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G3X Touch Voltage Question

Ed_Wischmeyer

Well Known Member
My RV-9A had the avionics installed by a "professional" shop before I bought it, and I've recently discovered that with the engine off, the G3X Touch shows the voltage to be a bit more than one volt lower than what I read across the battery with a VOM. Voltage on a cigarette lighter plug is about the same as the G3X Touch voltage, meaning there's probably a drop somewhere.

Engine running, I see 14+ volts, pretty normal.

Wiring schematics? Ha! None...

Any theories on what I should look for, next time the cowling is off?
 
Obviously next to impossible to be sure without a schematic, but it sounds like the GEA (or whatever is set to measure bus volts) is wired to an essential bus which is diode connected to the main bus.
Got a shot of your panel and switches?
 
Should be pretty easy for a “professional” shop to figure it out :D
The GEA24 can look at ‘bus’ voltage a couple different ways.
 
I'm kinda suspecting sensor calibration. Real voltage drop under almost no current flow would be very unusual. Does the drop change as you load the bus?
 
You can be sensing after a diode built into the aircraft AND be sensing with a diode protected EFIS.

My non-OP type EFIS has 2 power inputs.

The E bus is not browned out during engine start.

Would they not use diodes to protect each input and be powered by the higher voltage source?

Unless corrected up by the diode drop value, your voltmeter reads low.
 
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Voltage drop

To me this seems normal.
When measuring the Voltage on the battery AND the avionics is drawing AMP´s
there will be a Voltage drop in the cables.
Turn the landing light and the pitot heat on and you will see a higher Voltage
drop than 1 Volt.
If you pull all the circuit breakers, assuming no AMP draw, the Voltage in the cigarette lighter will be almost the same as @ the battery.
The master relay will produce a small Voltage drop.

Good Luck
 
Sorry. My point was, delta V without a decent draw would be weird. I was assuming only the GDU was on. Loosing a whole volt under that load only (plus contactor load/loss) would be a lot. A volt under 15/20 amps would be expected. See if the drop varies by load. Start with the bare minimum (pull breakers if you have to) and increase it with the known biggest draws. Let us know.
 
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