Lane B Engine ECU Nominal Current not reached.

I think this is a problem with double ignition coil SP-4 or the wiring connections to it.  This double coil is controlled by Lane B and fires the top spark plugs of cylinders 3 & 4.  I’m thinking that the ECU is not seeing the proper current when the coil is firing.  

Check the wiring connections at the primary side of coil SP-4 and make sure they are secure and fit tight.  Inspect the spark plug wires all the way from the coil to the top plugs at cylinders 3 & 4, and make sure they are screwed in tight at both ends.  Remove and inspect 3T and 4T spark plugs. Test coil pack SP-4. A problem with any one of these items would also explain the rough running when using Lane B only.  

You can also check the 10 amp fuse at position F6 in the Rotax Fusebox which powers SP-4, but this fuse will likey be OK or the engine would run much worse (or not at all) when selecting lane B only. If this fuse was blown, you would have no spark at cylinders 3 or 4 when selecting Lane B only.   Lastly, no, there are no clear definitions of the BUDS fault codes published.  

Thanks Jeff.  That gives me somewhere to start. I'm an old TV tech from the 70's and an electrical engineer.  If I have the schematics and a scope, I can usually find the problem with anything electronic.  I hope this is going to be one of those learning experiences where I'm going to have greater insight into the system. I will have some time this week to familiarize myself and do some testing.

Rob,

I forgot to mention that Rotax Service instruction SI-912 I-029 introduces a new style ignition coil and wiring harness that uses a new type of connector on the primary side of the coil.  If an owner wishes to implement this it would require 4 new double ignition coils and a new wiring harness.  It’s not mandatory and I suspect most 912iS owners won’t upgrade, but it hints that they may being seeing problems with the original style primary side coil connections.  More specifically, the old style power connection uses two individual Faston style connectors, whereas the new connector is a single sealed Tyco connector.  This is one of the reasons I suggested checking these connectors. 

Thanks Jeff.  That looks a little like a smoking gun.  I'm looking forward to diving into this on Saturday and see if I can see what is going on.  Looks like I failed to mention that I have more than 400 hours on the engine at this point for documentation, although I think it is in the buds dump.

That SI looks like a fairly significant amount of work and parts.  I'll see if I can coax it into working, then maybe order parts for a future repair.  

I looked through all of the schematics and the only common element was the B plug on the ECU.  I unplugged it and put it back on again and fired up the engine and all was good.  I’ll have to cut a couple of zip ties to pull it off and get a good look at it, but I think I’m on the right track.  I’m a little confused because the system I have seems to be what is the service instruction.  The 2021 schematics in the heavy maintenance manual seem to be different that want is in my engine.  
so I’ll finish the 100hr and annual and see if it will fly reliably