Alternator B Failure Diagnosis

Jeff Noted,

#1  Stator B open circuit output voltage is 3-6 volts 
#2  Stator B leads all show grounded ... to the engine case.
#3  Is there a possible ground failure that might be repaired?


#1.  That's Not Good!
#2.  There's Your Problem.
#3.  By all Means!  It is always a good idea to know what the problem really is before blindly throwing money at it.
       It may just be a pinched wire that needs some minor rerouting and a piece of heat-shrink applied.

Then again you may find the whole thing Fried Extra Crispy!
At least you will know what you have before you go spending the big bucks for no good reason.

Thanks Bill.  I’ll tear into it and see what I find.   A while back there was a mandatory SB to replace the Stators for a series of 912iS engines, but this one was not in that series.   There was also an information bulletin that announced the availability of a new ignition housing with an oil spray nozzle to better cool the Stator. This engine was built before that option was part of serial production so I guess upgrading is something to consider. But, it’s quite expensive. Considering both these bulletins, it might be that Rotax has had some failures of the B Stator.   This engine has 640 hours.   

I am facing a similar issue, replaced the Regulator B but that did not solve the problem. I ran into this post and wondering how you resolved this? 

I ended up replacing Stator B and that solved the problem. However, you should test your stator before replacing it.  The procedure is in the heavy maintenance manual, but you can start with unplugging the stator from the regulator (3 yellow wires in one connector) and testing each lead to engine ground. If one lead is grounded, you need go no further because it is shorted out. If none are grounded, then proceed with the rest of the test as outlined. 

At the same time we upgraded to the new stator housing and sprag clutch housing that integrates a new oil spray nozzle to keep the Stator cool.  See SI-912i-028

Also, we installed the new larger connector at the junction of the stator B leads and Regulator B leads. The new upgraded stator came with the new connector on leads, so the obvious thing to do was upgrade the connector on regulator B to match. I highly recommend this upgrade because the new connector is much more robust and actually acts as a heat sink for the regulator wires. The regulator is available with this new connector already installed, but it sounds like you just purchased one with the old connector. See SI-912i-024 for information on the connector upgrade.  Also, some here have posted part numbers to buy the Amphenol connector directly from Mouser electronics, which saves considerable money over buying it from Rotax and its the same exact connector.

The B alternator coils were severely blackened from overheating and had shorted to ground. Observed that the B regulator had overheated (it has a temperature witness strip applied to it). I have replacements to hand of those two parts, but am holding off reinstalling until I get advice from my ROTAX agent whether I should apply the housing upgrade. The motor has done 771 hours since installed in 2014.