Hello 915is owners/operators and Rotax maintenance technicians....
We have a problem with our flight school Kitfox with a 915is (710 hours). 
At this point, us iRMT's, with the use of a Type 3 dongle (Buds software) - have not determined a resolution.
I would appreciate any timely help you can offer. 

Stock flight systems EMU with start relay - not using Rotax “start power” switch. 
Normal start - below 2600 rpm - after approximately 15 second - both lane lights come in and engine quits. 
If I start at above 2600 rpm - engine continues to run. 

If we disconnect Stock EMS relay and use Rotax “start power” switch. Same problem occurs…let off the spring loaded start switch - both lane lights illuminate - engine quits. 

If we use “EMS back up” switch - engine starts normally, BUT
If I select the switch back to center off - battery is not charged (as noted by battery voltage slowly diminishing). 
If I leave EMS backup switch selected battery is charged.
This make no sense from our Rotax training. 

No faults noted on Stock EMU screen or Buds program? The Buds is indicating both generators and voltage regulators are outputting correctly. 
With engine running on Lane A (Generator OK indication on Stock EMU) - Lane B indicating standby.   If I switch Lane A off - Lane B immediately takes over - as it should. 
Bud’s data logs have been sent to (2) US service centers for review, but have yet to respond based on the weekend. 
With a spare new 915is in a box - we have replaced both the Stock EMU relay, the Stock EMU, AND the (3) fuse box relays = No change. 
  It also appears the fuse box and voltage regulator connections are different from my current 915is serial number, so a swap of the entire fuse box is not possible.
We have checked all electrical connections (plugs) throughout wiring harness, battery cables, and all the fusses in the fuse box. 
With our builders of Kitfox STi version with 915is arriving from round-the-world - filling the "Type" and Backcountry training schedule.
Cancelling or postponing training is a dissappointment to all. 
Any suggestions or similar experiences would be appreciated. 
  • Re: 915is generator start sequencing AND Battery chargeissues

    by » 9 months ago

    I believe alternator B is not producing power, because either alternator B or voltage regulator B has failed.  This is what’s I think is happening:

    You start the engine with battery power via the Start Power switch or the Emergency Power Switch.  Once running, the engine electrical power should be initially provided by alternator B, because alternator A does not come online until you reach 2500 (or maybe 2600) RPM for a few seconds.  If alternator B does not produce power, and Start Power is removed before Alternator A is online, the engine will stop just as you describe. 

    The failure of alternator B (theory) is further supported by the fact that when you do get the engine running by starting above 2600 RPM, then the battery does not charge if either the start power or emergency battery switch is not closed.  This makes sense because alternator B is normally charging the battery after alternator A connects to the engine (at which time alternator B is assigned to the airframe). With alternator B failed the battery will not charge. However, when you then close the emergency power switch you are connecting the airframe/battery to alternator A.  Alternator A is not intended to charge the battery because it’s dedicated to the engine loads and normally isolated, but it will do that short term as long as power requirements are low.  

    Your successful starts (above 2600) are possible because at that RPM alternator A is almost immediately assigned to the engine, likely before Start Power is removed.  Follow the prescribed procedure to test Stator B and Regulator B.  

  • Re: 915is generator start sequencing AND Battery chargeissues

    by » 9 months ago

    I agree with Jeff B. -- likely a failure in the Stator/Regulator B system.

    To troubleshoot: Disconnect the stator harness plug from the regulator and check voltage on the pins in the stator connector (pins 1-2, 2-3 and 1-3) with the engine running (BE CAREFUL!).  You should see an AC voltage on all three pairs of pins (measured voltage will vary with engine speed).  If you do, then the stator is likely OK and your problem lies in the regulator, or in the Fuse Box (blown B system fuse, bad relay, etc.).

    If your engine has had SI-915i-006 done (replacing the factory stator/regulator connectors with Amphenol ecomate RM connectors) then here is the assembly/disassembly instructions for that connector (in case you need to remove pins to access them for measurement):


    If your engine still has Deutsch DTM connectors on the stator/regulator harness, the instructions for that connector are here:


  • Re: 915is generator start sequencing AND Battery chargeissues

    by » 9 months ago

    Just one thing to add to Eric’s troubleshooting.  Be sure you check each of the three Stator B Wires to engine ground.  The reading should be infinite (no continuity to engine ground).  I’ve seen some stators that still make some voltage but have a grounded stator coil.   You can do this test first without the engine running (safer), and if you find a grounded stator wire the voltage test need not be done.  If grounded, the stator must be replaced.  

  • Re: 915is generator start sequencing AND Battery chargeissues

    by » 9 months ago

    Excellent point, Jeff.  Thanks.

You do not have permissions to reply to this topic.