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I am in the process of building a new panel for my Rans S-12S. I was careful to label each wire as I took it off, but found later that some of the labels didn't stick and just fell off. Got the latest copy of the wiring diagram from Rotax, also another simpler one online, and the original plan from back around 1998 when the engine was built. I have a few questions though.
1) Each of those diagrams is different in one way or another. (I am not very experienced in reading these so this may be a problem with my perceptions.)
2) Am I right in thinking that there are a number of different wiring systems that all interconnect when it comes to power and grounding? (ie.: Ignition system, avionics, panel lighting, engine instruments, to name the ones that come to mind)?
3) I've found some differences between my present systems and those in the diagrams. I don't have shielded P leads, but the diagrams show them shielded. I plan on replacing these.
4) I have two alternators and would like to use the external larger one as the Primary instead of the smaller built in one which feeds the battery and systems through the rectifier-regulator. I read an article that suggested adding an on-off-on switch to replace the current circuit breaker switches for each alternator. If I do this I assume I would have to put fuses or circuit breakers into each alternators line. But can I make this change and if so how should this ignition system be wired?
5) I have multiple circuit breakers and fuses (more then the wiring diagrams show). Can some of thse be pulled out. One diagram shows only two 30 amp slow blow fuses in the whole system. The newest shows a 50 A and a 5 A in the external alternator system and a 25 A in the internal system. Currently I have a 60 amp circuit breaker and a 7.5 breaker switch in the internal system, and a 30 amp in the expternal system. The wiring diagram shows a 50 and a 5 in the external system and a 25 in the internal system.

Can you see why I'm having problems with putting things back together. I can either try to put things back together as they were to begin with, or do it "right" according to one of the diagrams. But which one, and especially which fuses/circuit breakers, how many, and where do they go? Also, do I treat the ignition system as a seperate system? If so, where does it connect up behind the panel.

Hope you've held with me through all of this. If so, thanks. I look forward to any suggestions anyone might have to share.

Bill Nelson
  • Re: Rotax 912 ULS Electrical Systems

    by » 13 years ago


    Hi Bill,
    You should have an avionics tech make you a schematic; it would be worth the extra money. The avionics tech will make sure you have the correct circuit protection/wire size for your equipment. Don’t go to a car guy, he will not likely understand the aircraft requirements. For reference, Aeroelectric.com has some nice wiring schematics including Rotax 912 with overvolt protection.
    Do replace the P leads (ignition/”shorting wires”) with shielded wire.
    Also use shielded wire on the tach wire as it is such a tiny AC signal it needs the protection. A twisted pair works nicely or use plain shielded wire and use the shield as the grounding wire.
    The ignition should be considered separate and run directly to the switches. I recommend good quality toggle switches rather than the troublesome rotary switches.
    Running two alternators is just adding needless complexity and pilot workload. Just use the higher output alternator. If you are determined to use both then the “understanding the 914” article (Rotax-Owner/Support/Technical articles) talks about some of the challenges when trying to run two alternators and offers some solutions. Be aware it is a lot of work/complexity for very little gain.
    Rob

  • Re: Rotax 912 ULS Electrical Systems

    by » 13 years ago


    Thanks for the response and insights. I'm not planning on running both alternators at the same time though, just have the internal one as the backup instead of the primary. For that I need a switch (hopefully I can use an on-off-on switch) to switch between the two. I just need to know how big a fuse to use to protect each alternator, and whether I need more then one fuse to either, as the Rotax insallation book shows (a second fuse in the wiring to the external alt). If I can figure out how to wire this circuit then I should be able to set most of the rest up as it had been before.

    (I am assuming here that the ignition circuit includes the alternators and associated wiring and fuses, the P leads, rotary start switch, battery and master switch - which also relate to other circuits, and the magnetos.)

    Any other ideas?
    Bill

  • Re: Rotax 912 ULS Electrical Systems

    by » 13 years ago


    Hi Bill,
    The fuse/circuit breaker needs to protect the wire so it is hard to generalise without seeing your schematic. Have a look at AC43.13 for wire charts/circuit protection.
    The ignition system is only the P leads and switch(s). You don’t need to wire anything else as it is already connected on the engine.
    Rob

  • Re: Rotax 912 ULS Electrical Systems

    by » 13 years ago


    I have a question along the same thread. The issue I am up against is the regulator on a factory Rans Plane S6 SLSA had the two Battery Leads from the main battery are connected to the R and the B+ connections on the Regulator. The terminal that has "C" on it has power coming from the IGN Switch only when it is on.
    Rotax shows all the wires going to the master switch from the regulator and all being part of the same feed.
    I do not understand why they have the all the connections on the Regulator when it could just be wired inside. It might be because the Regulator is used for something else as well.
    I told by a AP Mechanic that If the "C" comes off or loses power the unit will go into full charge and over drive all the connected components.

    So is there some other function that this wire does that I am not seeing?

    I just want it so that it is right and the motor will go the 2000 hours with out so much as a hick up. I and 3 other plans that have the same set up that I need to go check and they are waiting on me to figure it out.
    Thanks to All who reply

    Rotax CA Replied to this question Off the list. I am posting this here so others can see what the correct wiring should be with respect to the attached photo below. Here is the answer from Kevin Kane

    Lee,

    "I've been studying the schematic and as long as the master switch is off (#16) there is nowhere for the battery current to go, so there is no way that the regulator (#s 5 & 6) could have a parasitic draw on the battery. Why the wire would have a switch in there is a mystery to me, especially without seeing the way it was setup in the aircraft your A&P saw. That being said I do not recommend you add a switch to line C from the regulator-rectifier between the plug and the 25A breaker. I especially say that after reading page 111, at the bottom-"NEVER SEVER CONNECTION BETWEEN TERMINAL C AND +B OF REGULATOR (E.G. BY REMOVAL OF A FUSE) WHILE THE ENGINE IS RUNNING. OVER VOLTAGE AND REGULATOR DAMAGE CAN OCCUR."

    I hope that helps and thanks for the challenging question,
    Kevin Kane

    Everyone Enjoy
    2011-12-04_11-35-04_703.jpg (You do not have access to download this file.)

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