Eriselle
Member
A few months ago, I noticed that the backlights on my Garmin GMC 307 would occasionally start blinking / flickering. It wasn't immediately clear whether this might be an error code being communicated or whether it was erratic. Eventually, I managed to capture a short video of it on my phone as it was happening, and I'll share that here:
https://vimeo.com/899272758/4a707aa982?share=copy
There was some debate amongst my airplane/airport friends whether this was an error code or not. The relevant Garmin manuals did not make any mention of this behavior, so I concluded something somewhere was just messed up. The autopilot functionality also did not have any issues whatsoever, so I doubted it was any kind of error code.
My first thought was the dimmer. However, it seemed to be operating correctly. The other lights that are dimmer controlled would dim and brighten appropriately, and no other lights flickered like the autopilot lights.
I found in the GMC 307 installation manual a mention that the photosensor that controls the indicator arrow brightness on the autopilot can also control the backlights that were having the issue, provided the lighting bus is providing below a threshold power level. The dimmer, even on the lowest value, still provides over the threshold, so I concluded that to test this I would need to pull the lighting bus pin from the high density DSUB connector from the rear of the unit. When I did this, the backlight brightness was indeed now controlled by the photosensor just like the indicator arrows on the unit.
At this point, there are three possibilities. First, the GMC 307 could have a defect and I would need to send it off to Garmin for service. Second, the wire itself could have a fault of some kind. Third, the lighting bus inside the Vans controller box could have some kind of internal fault.
One of the difficulties with diagnosing this problem is that it would only occur intermittently, and only when the engine is running. This informed me that it is probably caused by vibrations, and is most likely a physical problem. I tried to get a repro of the problem by wiggling, poking, tapping, and shaking things, to no avail. But, then I had an idea that maybe I actually could get a repro but maybe without the big engine vibrations, it was just too subtle to detect the lights blinking without the engine running. I own an oscilloscope, and I decided that maybe I'd be able to detect a blip in the continuity of power if I used it, better than trying to see a light flicker.
With the lighting bus pin already pulled from the DSUB connector for the GMC 307, I attached the oscilloscope probe to the lighting bus pin. I attached the probe ground to the aircraft itself. I then set a trigger on the oscilloscope to capture any significant drop in power and started fussing with the dimmer knob. Nothing. I jiggled the GMC 307 wiring harness cable.. BLIP. There it is. I found something wrong.
Under further inspection, it turns out that the lighting bus wire has a hole in the insulation because it is abrading on the rear side of the GTR 200 radio unit. The hard edge of it has completely worn a hole in the insulation of that one wire. The accompanying wires are well on the way to developing their own insulation problems. The wire is simply grounding out on the sharp-ish metal edge and that's what is causing the light to blink.
Attached are a couple pictures for your viewing pleasure.
This is a professionally built SLSA RV-12 legacy. It might be worth a look at your own wiring harness to make sure you aren't developing the same problem.
https://vimeo.com/899272758/4a707aa982?share=copy
There was some debate amongst my airplane/airport friends whether this was an error code or not. The relevant Garmin manuals did not make any mention of this behavior, so I concluded something somewhere was just messed up. The autopilot functionality also did not have any issues whatsoever, so I doubted it was any kind of error code.
My first thought was the dimmer. However, it seemed to be operating correctly. The other lights that are dimmer controlled would dim and brighten appropriately, and no other lights flickered like the autopilot lights.
I found in the GMC 307 installation manual a mention that the photosensor that controls the indicator arrow brightness on the autopilot can also control the backlights that were having the issue, provided the lighting bus is providing below a threshold power level. The dimmer, even on the lowest value, still provides over the threshold, so I concluded that to test this I would need to pull the lighting bus pin from the high density DSUB connector from the rear of the unit. When I did this, the backlight brightness was indeed now controlled by the photosensor just like the indicator arrows on the unit.
At this point, there are three possibilities. First, the GMC 307 could have a defect and I would need to send it off to Garmin for service. Second, the wire itself could have a fault of some kind. Third, the lighting bus inside the Vans controller box could have some kind of internal fault.
One of the difficulties with diagnosing this problem is that it would only occur intermittently, and only when the engine is running. This informed me that it is probably caused by vibrations, and is most likely a physical problem. I tried to get a repro of the problem by wiggling, poking, tapping, and shaking things, to no avail. But, then I had an idea that maybe I actually could get a repro but maybe without the big engine vibrations, it was just too subtle to detect the lights blinking without the engine running. I own an oscilloscope, and I decided that maybe I'd be able to detect a blip in the continuity of power if I used it, better than trying to see a light flicker.
With the lighting bus pin already pulled from the DSUB connector for the GMC 307, I attached the oscilloscope probe to the lighting bus pin. I attached the probe ground to the aircraft itself. I then set a trigger on the oscilloscope to capture any significant drop in power and started fussing with the dimmer knob. Nothing. I jiggled the GMC 307 wiring harness cable.. BLIP. There it is. I found something wrong.
Under further inspection, it turns out that the lighting bus wire has a hole in the insulation because it is abrading on the rear side of the GTR 200 radio unit. The hard edge of it has completely worn a hole in the insulation of that one wire. The accompanying wires are well on the way to developing their own insulation problems. The wire is simply grounding out on the sharp-ish metal edge and that's what is causing the light to blink.
Attached are a couple pictures for your viewing pleasure.
This is a professionally built SLSA RV-12 legacy. It might be worth a look at your own wiring harness to make sure you aren't developing the same problem.
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