CAN fault after cold start

When you start the 912iS, initially alternator B provides power to the ECU and other engine systems.  After a few moments, and reaching about 2500 RPM, alternator A is verified and then takes over engine functions and alternator B is then connected to the airframe.  It could be that alternator B is not initially providing enough power for the engine systems (which includes the fuel pumps) and so the ECU goes into fault on both lanes.  

When you hold the start power button longer you are providing battery power to the engine systems so this does not happen.  I have seen similar things happen when alternator B is failing, but if your airframe charging is good and holding about 13.8 volts during normal operation, then maybe you are simply releasing the start power button before the engine has sufficient RPM for alternator B to provide for the engine electrical functions.  Also, make sure you only have one fuel pump switched on when starting.  The fuel pumps are the biggest engine power draw and running them both during starting can cause this.  

Thank you for your reply.

After some research on the internet I also think the problem might be LFP battery that I use - especially because the voltage after start is slightly lower than it is with lead battery and therefore it falls into fault. After 10-15 seconds the voltage is back to “expected value” as for lead battery and everything id ok. 

do you think it might be the case?

David,

I would not think it’s a battery problem from what you are experiencing.  You don’t seem to have any problems when the battery is electrically connected to the engine systems (by holding the start button). It’s when you disconnect the battery early that you are having a problem. I believe that for one reason or another voltage is too low after you release the start power button. During that time your engine is likely still running on alternator B, but after 10-15 seconds it has probably switched to Alternator A (and the problem is avoided). Testing alternator B is easy, and I would want to rule that out.  

Also, it would be normal to need to hold start power for a couple seconds after the engine fires so alternator B can stabilize, but 10-15 seems too long. What I mean is that you may be required to hold it a second or two after you release the starter switch until the engine is stable. My start power is controlled by a maintained type toggle switch and it’s usually on for 5-10 seconds after starting the engine. 

You could also try to deliberately extend the situation as a test. Start the engine and keep the RPM low, about 2000 RPM, so the alternators don’t switch and instead the engine continues operating on Alternator B. If this extends the time you must hold the start power button to avoid this fault, that would point to a problem with the alternator B system.  

Ok, will try.

forgot to mention it's completely new engine - so would be surprised if the alternator is bad but it can happen.

i will also try a lead battery to confirm or discard my theory :)