Ignition Coil Primary & Secondary Resistance Testing

This video deals with How to test a Ignition coil on a honda gx120 but other coils maybe the same or very similar.

Primary coil resistance hook up 1 lead of the multimeter to the electrical tab and the other lead to the iron core making sure your meter is set to Ohms.

to check the secondary resistance hook up the multimeter to the iron core and inside the spark plug boot. Make sure readings are in the manufactures limits.
However I was suffering a phantom coil so sometimes this maybe tricking to figure out.

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[nicolaedinu]

After 1 week of specification searching this is the first video and documentation in general where I could find the coil resistance... The whole internet is void of this info... Nothing but generic codes... Thank you for this info!

[johncollins]

You're the first person to come close to showing what primary and secondary mean. So thank you for that!
You say a coil can pass the test but still be bad -- I paraphrase, and perhaps even misquote -- for which, apologies. I seem to have the opposite!! It works --- only when I removed the kill wire (then had to kill the engine by closing the throttle) -- but when I did your test the primary was showing 1.2 (and continuity beep), the secondary was showing 7.5 ..pfeef..nothing.. I've just pulled the wire from the cap hoping to bypass any problems the cap or its fitting to the wire might be posing, and middle, the business part of the wire which is tiny just seemed to come apart. The wire and it's best sheath (I suppose?) seemed okay. I had tested the kill/on/off switch and that was okay. So these HT leads are deceptively fat -- mostly rubber with one tiny strand running through the middle. How come the prong(s) on the coil in the cap actually meet them??? Well, the wire is moulded into the coil so I shall now search to see if any brave soul has made that venture! The only place I can find a coil with 52mm centres is... China. Cheap but 3 weeks. Still using sampans out there?
Well, to make the story even longer, a few years ago I put an inline so-called spark tester in, pulled and the thing started! Removed the tester. No start. Did this several times, each time it would only fire up with the tester in! What would the neighbours think?! Can't remember exactly but think I ..yes, I removed the cap, just left the little fixing coil. Forgot several times to my shock when starting up bold it down the hard plug. Strapped a bit of rubber around it. Finally put a new plug on last week. Lent it to neighbouring farmer ladies working. Came back not. Thought I'd solved the problem when I discovered the bulb wasn't sucking up petrol -- broken pipes renewed.
THE END (with apologies).

[sentrack]

I had very erratic coil resistances on my Echo coil. I came close to throwing in the towel and buying a new one for $80 but I thought I needed to understand how to remove the boot if I got a new one so I went through a dry run of removing the spring and boot and snipped off a little bit of the wire. Reinstalling the sharp point back into the into the wire and putting it back through the hole and testing solved the problem. Not sure but guessing the connection was poor there. Always redo the connection before getting a new one. Saved me $$$. Great video - thanks

[thomaswalz]

I'm in coil hell... I've three saws now that have good clean carbs, new plugs. When I spin the saw to see how much spark there is, I get reasonable spark. Once the plug is back in, the saws don't even kick, even with starting fluid. Not even a pop. I pull the plug, it's wet... which I believe is also a tell-tale.

All these saws were running great. While using them, they died, never to be started again. I need my big saws... I've tornado damage to clean up.

I just ordered new coils today... will see.

I appreciate you using a VOM... that's what I was looking for. Thanks.

[chicagobears]

Spot on brother. One of my coils was suspect; however, when tested, it would show that it was good. Your comment about a coil testing good, but still being bad was exactly my situation. Good video!

[jamesw]

The phantom coil you mention it seemed ok but was going bad.... coil packs are affected by heat. Usually they are cold when you test them with If you pop them in the oven at 100 deg C for 10mins and then test them, you'll find the phantom coil !!!

[philwinter]

Guys, remove the plug cap before measuring the secondary coil resistance as plug caps like the one in the clip are usually suppressor types ie there to suppress RF (radio frequency) noise. That's the noise or video interference you hear/see on a radio/TV etc. These type of caps are either, zero ohms, 5 K (5 thousand) ohms, 8K or 10K. You should also check the plug caps are good, ie not open circuit or more than 10K.

[TIMMEH]

Thanks for that info at the end of the vid, I had one doing just that. The spark would go weak after 10 minutes of running, gave the symptom of losing fuel. Changing the plug for a new one which made it run for longer was was the only clue to what was wrong.

[robertsmith]

I like to unscrew the wire and check separately from the coil. When testing the coil if it's acting up when the saw is warm only - just use a heat gun set on low and heat the coil - then test again I've caught many bad coils that way. Use Stens or OEM - the really cheap chi coils will fail 90% of time.

[BurtBowers]

So if the coil is bad then the mower wont start at all? I replaced the carb, the plug, new gas line, flushed out gas tank, only thing must be is coil or valve adjustment?

[PRN-Life]

Thank you this was Actually helpful ! Now i understood the connections, ive been troubleshooting my freshly bought hva 444sg chainsaw from 1986 with no spark issues i already have a hva 44 from the same era with the exact same ignition coil that one works perfectly and the 444 had no spark at all after testing with the multimeter it actually reads almost the same digits ! So then i know there might be some Tore cables Or the Generator/alternator in the 444 sonil switching the generator to see what the difference is Hopefully i dont have to redo all wiring in the saw .

Thanks for the tip this saved me roughly 180$ for that igintion coil they only made that coil for the 44/444 Truly Annoying .
Regards Paul-Roger

[Fatamus]

If the coil works when it is cool and dose not work after it warms up is most likely the expansion of the material as the coil warms up. To help explain, imagine a break in a wire which it in a tightly confined space (like wraps in a coil). While in the cool state the wire contracts which makes the connection between the broken piece(s) of wire but as the coil heat up and expansion of the materials pull the wires away from a sufficient contact. At first the coil may run within spec but as the it heats up the coil may fail right away, or lose power, backfiring and will eventually stop working and you and your Harley are stranded on the side of the road.

[hellomoto]

This is a great and old school bench test. If it tests well on the bench, that's where it's nice to have a lab scope to check spark firing patterns (automotive)

[CHRISTOPHERCARMONA-bh]

I purchased a brand new quarterback for a 511. It worked one month and it stopped working. What caused that?

[ronrobertson]

I have two sthil ms250 chainsaws. Both will not show spark. Have replaced one with a new one and still no spark. All wires appear to be correctly wired, I found the resistance specs in the owners manual, Resistance from spark plug boot to ground is 1.5 to 12k ohms and ignition lead to ground is less than 10 ohms.

[mattb]

I think my SRM 230 coil is bad. Pulled it. One side has a measurement of 330 ohm, the other side is 2.2k ohm.
Doing the sparky test yields no spark 99.9% time, gave me one tiny spark during one of the pulls, but it's mostly dead. The trimmer started like a champ for the season, but died within the first minute during warmup, then stayed dead. I also checked the Stop switch and wire inside the handle, and these look good and have continuity with switch is in Stop. Had to convince myself it was ok to buy a $43 OEM part.

[gregorypiercesr.]

I was getting the same reading of 10.ohms and no spark, same exact looking coil on a Kohler RH265 engine

[bearrh]

what would cause a coil to fail repeatedly within a short period of time after replacing it with factory parts not aftermarket junk? Thanks

[phillipcoiner]

You should check with a Megger to tell if insulation breaks down.

[frontlinemedia]

This is very interesting because I have been watch numerous videos and ppl are checking 2ndary differently. One person checks the iron core another checks the pin. So everyone isn't right