FX-3 MAIN volts indication issue

[Cactus Charlie]

My FX-3 shows Volts-1 about 0.6 with IBBS on, Master off, and all breakers in. I have looked at the logs from several other FX-3 and they all show this anomaly but with some variation in the voltage.

This has bugged me for years but I recently took lots of measurements which I think indicate a voltage offset in the ground references provided to the GEA 24. I'm still looking through my measurement data and I have some ideas of what to test next.

One test case on my FX-3 was interesting and I'd like to know if other FX-3 (or EX-3) show the same anomaly -

IBBS On, Master Off - record and report Volts-1 (main bus) and BAT Amps
Pull GEA circuit breaker - record and report Volts-1 (main bus) and BAT Amps
Push GEA circuit breaker - record and report Volts-1 (main bus) and BAT Amps

 

[Neal]

There's been talk of parasitic drain when a FX-3 is in the hangar and why most have to leave it on a charger unless using an EarthX battery. I wonder if this is related and what may be that drain?

 

[Cactus Charlie]

The main battery on an FX-3 has three parasitic loads. My measurements show the currents taken by those loads are:

40 μA - GPS keep alive
115 μA - left ignition module
115 μA - right ignition module

I do not believe these parasitic loads are related to the anomaly in indicated Volts-1.

 

[ve6yeq]

Can you pull different breakers and try to narrow down where it is coming from? I have not searched the EX-3/FX-3 electrical wiring diagrams, but I would guess there is a pull-up resistor or similar somewhere connected to the wrong bus...

 

[Cactus Charlie]

Can you pull different breakers and try to narrow down where it is coming from?

I have data for many different CB pulls but before I share that I want to know what others see. I'm particularly interested in indicated BAT AMPS with GEA CB pulled as that result was quite surprising.

I currently suspect a problem with the ground path of the IBBS but I'll have to insert a breakout connector to get more data.

I have already established that there is a difference in potential between the braid that grounds the transponder chassis and the bracket that mounts the Aux 12 VDC outlet.

Don't forget that GEA, GDU, and GSU breakers only feed power from main bus and should not be passing any current with main bus dead and those LRU powered by IBBS only. IBBS power does not go through any breakers.

 

[ve6yeq]

If I look at the G3X Touch data log from the first leg of the day on Monday, I see 0.4-0.6 Vdc on the main bus when the main bus switch is off, and the G3X Touch is running from IBBS.

My preflight procedure is to first boot the G3X Touch off the IBBS, do inside checks while waiting for the boot and for ADAHRS to complete the self-test, then power on the Master so I can preflight pitot heat (see current rise), strobes, nav lights, wig/wag, stall horn, and trim. I then turn off the main bus and leave the G3X Touch running off IBBS while I complete the outside walk-around so that the GPS and SXM weather have time to "lock" and refresh their data (If I had the GNC355A on the IBBS, it could also GPS lock, refresh, and I could load flight plans before engine start).

In the logs, I can see it at about 0.5 Vdc before I ever turn master on, and after I turn master off, it drops back to 0.6 Vdc, eventually decreasing to 0.4 Vdc just before I turn master on before engine start.