912iS Dual Flashing Lane Light at Startup

Mark,

I’m not 100% sure, but with both lanes flashing and Generator B in failsafe, I would first suspect that alternator A or regulator A may not be producing the required voltage to run the engine electrical loads.  When the engine first starts (and start power is turned off) generator B powers engine electrical loads. Then after a few moments the ECU verifies voltage on Alternator A and then switches over so the engine is powered by A, at which time alternator B is connected to the airframe.  It sounds like alternator A may be failing the voltage test, so the engine stays with alternator B (failsafe). A couple questions. 

Is this happening with the start power switch still on, or after you turn it off? (Assuming you have a manual start power switch).  

Do you have voltmeters on both the airframe electrical buss and the engine electrical buss?  If so what voltage are you seeing on each.  Also, what is the amp meter showing.  

Jeff, et al-

I started simply and went about troubleshooting the fuse box first. I physically verified the condition of each fuse (all were good), then tested resistance on the normally closed pins of each of the K1/K2/K3 relays (85/86). All 3 relays tested ~105 ohms, so it wasn't the meter coil.

BUDS showed Generator Manager in Failsafe fault (see attached pic) so I was inclined to think it was a bad relay regardless of the resistance tests. 

I purchased 3 new relays and the issue was resolved after replacing the K2 relay. No lane lights illuminated at startup, no fault registered on the BUDS and after exciting the gens at ~2500 RPM the A gen activated and bus voltage registered in range (see attached pic).

Upon starting the aircraft today, the issue resurfaced. I don't know what is causing it, and I don't know why it is intermittent.

Thanks in advance to folks on this forum.

Mark,

The next thing I would do is check the output of Generator A and Regulator A.  Start with disconnecting the grey plug between Regulator A and Generator (Stator) A. Then, with the engine off, test each of the three leads that go to the stator to engine ground.  If any of the leads are grounded, the stator is shorted out.  If that test passes, test the stator leads to each other in all three possible pairs.  Each pair should have a similar reading, something like 1 ohm, but consistency is more important than the acutal number. If you find a pair that reads open, then the stator has failed.  If the stator passes these tests, plug it back in to the regulator, start the engine and test the output voltage of the regulator.  You can test it at the fuse box on terminals A and A+ with the wires still connected to the studs. You are looking for voltage around 13.8. 

Generator B and Regulator B are obviously working, you can see on the BUDS screen that Alternator B is active and providing voltage to the Lanes.  So, either Generator A is not producing the required voltage consistently, or the fuse box is not seeing that voltage.  This could be a problem in the Fusebox, but I think that is less likely than a regulator or stator failure. If your Alternator A and Regulator A test out OK, the next steps are harder and you may have to send your fusebox to a Service Center for testing. It's possible that you could have a cold solder connection in the fusebox that is intermittent. 

Thanks, Jeff.

Regulator A failed. I replaced it and the issue is (happily) resolved. 😁

Thanks for reporting back Mark. Failure of the A regulator is fairly uncommon.  Generator A is pretty well optimized for the engine electrical requirements, so that regulator does not build up much heat. If you have not already done so, I would recommend still doing the resistance tests on the stator as I outlined earlier, just to make sure the problem was isolated to the regulator.  It only takes a few minutes and is good insurance. Happy flying!