I’m looking for help identifying the function of the three relays in the fuse box.  I’m trying to track down why my fuel pumps have started to continue running after engine shutdown and am suspecting welded contacts.

I found this thread on the forum - good explanation, Bill, but nowhere in the Rotax manuals can I find a description for the operating sequence for application and disconnection of start power to the A and B regulator buses.  And nowhere can I even find these relays identified or mentioned, including in the IPC.  They appear to be plug-in relays, so should be easily replaced.


I have an RV-12iS, and Van’s provides the attached simplified schematic in which I am assuming that two of these relays are the K1 and K2 relays shown, but I’m at a loss regarding the third relay.

Can anyone help me with this identification?

9959_1_20211003_183041122_iOS.jpeg (You do not have access to download this file.)
9959_1_IMG_1071.jpeg (You do not have access to download this file.)
  • Re: Fuse Box Relay ID

    by » 11 months ago

    Hi Bob

    Before you start to try and "repair" a 5000 USD part, the fuse box, i will ask for a warranty if it was me.  Fill in a customer service information report, CSIR, and report exactly what is happening.  The parts, other than the fuses themselves, have no replacement part numbers and they do not want people working inside of the box.  That said if you believe that is the issue then put in a request with one of the independent Service Centres for Rotax.  

    you can find copy at:  https://www.rotax-owner.com/en/support-topmenu/csir-service-information-report

    Additionally at: https://doc.rotaxirmt.com/sir


  • Re: Fuse Box Relay ID

    by » 11 months ago


    I don’t have an internal wiring diagram for the Rotax 912iS fuse box, I don’t think one is available.  However from my experience I believe the the positive lead to each fuel pump comes directly off of a fused terminal in the Rotax Fuse Box and is not switched.  These positive leads have power anytime the engine is running or the start power or emergency power switches are turned on. In other words, anytime the engine electrical system has power, the fuel pumps also have power on the positive lead. The fuel pump switches that the pilot controls interrupt the negative lead between where they are connected to the regulator “A” ground plate, and the pumps. Your RV-12 diagram supports this, as it shows the switches on the negative side.  There are no relays directly involved in the operation of the fuel pump, I suspect the relays your are looking at control the engines automatic alternator selection and routing of power to the airframe power buss.  

    If wired correctly, even if the switched side (negative lead) of the pump was stuck closed the pumps should not be able to run with the engine off unless the start power switch or the emergency power switch is closed.  There simply should not be power at the Rotax fuse box if one or more of those three conditions are not met.  And since the positive lead to each pump comes from this fuse box there should be no power at the pumps once the engine stops unless you have left on your start power or emergency power switch.  Is your start power switch integrated with your lane switches with a rotary key system, or do you have the standard separate switches for the lanes, start power and start button?

  • Re: Fuse Box Relay ID

    by » 11 months ago

    Hey Bob,

    After my last response I took another look at the RV12 912iS electrical diagram you posted.  I believe the K1/K2 Relay (that you framed in red) is part of the Rotax fusebox. This Relay appears to control switching Generator B buss to power the airframe loads once the engine is started.  However, if this relay was stuck in the closed position then buss A wound not get “start power” so the engine would not start, and if stuck open the aircraft power buss would not get charged after the engine was started.

    I think it’s more likely that the solid state start switch (right below the K1/K2 relay in the drawing) is staying closed, either because it’s failed closed or because it’s being commanded closed by the start computer.  That relay would not be in the Rotax fusebox, and is a third party item outside of the Rotax scope.  I’m not familiar with those items, but the solid state start power switch is the only component in the diagram that can connect both the positive and negative sides of the airframe power to the Rotax fusebox buss A. And that would be required to keep the pumps running after the engine is stopped.  If it’s staying closed you would have continuity between the two ground busses (A and B) with the engine off.  That would be a simple test.  

  • Re: Fuse Box Relay ID

    by » 11 months ago

    Thanks for your feedback guys.  My thought was that if the contact I’ve circled below was welded in the other position it would allow battery voltage to get to the the A bus, through the B bus, thus powering the fuel pumps.  But yes, I later realized that the A regulator, not being connected to the airframe ground, would not allow current to flow back to the battery.

    Van’s thinks there’s an issue with the start computer in that it’s not allowing the start power solid-state switch to drop out.  I just replaced the start computer back in October when they revised their HIC module, and all has been well until recently - fortunately I kept the original.  Continuity between the regulator grounds would help confirm that possibility, plus Van’s added a diagnostic LED to the HIC module board to help diagnose issues such as this.

    I’ll report back once I can get out to the hangar.

    As a side note though, many of us choose to build our own planes because we want to understand all of the systems.  It’s perfectly understandable that Rotax wants to protect their IP, I get that, but it doesn’t seem they would jeopardize that by providing us with an explanation of what’s happening with power transfer between these elements during start, run and shutdown.  It could avoid making us risk damage when removing parts to send them for diagnosis only to find there’s nothing wrong with them.  Enough said.  Hopefully will be able to report something soon. 

    36821_2_IMG_1071.jpeg (You do not have access to download this file.)

  • Re: Fuse Box Relay ID

    by » 11 months ago

    Problem found.  Embarrassed to say, the enemy was me!

    First checked continuity between the two regulator grounds - dead short!  Opened the canopy and what did I find?  The emergency power switch was on!  See attached.  It ties both regulator grounds together and connects battery power to the A alternator/regulator bus in the event of a loss of the A alternator or regulator.  So, my failure diagnosis was right, it was a “stuck” contact, just not a failure.

    It first occurred last Saturday during one of the five Young Eagles flights I made.  It’s possible it was accidentally switched by one of the young eagles while entering or exiting, but as PIC, it’s my responsibility, so lesson learned.

    Sorry for the false alarm.

    36830_2_IMG_1071.jpeg (You do not have access to download this file.)

    Thank you said by: RotaxOwner Admin

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