FADEC interface

Did you interface via the Garmin GEA 24 as shown in the diagram in the Garmin G3X installation manual?  “Figure 32-2.2 GEA 24 - Rotax 912iS/915iS FADEC Wiring Example” in the _ap version of manual.

Yes, I interface thought GEA24, and use that schematic. The only doubt I have is about where to attach the Garmin Shield Block Ground (see picture), as the Rotax connector is plastic. Should I ground it to the airframe?

First, an easy mistake is to forget that the ECUs must be powered on in order to receive FADEC information at the G3X, but I’ll assume you know that and have verified power at the ECUs and the GEA-24.  So the next thing to look for is a CAN bus problem.

You are dealing with two different CAN bus’s for the GEA-24.  The first is the Rotax bus, which connects the ECU to the GEA-24 as per the diagram you posted.  The second is the Garmin CAN which connects the Garmin G3X to its remote units.  In the photo you posted, the GEA-24 shows as connected over the Garmin CAN, so that would point me towards the Rotax CAN as the problem. 

The CAN wiring must have a 120 ohm terminator (resistor) at each end, but no others in between. Two 120 ohm resistors in parallel gives you 60 ohms between the CAN LO and CAN HI wires, and that’s the first thing you should test. The Rotax connections have the 120 ohm terminations built in, so if you wired it as per the diagram you posted, then you should be able to unplug connector P244 at the GEA-24 and test between pins 33 & 17, which should be 60 ohms nominally. 

Regarding the ground wire at the plastic GEA-24 connector shell, this can be left off, or tailed out and connected to the metal case of the GEA-24 at one of its mounting screws.  The important thing is that the ground wire is attached at least at one location. You are just draining off any static from the cable shield.  I personally ground at each connection.  

Thanks Jeff for you reply. I already checked the CAN bus resistance and it’s OK.
X3-1 2 and 3 works as per manual, but I just realize that there’s no power to HIC A and B at pin 1. Re-checked all wiring and fuses, but still no power. Any thoughts?

With pin 1 dead on both HIC connectors you won’t get any information over the Rotax CAN. With BOTH lane switches dead, and power verified at the X3 pins, it makes me think it could be a ground issue. Have you verified the ground bus’s at regulator A and B are normally isolated, and then connected together when the start power or backup power switch is closed?  What ground reference are you using when testing HIC A/B terminal 1?  The attached full wiring diagram was posted a while back, it’s for a 912iS but still may be helpful as the systems are similar.