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Hi all ! George in Utah here flying legacy Sonex 1020 and Aerovee #356 normally aspirated. At runup the other day, noticed a larger RPM drop and rough running on the secondary igniton. Figured a fouled plug, so made an uneventful takeoff and flew for 45 minutes. Did not do an airborne mag check. After landing, decided another runup was in order and when switched to secondary, Very rough and could barely keep running. EGT on cyl #2 was falling off the scale on the secondary, obviously not firing. I'm not very savvy about electronics, so I'll tell you what I did to diagnose the problem as best I can.
1) Looked for anything obvious, wiring , loose connections, etc. and found all looked as normal.
2) Replaced #2 sparkplug with a new one. The "old" one ( 5 hrs time) looked fine but sooty.
3) Pulled ALL the ignition wires and checked resistance : #1 -10K ohm at 24" , #2 - 13K ohm at 32" , #3- 13K ohm at 30 " , #4 - 14K ohm at 27". These values seem okay ? Didn't seem these values would explain #2 cylinder not firing.
4) Trigger cap magnet still magnetized? Yes.
5) Resistance check of both coils ( the co-pilot side one dedicated to #4 and #2, the pilot's side is #3 and #1)
1 - Primary, terminal to terminal at 3.3 ohm on both coils.
2 - Secondary, a meter's probe in each ignition wire hole = 13.4 K ohm on co-pilot side ( cyls. #4 and #2), and no reading at all on the pilot's
side coil(cyls #3 and #1). This was the OPPOSITE of what I was expecting (assuming 13.4Kohm is good and 0k ohm is bad). I was
expecting a bad #2,#4 coil !
6) Turned on Master and secondary switch and checked for 12v at top terminals. Yes, and coils warmed up quickly.
7) Decided to measure resistance at ignition wire holes again just for grins. No change on co-pilot side, still 13.4K ohms, but now on pilots side
read 27.3K ohms! VERY confused now ! Another check after about 5 minutes or so and still 13.4K ohm on co-pilot side, but now back to
ZERO on pilots side. What am I missing here?
I am not sure how to check the triggers, so didn't go on any further. It seems I DO have a bad coil ( chime in here if you agree or not), but it is not the one that would explain cyl #2 not firing. And, does only one half of a coil go bad ? I'm at a loss and could use your help in diagnosing the problem. Can I afford to just buy new coils, triggers, wires and trigger cap? Sure, but would rather not. I hate to think of these parts as just consumables needing to be replaced every so often. BTW, my Aerove has over 320 relatively trouble free hours on it.
Thanks for your help in advance. George.

The secondary should have less turns, and probably less resistance. So the 0 ohm may be ok. I've not measured mine.

How are your coils grounded? Just through the firewall mount or do you have a dedicated ground? If through the firewall maybe that ground connection has degraded with age. You could run a dedicated ground to the coil mounts and see.


You can see my dedicated ground wires.

If just the #2 temp falls off I'd suspect a coil as opposed to a trigger. You could swap coils to see if it follows the coil. Or swap the #4 and #2 plug wires at the coil.

Hey Guys/Bryan,

Bryan Cotton wrote:The secondary should have less turns, and probably less resistance.

That's backwards isn't it? Isn't the "coil" is a step up xformer with many more turns on the Secondary?

I'll be at the hangar tomorrow and will measure my 3 Ohm Dyna Coil if this hasn't been resolved.

Wes

WesRagle wrote:Hey Guys/Bryan,

That's backwards isn't it? Isn't the "coil" is a step up xformer with many more turns on the Secondary?

I'll be at the hangar tomorrow and will measure my 3 Ohm Dyna Coil if this hasn't been resolved.

Wes

Whoops you are right, I goofed.

sonex1020x wrote:At runup the other day, noticed a larger RPM drop and rough running on the secondary igniton. Figured a fouled plug, so made an uneventful takeoff and flew for 45 minutes.

It was hard for me to read past this line. Not trying to be difficult but this seems to me to not be a great idea.

Certainly you are the PIC and I respect that, but flying a plane with a known ignition issue is not something I recommend ...

Best,

Dale
3.0 Corvair/Tailwheel

sonex1020x wrote:3) Pulled ALL the ignition wires and checked resistance : #1 -10K ohm at 24" , #2 - 13K ohm at 32" , #3- 13K ohm at 30 " , #4 - 14K ohm at 27". These values seem okay ? Didn't seem these values would explain #2 cylinder not firing.
George.

If my memory serves me correctly from my old school mechanic days you should expect 4000 to 8000 OHMS per foot of ignition lead.

Thanks Dale, but looking for helpful information, not lectures. George.

sonex1020x wrote:Thanks Dale, but looking for helpful information, not lectures. George.

George,

You are PIC and have the right to fly a broken airplane. It wasn't meant to be a lecture but a warning that others will hopefully adhere to.

If you want my personal helpful information ... install a Corvair conversion! 8~)

Dale
3.0 Corvair/TailWwheel

Hi Guys/George,

I measured the resistance of a known good 3 Ohm DynaCoil. The primary measured 3.0 Ohms and the secondary as measured tower to tower measured 12.6 KOhms. Measuring from a tower to the core laminates measured OPEN.

EDIT: I contacted tech support at Dyantek and they said that the above measurements are the suggested trouble shooting procedure for their coils. The 3 Ohm and 12.6 KOhms are as expected.

Wes