Failed Ignition Coil

[sonex1374]

My Jabiru 3300 failed a mag check on my last flight's run-up. The engine ran fine, but one mag was completely in-op. Troubleshooting the problem I determined that the problem was within the magnetron ignition coil unit, and not the starter switch, mag wiring, spark plugs, spark plug wires, distributor cap or rotor. My engine is a late-model roller-cam engine, with 300 hours on it.

Removing the coil allowed me to see the problem clearly. The distributor wire had broken at the connection to the coil housing. The distributor wire exits the housing at a molded shoulder. This stabilizes the wire and prevents it from disconnecting from the coil innards. The wire also has a built-in strain relief rubber boot, but the boot also connects to the shoulder. This places all the load from any vibration in the wire at the junction of the body and the shoulder, and that's exactly where the failure occurred.

Jabiru uses an ignition coil sourced from a Honda generator, but these coils are similar to many tractor and other small engine ignition coils. It is possible to buy similar coils off eBay or other suppliers, but with so many imitation parts out on the market, there's no telling which are genuine. There are several anecdotal stories of imitation Honda coils being used on Jabiru engines, only to fail in short order. I ordered an authentic coil from Jabiru USA, just to be safe.

To prevent this type of failure from happening again, I will stabilize the distributor wire to prevent vibration from working on that shoulder. I suggest inspecting these junctions periodically to look for signs of cracking, and excessive movement in the distributor wire likely indicates a crack has already occurred.

Jeff

[WaiexN143NM]

Hi Jeff,
Thanks for the post. We are doing the annual condition inspection now. We will look at our coil wires over closely. And support them better.
In fact I was just on the jab USA site looking up part numbers for a coil , distributer cap, and rotor to take along as spares on a cross country.
The coil should be part# PI 0525.
Can a new coil wire be installed? It’s just a pin or screw type pin in the boss if I remember. I’ll take a look tomm morn. I believe I put plastic stay straps 2,around each coil wire and around the coil. I was afraid if the wire fell or pulled out in flight. Then a little black rtv where the straps crossed on the wire.
WaiexN143NM
Michael

[sonex892.]

I was recently testing a prototype cold start system for a friend. The cold start was working great but the modified coil a cheap Honda copy partially failed after about 3 hrs. Rough running picked up during pre-flight run ups. I'm not sure what caused the failure but decided its not worth using anything other than a Honda. I'm flying again with my original coil and waiting on a genuine Honda coil to arrive so I can continue the cold start test.

Here is a UK site with a lot of helpful Jabiru replacement part numbers http://www.jabiruowner.co.uk/ under alternate jab spares.

Also an old UK service bulletin recall for dodgey coils from 2003. Funny I had never seen this in Australia http://contrails.free.fr/engine_jab_coil_letter_en.php

Steve
Sonex 892

[RodgerC]

This may or may not be relevant, but here goes....

I recently suspected a dud magnetron on my AeroVee due to a lack of spark during run-up on cylinders 1 and 3. The lower magnetron was the “obvious” culprit.

After several frustrating hours of trying to remove it, I decided to adjust it’s air gap and try it again, just in case the gap (too close/too wide) was the problem.

Upon restarting it still ran like a hairy goat on mags only and I was resigned to parting with a year’s pay to import a new unit from Sonex. But habitually I did another magneto check just before shutdown and voila, it was all working again.

I am yet to confirm the cause, but I now suspect that one plug had a lead deposit that was shorting it out and as a result, neither of the plugs being fed from the lower magnetron were sparking. Had I simply cleaned the plugs, I may have saved myself some frustration.

At the time I didn’t suspect a fouled plug because two cylinders were not firing rather than just one.

Hopefully this hypothesis will result in comment from someone with greater understanding of the magnetrons than that which I possess.

[N111YX]

Thanks for that great info, Jeff. It's too bad that your coil went bad so early. It's still shiny!

You may recall my need to replace one of mine (after you loaned me your battery charger to get started in Lakeland, thanks!) and the one ordered from Jab USA came with a note that it's a even tighter fit than the original coil. I recall some tension, from nearby structure, on that cable area that you show and I plan to keep an eye on it.

[WaiexN143NM]

Hi all, Jeff,
You should be able to pull out the old wire. There. Is a serrated pin in the boss. Source a ne w wire at the local auto parts store, and you’ll have a spare coil.
Or is there enough length to cut the wire at break, reinsert, and still long enough to reach distributer cap?
Michael

[sonex1374]

I received the new coil from Jabiru USA. It is outwardly different than the old coil. Markings include Honda TEK FL833. Although the dimensions are a bit different, it fits just fine and is now installed and running well in my plane.

New Coil on Bottom:

Comparison of External Dimensions:

New Coil Markings:

Jeff