Coil Resistance Specifications Changed?

·         John Womer

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Two questions... one general, and one specific to my airplane.

Section 3.13 Table 11 in the latest revision of the Engine Maintenance Manual (JEM0002-6) lists different Primary Resistance and Secondary Resistance specifications than listed in the same table of earlier versions. In addition, those different specifications do not match those in Section 9.21 of the same manual.

Do I assume correctly that this is a misprint?

My 2010 J230-SP with engine S/N 33A2072 has Honda ignition coils. When we recently my my coils, the secondary resistance on both was within specification at 6.8k Ω, but the primary resistance on one coil was 1.1Ω and the primary resistance on the other coil alternated between 1.1Ω and 1.2Ω. My engine is running smoothly, and I don't detect any issues when I do a mag checks before takeoff and in flight. What does it mean when the primary resistance is "slightly out of spec?" What problems can it create?

Thanks.

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2010 J230-SP

·         Doug Smith

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Hi John,
Looking at the table of changes for that manual Section 3.13 seems the most current; I suspect the values in 9.21 are old: I'll pass that on to the manual editing squirrels and confirm.

What quality of meter are you using to measure the resistance and are your batteries fresh? Readings of a fraction of an ohm are getting close to the limit of resolution for most meters so it'd be worth checking yours is reading correctly.

Can you confirm that your coils are the black ones with the square bulge on one side?

Ultimately if they're out of spec they'll need to be replaced but we'll confirm those figures first.

Regards,
Doug.

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·         John Womer

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I used an Extech Mini Digital Multimeter (street price $75 USD / $105 AUD) so even with a fresh set of the finest AAA batteries my local hardware store carries, we're not the accuracy of a Fluke or Simpson meter!

I have the square Honda coils like those show in Figure 44 on Page 74 of JEM0002-6.

Re "Looking at the table of changes for that manual Section 3.13 seems the most current..." are you saying that the Primary Resistance specification has changed from 0.8 Ω to 1.0 Ω (as specified in JEM0002-5 and earlier versions of the manual) to 1.9 Ω to 2.1 Ω (as specified in JEM0002-6)? The table of changes does list "Revise electrical system specifications (section 3.13)", but that is a BIG change!

Or is it that the Honda Coil PI0524N listed in JEM0002-6 is a new and different Honda Coil?

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2010 J230-SP

·         Doug Smith

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Well a $75 (US) meter isn't bad by any means; it should be reading OK if the batteries are good... but it'd be worth checking with another meter if you can borrow one.

The official word is:
The measurement in the current manual Section 3.13 is correct for the current Honda coils PI0525N that go through QA here before sold. The secondary measurement does not hold for any Honda coils that have different leads or extensions not supplied by Jabiru. The measurements from the earlier manual (shown in 9.21) are for the earlier Honda coil PI1524N.

The different part numbers (different coil models) would explain the large difference.

At 1.2 ohms you're still outside the limits but not by much: cleaning the contacts and fresh batteries in your meter might get you over the line.

D.

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·         John Womer

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I'll will make sure all of my electrical contacts are clean, and I'll recheck my coils with another meter.

Re "The official word", I think I understand what you are saying, but just to make sure I do, let me put it another way...

1. All of the resistance specifications listed in the current manual, JEM0002-6, Page 21, Section 3.13, Table 11 are correct for the part numbers listed. The resistance specifications listed under Honda Coil P/N PI0525N pertain to that part number only. They do not apply to previously supplied Honda Coils P/N PI1524N.

2. All specifications apply only to unmodified parts as originally supplied by Jabiru. Any modifications (different leads, extensions etc.) will result in different measurements

3. The resistance specifications for Honda Coils stated in JEM0002-6, Pages 74-74, Section 9.21 apply to Honda Coils P/N PI1524N only.

Correct?

One last question on this topic. I have looked elsewhere online but I could not find the answer. (And I am not asking to avoid replacing my coils.) What happens if the primary resistance of an ignition coil is slightly high? Does it result in too much or too little energy being passed on to the secondary coil? Does it create additional heat which could eventually break down the windings and cause an internal short?

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·         Doug Smith

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Yep, that's right. It's been added to the list for the next manual update so it should all be a bit clearer then.

I'm not an electrical guru by training but working from first principles, higher resistance should result in less current through the primary coils which will then follow on to less output from the secondary. I would expect a mild drop in performance, probably only measurable at low (cranking) rpm. For example you might find those coils need 300 RPM to produce a spark equivalent to another coil at 280 RPM. I would not expect any significant heat build up.

D.