This just happened to me and I'm not sure if it was just a coincidence or not. During the engine start checklist I got everything ready, engine was running and generator was on, but when I turned on the avionics switch which powers the radio and transponder they didn't boot up (but GPS did). I switched it off and on again, no luck. Checked the circuit breakers (radio and xpdr) and reset them, no luck. Tried doing it again without first turning on high load circuits (beacons, socket, etc), no luck. Then my CFI suggested to shut down everything, switch on only Master, and switch on avionics with battery power. It worked, and we started the engine up, switched on generator with the avionics already on and it was fine. Didn't have much time to troubleshoot (paying for a lesson).
Is there anything possibly that might cause the Rotax 912ULS to provide unstable power that would cause avionics like a transponder and radio to not turn on, but not so unstable that they turn off once already on? Or is this just a coincidence and the Rotax power supply had nothing to do with it? Voltages were always in green. Don't have an ammeter. Battery is lead acid so power should be stable.
Master on, engine start, generator on, avionics on = avionics not working
Master on, avionics on (they work), engine start, generator on = avionics still working
