Need help troubleshooting a sleeping ECU

The round connector with yellow wires won’t have power until the alternator (engine) is running, that is the AC voltage from the alternator to the rectifier/regulator.   I would start with the following tests, it sounds like you have done some of them, but work through them in this order.

With ALL switches off:  Verify there is NO continuity between the two (A, B) regulator ground plates.

With ALL switches off:  Verify there IS continuity between the regulator B ground plate and the airframe ground.
 
With ONLY the Backup Battery switch on:  Verify there IS continuity between the two regulator ground plates.

With ONLY the Start Power switch on:  Verify there IS continuity between the two regulator ground plates.

With ONLY the Master Switch on: Verify there is 12volts on the X3 connector Pin 3 (horizontal pin) in reference to the regulator B ground plate (airframe ground).

With the Master Switch AND the Backup Battery switch on:  Verify there is 12 volts on the X3 connector Pin 1, in reference to both of the regulator ground plates. 

With the Master Switch AND the Start Power switch on:  Verify there is 12 volts on the X3 connector Pin 2, in reference to both of the regulator ground plates.  

Without a schematic of the Fuse Box it's hard to know precisely where to look, but I would also check continuity of fuses in the Fuse Box.  Given that the A side appears to be the trouble, I would suspect the 20A fuse (F20) next to the large "A+" stud, and the small green board-mounted fuse (F24) near the "A" and "A-" studs.  Perhaps also the 7.5A fuse (F3; in the path for "Selector Switch A") at the left-most end of the long row of fuses.  All fuses should be a dead short with your meter in continuity mode, or nearly zero ohms in resistance mode.

This information is in the Heavy Maintenance Manual (64Mb PDF) chapter 76-10-00.  The Fuse Box section begins on page 29 (page 345 of the PDF).  Manual revision pages are here (5Mb PDF).

All 7 continuity and voltage checks done, I'm seeing the results that Jeff said to expect.

All fuses Eric mentioned are good. I also pulled and visually inspected every other fuse in that row, even though the LEDs already indicated that they're good.

Still no lights on the A side of the fuse box, and no ECU power draw. Is the A side supposed to be lit before engine start, or does that side only come alive after GenA kicks in?

More ideas?

The LEDs on the "A" Generator do NOT illuminate before START.
The ECU Power is sourced at the FUSE Box  X1(A) and X2(B) connectors and delivered to the ECU through the A1, A2 and B connectors.

DO NOT go reseating the A1, A2, and B connectors without a VERY GOOD reason.
They have a 20-reconnect Life Limit.

Bill Hertzel wrote:

The LEDs on the "A" Generator do NOT illuminate before START.
The ECU Power is sourced at the FUSE Box  X1(A) and X2(B) connectors and delivered to the ECU through the A1, A2 and B connectors.

DO NOT go reseating the A1, A2, and B connectors without a VERY GOOD reason.
They have a 20-reconnect Life Limit.

Ok, that's really great to know. I can stop worrying about the A side of the fuse box.

And yeah, my ECU connectors were installed on Oct 6 and haven't come off.

So if my A side is fine, and my fuse LEDs are lit, which indicate that my ECU is powered up, why am I only seeing 0.35A total system draw with Start and/or Backup power on? Should I not see ~1.5A?

This all started with a lack of CAN data on the Dynon, which I and Dynon deduced was a sleepy ECU. But Bill is indicating that the ECU and fuse box are fine, my voltages and continuity appear fine. But there's still the lack of amp drain.

This is getting frustrating. I just want to see oil pressure on the screen so I can get on with my first engine start.