Monday, July 11, 2011

Magneto mysteries

I'm completely flummoxed by the wiring of the one magneto in my RV-7A and maybe you can offer some advice.

I have one Lightspeed electronic ignition in the RV, and one magneto. I'm not using the fancy and pricey rotary switch that is so common on airplanes; I'm using a simple toggle switch to activate the Lightspeed, and one to activate the magneto.

The Lightspeed wiring was pretty simple: run a wire to from the Lightspeed control box to a switch, wire through a pullable 5 amp circuit breaker to battery power.

The magneto is a little bit different. It doesn't use ship power, of course. It generates its own power and, thus, spark. Simple.

I've read Bob Nuckoll's discussion of the magneto; I'm not understanding its application.

Here's how I have it wired. Tell me where I went wrong. I made a so-called P-lead, which uses a single connector shielded cable. The conductor wire attaches to a post on the magneto. The shield is pigtailed to an 18g wire, which attaches to a ground stud. I used the construction practices as specified here, except that I soldered a wire to the shield, rather than crimp a connector onto the shield. I figured it would be more durable.

And so, here's how it looks on my magneto. You can click the image for a larger version:

My understanding is that when the magneto is grounded, it is, basically, off and when it is not grounded, it is on and capable of providing electrical power to the spark plugs.

I was under the impression that it is grounded when there is connectivity between the two posts/wires. The problem is from what I can tell there is always connectivity between the two posts and, subsequently, wires; at least there is when I measure it.

I ran this wire to a toggle switch on the instrument panel, figuring that providing connectivity between the two wires "grounds" the magneto, rendering it inoperative.

Here's the wires at the toggle switch:

So, under my theory, flipping the toggle switch upside down, the "down" position establishes a connection between the two wires which grounds the magneto, making it inoperative.

But because the two wires appear to be always with connectivity, because there's continuity between the two posts as I measure it, this would appear to make the switch itself irrelevant, which means I've done something wrong in my theory of how this works.

The problem is, of course, I don't know what it is.


  1. You say the two posts have continuity with no wires attached? If so, does this exist in every position of the mag or only when it's rotated to certain ones? Is there an impulse coupler on that Mag?

    If the continuity is only when you have it wired, the problem is with your wires or switch. Check the shielding to make sure it is not touching the wire.


  2. It does have an impulse coupler and there is continuity between the two posts without wires. When removed from the posts, the p-lead checks out fine with no continuity between conductor and shield.

  3. Bob - the coil in the magneto will "appear" to keep the two posts (P-lead and ground) connected at all times. You can't measure this with continuity. It's the same reason you can't time a magneto with a continuity tester. A "buzz box" uses AC not DC to test when the points are open on the mag.

    Your description sounds correct, so if you've timed the mag, it should work as advertised.

    - PAF