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Joined: Aug 2020
Posts: 4
Novice
Guys, I'd be interested to hear what resistances your chainsaw has on the primary and secondary windings of the coil module.

For any popular leading brand - Stihl, Husqvarna, Poulan, etc. - doesn't matter which.

Mine is a 16" McCulloch CS420T (2015) and I think the coil could be on its way out, based on a direct comparison of spark intensity and consistency between a neighbour's Stihl and mine in a darkened workshop and the plug grounded against the chassis. That is, my sparking looks a little weak and erratic, and needs more speed on the pull than his to kick it off. Although once it's going it accelerates smoothly and up to high speed without any trouble. And it can idle or run at moderate speeds without any roughness. But then again, it can be idling nicely and then suddenly cut out. Intuitively, it doesn't look to me like a fuel problem.

The Stihl coil resistances are 1.5 ohms (primary) and 10,000 ohms (secondary). A good voltage step up ratio.

My Mac resistances are 800 ohms (primary) and 1250 ohms (secondary). This seems really odd to me. They seem way too similar to be by design, but I don't have the specs for this model. I have checked the grounding of the iron core onto the engine chassis and it's OK.

However, I'd prefer to be sure the coil is truly intermittently faulty before purchasing a replacement part. So some typical data may help me decide this issue.

Thanks.

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Joined: Jan 2015
Posts: 1,487
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SENIOR TECHNICIAN
Most modern magneto coils contain electronic triggers and the primary winding can not be reliably tested with an expensive coil tester designed to test these coils. For most techs it is just a Go/No Go testing; otherwords it either works correctly or it don't.

Joined: Aug 2020
Posts: 4
Novice
Thanks. OK, sure. However I heard it related that coils can function when the machine is cold, and then play up ii.e. fail to provide a voltage switch to the HT cable) when warmed up. But then start working again when the machine cools. I don't know how common that behaviour is, but it suggests there could be an intermediate state between go/no go, but whether that is a failure of the solid-state switch circuitry embedded in the resin block, or of the primary wire itself I wouldn't have a clue. However due to the high voltages present across the secondary winding and being comprised of much finer wire gauge, I suspect that the primary side is more likely to be reliable 'in principle' and the secondary winding the one subject to failure.

So I would be interested in any other related observations, before making a final decision on what to do in this case . At least the saw is working for now and can be started and run most of the time.I am probably bound to replace the coil module anyway in due course and if so, I'll pull the old one apart to suss out what's happening inside it and add to the knowledge bank.


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