I’m working on a 912iS aircraft that is not charging the battery.  I have the problem diagnosed to the alternator B stator, but I’m hoping to get a couple of you to shoot holes in (or confirm) the diagnosis before I order a stator assembly. Originally I missed diagnosed this as the Voltage Regulator B, so that has been replaced and did not solve the problem.

These are the test results:

1. Stator B ohm readings are between .5 and .7 ohms measuring between any two of the three yellow leads with the connector unplugged from the regulator.  This is similar to Stator A (and misleading).

2. Stator B open circuit output voltage is 3-6 volts between any two of the stator poles (yellow wires).  Stator A open circuit voltage is 35 volts.

3. Stator B leads all show grounded with readings between 1 and 2 Ohms from any yellow lead to engine case.  Stator A leads show infinite ohms to ground.

My diagnosis is that Stator B has gone to ground, even though the windings are not open.  So, voltage is being shunted to ground inside the stator.  Is there any reason to take the alternator apart before ordering the Stator? Is there a possible ground failure that might be repaired? I would not think so.  





  • Re: Alternator B Failure Diagnosis

    by » 2 years ago

    Jeff Noted,

    #1  Stator B ope
    n 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.

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.

  • Re: Alternator B Failure Diagnosis

    by » 2 years ago

    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.   

  • Re: Alternator B Failure Diagnosis

    by » 5 months ago

    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? 

  • Re: Alternator B Failure Diagnosis

    by » 5 months ago

    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.

    Thank you said by: Balaji Thiagarajan

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