Why do coils burn out?

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A new coil that only ran for a short time most likely had a factory defect (or was just poorly made). Having more or less resistance for the spark to overcome will increase either the current or the voltage the coil sees & stress it beyond it's design. That would generally be caused by the wrong type of lead/plug or excessive gap
 
in the automotive world its typically caused by not changing the spark plugs, over time internal resistance rises overworking the coils but poor designs lacking proper sized heatsinks or bad potting material expands/contracts too much damaging the wiring. Even a engine overheat fails coils!
 
Something interesting about coils is that to high of a resistance load can allow the voltage in the coil due to lack of loading to rise high enough to 'punch through' the insulation on the copper windings. This doesn't happen very often on a single cylinder engine but a multiple cylinder engine running while spark testing it could happen. Against my intuition but I believe to be true from reading about it is that coils can take a shorted load than one that is to high. Spark gap serves as a voltage limit because once spark starts voltage rise stops.
 
I'm wondering if the m-tronic saws don't like driving the carb solonoids harder once the saw is modded and it is burning out the electronics rather than the coil firing portion of the electronics.
 
I'm not sure I would over analyze on a sample of one, like Husqvarna did with their early offerings, Stihl will go through a learning curve and sort them out. I would find out if there are updates or their equivalent to service bulletins for clues :) But suspect if this is something common, Stihl either is aware of if it's systemic or already has a solution for.
 
I'm not sure I would over analyze on a sample of one, like Husqvarna did with their early offerings, Stihl will go through a learning curve and sort them out. I would find out if there are updates or their equivalent to service bulletins for clues :) But suspect if this is something common, Stihl either is aware of if it's systemic or already has a solution for.
This seems to be common on modified saws. I don't think Sthil has this on their radar.
 
The first reason is that in newer saws, the coil and other elements are to break. And the second reason is overheating, the coil is also supposed to give off heat and when it is covered with oil and sawdust, it burns out. I have various chainsaws, even 40 years old, I have never replaced the coil
 
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