I had a similar problem with the Grand Rapids EIS 6000 engine monitor and TruTrak autopilot. I found someone with much, much more electronics knowledge than
me to put an oscilloscope on various wires. We found that during transmission an alternating current (very low) was ‘riding’ on the DC power line into the EIS. We found other issues with the fuel probes, EGT’s and the autopilot, but I can’t remember exactly
what they were.
We solved (to the annoyance level) the problems by installing a filtered connector on the EIS and the autopilot:
Catalog:
http://www.digikey.com/Web%20Export/Supplier%20Content/AmphenolCommercial_664/PDF/Amp_CP_FILTERD-CONN_CAT.pdf?redirected=1
We used a FCC17-B25AD-250, only because that’s what he had in his bin. It worked fairly well. I’m sure if we would have done more work we could have eliminated
the problem, but it was very time consuming. The only time I have a problem now is transmitting on the higher frequencies, >135.00 on one radio. It only affects the EGT’s by 5-10 degrees inflight. Of course, it is much worse just sitting in the hangar.
I am sure this problem has something to do with routing. I used RG-400 everywhere, but inevitably wires get next to each other. I did not shield everything.
I only shielded in accordance with the manuals. Neither TruTrak or Grand Rapids recommended shielding. At the same time, none of my RV friends have these kinds of problems, so an all fiberglass airplane may make it much more difficult to eliminate all the
RF problems.
Steve
From: Lancair Mailing List [mailto:lml@lancaironline.net]
On Behalf Of Shane
Sent: Friday, April 25, 2014 10:53 AM
To: lml@lancaironline.net
Subject: [LML] G3X Fuel probe interference
We have installed a G3X dual screen system in a Lancair ES and have a strange occurrence happening.
The system uses the standard EI capacitance probes and fuel level indication is very accurate. What is happening is when we key the mike to transmit, after a few seconds the fuel
indication drops to zero. After we stop transmitting the level returns to normal. We have used shielded wiring, antenna coax is routed down the opposite side of the fuselage from the probe wiring, all the LRU’s in the G3X system are centrally grounded along
with everything else in the system. We have tried everything we can think of as far as trying to isolate and shield all the different components, but nothing works. The rest of the system works perfectly.