (limited) aviation experience, but also in wider electronics experience, then this looks like a classic noise or earth-loop problem. The trouble is, how come it is only happening after 25-hours (hence question 1).

What style of connectors are you using, if any? And what spec wire have you used?

Lets take one problem at a time - oil pressure sensor say. If we can work out how to correct that, the solution might be the same for the other problems. Or, it might be you have a series of coincidental problems, in which case doing them one-by-one is still a good idea.

First, check to see what oil pressure sensor you have. In mod 2008, Rotax supplied sensors changed from one-wire to two-wire type, which type do you have? The new type is Rotax P/N 956413.

The oil pressure sensor is just a variable resistor, with resistance varying with pressure. If you have en earth problem, then changes in resistance in the earth return can play havoc with the resistance overall, making the reading fluctuate. Measuring the resitance is not really going to help, the values are too small to read accurately, and just by measuring it you are changing it anyway.

You say you have done some trouble shooting with earth wiring - can you describe what have you done in more detail?

If the fluctuating reading happens on the ground, then the best next step is to temporarily fit a variable resistor in place of the sensor. WARNING, if an oil problem happens during this test, you might seriously damage your engine. Anyway - connect in a variable resistor to the RDAC and see if it settles the pressure setting down. If it does, then it is a sensor problem. If it doesn't it's a further wiring, RDAC or Ultra problem, and we can carry on from there.

See how you get on and let us know,

Cheers

Simon C
~~~~~~
G-MOOV Syndicate

johnM · Post by **johnM** » Fri Jul 24, 2009 11:42 pm

Hi Simon
Thanks for your reply. I have had the problems for around 8 months now and to be honest in that time I have become confused as to what I have have not done and in what order. However I have tried the following to correct the fluctuating oil pressure reading.

1. Three different oil pressure senders. Fluctuations.
2. Connecting sender to Rotax gauge. No fluctuation
3. Connecting sender to MGL TP1 gauge. Fluctuations
4. Using separate 12v source to power RDAC. Fluctuations
5. Changing RDAC. Fluctuations
6. Changing earth connection. Fluctuations.
7. Connecting oil pressure wire to oil temp input on RDAC. Oil temp
reading(which is really oil press.) then fluctuates.
8. Connecting multimeter to sender to measure resistance with engine running.
No fluctuations
9. Looking at probe page on EFIS. Fluctuations in input reading.
10. Substituting 70 ohm resistor for sender. Reduced fluctuations but still fluctuates around 0.4 bar

I am now touring France and on long flights I am finding the water temp reading and compass reading fluctuating, but only after about an hour's flying.
I am not an electronics guru and am at a loss as to where to turn. The only part left to change is the display but I am told that the problem cannot be there as it does not process the signal.
Any ideas?

Simon Clifton · Post by **Simon Clifton** » Sat Jul 25, 2009 8:13 am

John

I agree that the cause of the fluctuations must stop at the RDAC. On the other hand, you do seem to have done all the sensible things I would do and you have not fixed it yet, so it might be worth swapping the Ultra just to confirm that (assuming it won't cost anything).

Your symptoms are defintately characteristic of an earthing or power supply problem.

Picking up from point 10 on your list where you have fitted a 70 Ohm resistor and still seen fluctuations, that is the best test to work with. That effectively isolates any engine installation or sender issues. How long was the wire from the RDAC to the resistor? Did it use any of the original sender wiring? If you can get to a point where the RDAC has just the resistor and very short good wiring to the resistor, then you can at least be certain where to look next.

Here is the ultimate test:

Remove the RDAC from the plane, disconnect all sensors. Fit the 70 Ohm resistor on some good short temporary wiring. Run the RDAC and Ultra from a temporary battery. If you still have fluctuations then I'd ask for my money back!

Assuming you don't, then start putting it back together, starting with using the plane power supply but keeping everything else the same.

Assuming you maintain a steady reading, try starting the engine. Still steady? Re-attach the sender, then its wiring harness.

My hunch is that the fluctuations will come back when you start using plane power or plane sensors, and you have an earth loop problem.

If I am right, then the next thing to do is inspect your wiring (and I know you origially asked for someone to take a look anyway) - what general principles and specific techniques have you used in this area?

Cheers

Simon C
~~~~~~
G-MOOV

tnowak · Post by **tnowak** » Mon Jul 27, 2009 1:11 pm

John,

I am not familiar with your unit at all, but it sounds like you could have a problem with the "processor" which is affected by either temperature or vibration. The fact you can use the unit for an hour or so without problems would tend to indicate a very intermittant problem.

If you had earthing problems it is most likely you would see problems almost as soon as you start using the unit. It is possible you may have a supply problem (noise or spikes), but as previously suggested, that can be eliminated by using a temporary separate battery supply.

Just out of interest, does the compass feature use any external sensors?

A dry joint on the PCB or a temperature sensitive component could cause the problems you describe.

Have you tried a"tap" test? Gently tap the unit (small hammer only!) to see if you can make the symptoms appear.
Regards Tony Nowak

johnM · Post by **johnM** » Mon Jul 27, 2009 4:04 pm

Hi

Thanks for the suggestions. Firstly I should say that Paul Sistern at Parts for Aircraft, the UK distributor, has been brilliantly helpful. Unfortunately he is not a technician and seems to get little backup from the factory.

Yesterday the problems got worse. I landed after a one hour flight. Restarted after 10 mins and while taxiing I lost all readings from the sensors wired to the RDAC and got the message "RDAC Fail". Five minutes later they all came back.

What is curious is that the compass is wired straight to the Ultra and not the RDAC and yet it seems to wander around on occasions.
When I tried using a resistor in place of the oil pressure sensor I put one end into the input and the other was attached to the firewall with a crimped connector i.e. no wires were involved.

The wire I am using is 18g tefzel( is that the name?) wire from Aircraft Spruce.

The oil pressure reading fluctuation usually commences around two minutes after start-up. The water temp reading is now fluctuating violently. It will go 60 - 25- 48 etc at 1 HZ intervals but is steady with the engine off. Would an earth loop problem cause such violent movements?

I have not tried the tap test. I have two sledgehammers. Would you suggest the large or the small and how small should the pieces be at the finish? !!!

bertdeleporte · Post by **bertdeleporte** » Mon Jul 27, 2009 7:02 pm

Hello,

maybe my question is an idiot one, but do you "filter" your power supply (parallel fitted ceramic condenser on power supply terminals of RDAC and display units)? I got some fluctuations once on a industrial computer reading LM335Z gauges (temp sensitive transistors), they dissapeared with fitting a filter (100 µF if I remember well, I can find out if you want, "experts" advice welcome).

I cannot tell you more, I solved the trouble, I did not inqured further, the system is working fine for five years now. The guy who invited me to filter the power supply told me this maybe due to the mode of passing from AC to DC of the transformer/rectifier, which is done now by "slicing" the different phases of the alternator, instead of working with diodes, which induces a lot of electric noise. The high frequency (several kHz) noise can produce unexpected "collateral effects" due to the RLC behaviour of analogic components.

As Tony and Simon pointed out, a weak weld joint or a sensitive component in the circuit can cause troubleshooting with dynamic coupling. By adding on condenser on the circuit you may avoid the coupling of the gauge and another part of the circuit, it worth make the test, it will cost you abt £0.01. The live plugging of a meter makes the problem vanish, which could go towards the dynamic explanation, on the other hand, the alternate supply did not give dramatic results...

Bertrand

tnowak · Post by **tnowak** » Mon Jul 27, 2009 8:23 pm

John,

Does the unit appear to work okay without the engine running? If so it could well be a problem with the supply voltage. As Bertrand mentions, a simple filter may cure the problem.
May be worth contacting the manufacturer to see what criteria are necessary for that unit to display "RDAC fail". It obviously isn't happy with something but can detect what doesn't meet specification.
Tony N

Simon Clifton · Post by **Simon Clifton** » Wed Jul 29, 2009 8:58 am

John

Do the out-of-plane test, and tell us what you see (ref my earlier post of Saturday).

At that point we'll know which hammer you should use.

Simon C
~~~~~~~

Pete · Post by **Pete** » Thu Jul 30, 2009 3:37 pm

As there is no mention of rev counter fluctation, I think it's reasonable to assume that the UltraXL is working fine.

The common thing in all your problems are that they are analog senders, these things all generate very low voltages. I have no experience with the Ultra, but I would expect that all these senders would have a 2 wire attachment, and both wires are isolated from ground ie the RDAC will be measuring the voltage between the two wires - rather than measuring the voltage with respect to ground.

So rather than looking for a poor connections, you could be looking for a connection between ground and the sensor wires. If such a path exists then you will be getting current flows through some funny earth paths, this current flow could then be influenced by electrical instruments, transponder, lights radio etc.

Good luck, faults like that can be a pig to locate

johnM · Post by **johnM** » Thu Jul 30, 2009 9:20 pm

hi

The oil pressure sender has only one wire. The earth is through the body. I am planning this weekend to remove all the other senders and just leave the oil pressure sender connected to see if other senders could be causing interference.
The water temp sender is also fluctuating( but not the identical CHT sender) but is steady with the engine off. Which suggests what?

cheers
john

Simon Clifton · Post by **Simon Clifton** » Sat Aug 01, 2009 9:44 pm

John

My guess is there is a earth loop between the RDAC and your one-wire sensors.

I don't think it is likely you are getting interference from your other sensors.

The test of powering the RDAC out of the plane and with special wiring from a battery will isolate the earth-loop path. At the least, this test will prove you do or don't have an earth loop problem.

Hope your testing goes okay,

Simon C

johnM · Post by **johnM** » Sun Aug 02, 2009 7:40 pm

hi

I have now tried removing all the other sender wires to eliminate the possibility of interference. No change.
I have also tried running the RDAC earth direct to the battery( the manual says connect it to the engne block) and that makes no difference either.

I am losing the will to live over this one!!