Key Ignition vs. Toggle Switches

[jrevens]

Hi Grover,

I think you'd find a lot of useful information about this in the "AeroElectric Connection" by Bob Nuckolls, if you haven't already looked at that. I'll try to explain how he has suggested wiring that circuit. It's probably the simplest circuit on the airplane, so my long explanation will probably be over-kill. You can use single-conductor shielded wire for each ignition module. 16 or 18 gauge would be fine. I used 18 gauge. While shielding isn't necessary on these circuits, and even 18 gauge is bigger than it needs to be, that type/size of wire was suggested for robustness, as well as being able to cleverly use the shielding as the ground conductor. So, a pigtail is attached to the shield on each wire, and each pigtail goes to it's respective common terminal on an ignition switch (toggle, rocker or whatever). Each center conductor attaches to the N.C. terminal of it's switch. At the ignition module on the engine, the center conductors of those 2 shielded wires each goes to it's respective pin on the module connectors (the brown wire mentioned by Larry - refer to Rotax manual). The shields from your 2 wires are then connected to a grounding point ON THE ENGINE, again with pigtails. So, when a switch is "off", contact is made between the common and N.C. terminals, which connects the ignition module pin, through the center conductor to the switch and from there through the shield back to ground at the engine. When a switch is "on", that pin is disconnected from ground as the N.C. contact becomes "open". Again, you don't have to use a single conductor shielded cable - you could use a 2-conductor un-shielded cable for each module (or other combinations of conductors with or without shielding if you wanted to), but the shielded cable is generally a little tougher and more resistant to intolerable damage. It's a nice, clean way to do it, and you've also got shielding, whether it's needed or not.

[SS7Flyer]

You can run a very simple on-off-start ignition switch when using toggles for the ignition boxes.

[jrevens]

You can run a very simple on-off-start ignition switch when using toggles for the ignition boxes.

You can run an even simpler momentary contact on-off keyed switch for the start function, along with the ignition switches. That happens to be how they do it in the RV-12s, and that's what I did too. In fact I used the same compact, light weight switch that the RV uses too... pretty much like what is used on many alarm systems. The 2 ignition toggle switches are completely separate from that start switch. Which ever you use, you want to be sure to use an arc-suppressing diode on the start solenoid/contactor coil circuit to protect the contacts in the start switch from a possible large back emf pulse created when the coil is de-energized and the magnetic field collapses.

[Flybyjim]

I am building the ss7 but have not yet ordered the firewall forward kit, is this not all included with this kit?. I would like to think Kitfox has these answers all worked out with this kit. The last kit I built, not a Kitfox had all the parts supplied with the two toggle switches and a start key switch.

[jrevens]

I am building the ss7 but have not yet ordered the firewall forward kit, is this not all included with this kit?. I would like to think Kitfox has these answers all worked out with this kit. The last kit I built, not a Kitfox had all the parts supplied with the two toggle switches and a start key switch.

My kit came with a standard ignition/start key switch made by ACS - a copy of the original Bendix switches. Some of us prefer to have separate switches for ignition and start, rather than the combo switch. There are people who claim to have had ignition module failures that they attribute to the combination switches, including at least one well-known Rotax supplier. This has been discussed on this forum in the past, Jim. There are some other advantages to having separate switches also.

[GWright6970]

John,
I purchased Bob Nuckolls' 12th Edition from him. It REALLY helped me with a lot of things, but I needed a diagram better than his Rotax 912 Figure Z-16; with Notes 2 and 3 not helping much to clarify things. I have no idea what that box is that the ignition module wires go to...I have a guess, but I hate guessing!

I have been blessed by two builders who have allowed me to photograph their flying aircraft. In one of these aircraft the ACS OFF R L BOTH START switch is also installed with two toggle switches. On the other side of the panel, the ACS switch is clearly wired to the R and L contact points with wires appearing to be coming from the two toggle switches next to it. I am guessing that the toggle switches are installed in series between the ACS switch and the ignition modules. I do NOT see shielded wires... Much less the shielding being grounded. This got my curiosity really going to say the least....hence my original question.

I have read and understand both sides of the ACS switch debate. I also have listened to discussions on the benefit of turning over the engine without the modules "on." I wish to therefore use BOTH the toggle switches and the ACS switch. I do have the ACS diode to install on the starter solenoid.

(So I also gather that the shields are to protect the modules from electromagnetic compatibility and electromagnetic interference that can be introduced by the brown ignition wires.)

SUMMARY
From what I now gather from my Rotax 912 Installation Manual and your reply, I can run a shielded wire from each module to it's toggle switch, connecting the shields as described.... The ACS would then be wired as shown in the Kitfox manual except that the two "brown ignition" wires are attached to the toggle switches instead of the ACS switch. I will also install the diode.

Thank you!!!

[jrevens]

You're welcome, Grover. Just one additional comment - I probably wasn't clear enough, but shielding is probably not needed on the ignition wiring for these Rotax engines. Again, the shielded cable is a little more robust and perhaps less vulnerable to serious damage than un-shielded cable. It's nice to be able to use the shield for the ground path on those particular circuits is all, and additionally you have shielding whether needed or not... (just in case?). FWIW, I wouldn't be a fan of using extra switches in series with the combination switch contacts. You're just introducing another potential failure point in the circuit, with no advantage at all that I can see. You would have 4 sets of contacts that have to be made instead of just 2, in order to make sure your engine is "safe" from accidental ignition. Simpler is better in this instance, IMHO. It sounds like you're not going to do that, and I believe that is what you are saying in your summary... that you will use the ACS switch, but just not use the ignition switching feature of it, only using it as a start switch. Is that correct?

[GWright6970]

(?... that you will use the ACS switch, but just not use the ignition switching feature of it, only using it as a start switch. Is that correct?)

John,
You are correct. I will NOT be running the module brown wires on to the ACS switch. I will run the module wires to the toggle switches ONLY.

I will be using the shielded wire and grounding both ends... Even though I read an EAA Sport article that said to ground only one end.

Again, thank you!