I've been scratching my head a bit this weekend trying to finish up my wiring. I've noticed that if I compare the wiring schematic shown in the rotax manual to the one kitfox provides there is a discrepancy. I'll get into detail - but in short - kitfox has a single fuse on the 12v input to the switch protecting both start options (normal start power and backup start power). The Rotax manual has individual fuses to each on the 12v.

Let me talk out loud here to provide more detail/context. But first, I'm no expert, so maybe it doesn't matter?

Here's the Rotax manual bit (cut out the stuff that doesn't matter to this discussion). It's figure 3.14.

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Here, the first big difference is they are using two switches rather than one. Lets ignore that for now. If we break it down a bit, lets first look at the S2 circuit. Here we have F2 connected to 12v which then feeds into X3-3 and switch S2 when on (momentary) completes the connection of X3-2 to X3-3 (or X3-2 to 12v to be more exact). The way I see this is that when we are holding on switch S2 we are getting the 12v required on X3-2 for starting the engine, once we let go and the engine (eventually) switches alternators, X3-3 will be providing the charging and bus current back to the system, with the F2 breaker protecting it all. If F2 blows for any reason you would loose the alternator to your main bus and charge as well as the ability to start the engine using S2.

So now lets assume F2 is blown. We could use S3 to pull 12v from the battery to start the engine using X3-1. I would assume here that the use of X3-1 also tells the ECU not to switch alternators and only to keep essentials running on the first alternator. (Even if that's not the case it doesn't matter here).

Now we clearly see the opposite here too. If F1 was blown, you can still use S2 through F2 to start the engine.

Lets now look at the Kitfox schematic. I hand drew it out to resemble more closely how Rotax drew it out (but should be the same - hopefully I didn't screw that part up).
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Lets look at this and try to compare it. First if we look at X3-3 output back to the main bus/12v it's protected with a 30a fuse F2, which is the same as Rotax. Next, lets compare the S2 Rotax circuit. The Kitfox schematic we see that the DPDT switch connects X3-2 to 12v through a second (F1) fuse, which then loops back to X3-3 through F2. I think electrically this bit works the same, you just now have two fuses.

Now if we look at S3 from Rotax. The Kitfox diagram has X3-1 connect through the switch to 12v which is protected by F1. On it's own, this is the same as Rotax. (Side note, Rotax uses two 30A fuses, not a 30A and a 20A. For simplicity I show both as 30A here).
So far so good. The problem that I now see is that if F1 in the kitfox diagram blows you have no way to use either start function and if F2 blows you can still use both.

Thus, my concern is what issues or possible problems could arise with the Kitfox diagram as is? I suspect there are some very good reasons Rotax has things protected that way, but nothing I can find says why (surprise). 

Finally, not being an expert, the only way I can see to use a DPDT switch and have the same protection and usage as Rotax is shown below. You need a third 30a fuse and need to move the existing F1 in the kitfox diagram. Basically you have fuses on both the X3-1 and X3-2 lines rather than just the power input to the switch.

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Now maybe this is all for not and there is no problem - but I don't like not knowing and just blindly following because it's been done before.


  • Re: Start and Backup Power Protection

    by » one year ago

    Well, here goes. Fusebox pin 3 is the output of the alternator.  The purpose of F2 is to protect the alternator, as the alternator output is supplied to the 12v buss through F2. So the connection shown on Rotax diagram at pin 3 of fuse box harness connector is just a easy place to supply 12 v to the start power switch.  A  backup power switch is added to supply buss voltage to the relay box in the event there is no alternator power available  (A and B alt both inoperative), and at the same time make  Aircraft ground connected to EMS ground because if we want to use battery alone power to run the computers we have to connect EMS ground to battery ground, (aircraft ground). F1 is to protect the battery/main buss from a catastrophic short in the system. It is true that a defect could blow F1 and the aviator had better find a landing spot. I have my system wired almost as shown in the Kitfox firewall forward manual which is document P/N 64913.500. I built my SS7 kit in  2012-2013. have 280 hr on it now. 

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