Rambling on about coils

[Old2stroke]

I am surprised at the number of faulty coils that are being replaced and wondering what some of the causes for coil failure might be. They should last for the life of the engine. Of course, modern coils aren't just coils anymore, there are semiconductors and a few passive components that replace the function of the old points by controlling the primary current of the coil and they are all potted together with the coil to produce an ignition module. Semiconductors have a fairly unpredictable failure time and probably semiconductor component failure could be the cause of most ignition failures. Top quality components such as the ones in the many modules in our cars have become super reliable but who knows what quality standard is used in a chainsaw module. Heat is the big enemy of semiconductors and is a difficult problem to solve in chainsaws. The air coming off the flywheel helps to keep the module cool while the saw is running but as soon as it is shut off, the module tends to soak up heat from the saw and will be real hot if the saw is restarted after a few minutes. This is a normal mode of operation for a chainsaw so heat could be an issue, which means the semiconductors have to be carefully selected to withstand the highest anticipated temperature. Back to quality of components.
There is another issue we should be aware of. When ignition occurs, the secondary coil voltage will rise up to the level that is required to fire the plug (about 20,000 volts) but will never go higher than that after the plug fires. If the coil lead is removed from the plug there is no path to ground for the spark energy and the coil voltage will spike MUCH higher than normal. It could get so high that it will break down the insulation in the secondary winding and ultimately lead to coil failure. Some modern coils may have over-voltage protection built into them, but it is wise not to trust this feature so a golden rule should be to never pull the engine over fast with the plug lead off unless the ignition switch is also off. When you are checking compression or just trying to clear out a flooded engine with the plug out, is your ignition switch off?

 

[pioneerguy600]

The saws and engines with the module under the flywheel suffers this fate the most, no cooling air under there at all running or not.

 

[crabby cooter]

put a new plug in once in a while. old plugs need more voltage

 

[bang]

Chainsaws are a hobby for me and 2cycle kart racing engines are my specialty. I can say without doubt that the coils in a kt 100 Yamaha with tci degrade from use.
I check all coils for resistance at every rebuild and it drops each time an engine is checked. I have a minimum acceptable resistance I will allow and although the engine will still run and sound normal the power will suffer at high rpm (I dyno engines after rebuild). A new coil sitting on the shelf doesn't loose resistance so it is fairly certain that heat and use break down the coil and vibration is what kills it.

 

[Chris-PA]

Chainsaws are a hobby for me and 2cycle kart racing engines are my specialty. I can say without doubt that the coils in a kt 100 Yamaha with tci degrade from use.
I check all coils for resistance at every rebuild and it drops each time an engine is checked. I have a minimum acceptable resistance I will allow and although the engine will still run and sound normal the power will suffer at high rpm (I dyno engines after rebuild). A new coil sitting on the shelf doesn't loose resistance so it is fairly certain that heat and use break down the coil and vibration is what kills it.

Between which terminals do you measure the resistance?

I ask because you may just be seeing a characteristic of one flawed design. All electronic components degrade over time, and this is a harsh environment, but the failure modes vary and not all of them can be measured by a simple DC resistance check. The high voltage coil for example is just a continuous copper wire. If you can actually see the resistance in ohms dropping on a low voltage DVM then that a major defect. The wire isn't changing, it would have to be insulation breaking down rapidly.

 

[bang]

If the coil is properly grounded to the engine I test from ground to the end of the plug wire with the boot removed. If the coil is out on the bench I contact the green pigtail to the arm and attach one lead there and the other lead to the plug wire. These engines are rebuilt ideally at 8 hours of use(some more some less). I wonder if a coil from a chainsaw was monitored and notes maintained as we do with a KT100 if you wouldn't see the same degradation. I'm guessing chainsaw coils don't get attention until they quit. When a KT coil fails it's usually the small ground pigtail or the tci wire breaking inside the insulation near the junction at the coil. To combat that we put a large dab of silicone where the wires exit the coil.

 

[Chris-PA]

If the coil is properly grounded to the engine I test from ground to the end of the plug wire with the boot removed. If the coil is out on the bench I contact the green pigtail to the arm and attach one lead there and the other lead to the plug wire. These engines are rebuilt ideally at 8 hours of use(some more some less). I wonder if a coil from a chainsaw was monitored and notes maintained as we do with a KT100 if you wouldn't see the same degradation. I'm guessing chainsaw coils don't get attention until they quit. When a KT coil fails it's usually the small ground pigtail or the tci wire breaking inside the insulation near the junction at the coil. To combat that we put a large dab of silicone where the wires exit the coil.

Yeah, that's weird. You're measuring the resistance of a coil of copper wire - there aren't even any semiconductors in that part of the circuit. The only way the resistance can change is if the insulation is starting to conduct or break down (charring, carbon trails, etc.), there is very conductive external surface contamination (again, maybe burning and leaving carbon trails), or the windings are starting to short. None of that should be a problem in a modern coil design, unless it's been placed in some really horrible location were it's just getting hotter than the potting or insulation can stand.

It's more likely to have failures on the primary side where the transistors and control parts are.

 

[bang]

Probably couldn't be in a worse environment afa heat is concerned. It's in a totally sealed area with no fresh air as delivered new. I put an air gap under the cover that helps but doesn't solve the problem.

 

[aokpops]

I don't know as much as you guys do about coils , for whatever reason I go threw more coils than spark plugs . One thing that seems to help is get the air gap as big as you can . I was told put businesses card in the gap was getting a very short coil life . The last few years been widening the gap ,today I found the max for a chainsaw .020 at .017 ran good . Would like your thoughts on this theory

 

[Chris-PA]

I don't know as much as you guys do about coils , for whatever reason I go threw more coils than spark plugs . One thing that seems to help is get the air gap as big as you can . I was told put businesses card in the gap was getting a very short coil life . The last few years been widening the gap ,today I found the max for a chainsaw .020 at .017 ran good . Would like your thoughts on this theory

I had an old McCulloch ignition fail on my SE3420 (PM605), but they are known for that and they are getting old now. Other than that I
I've never had one fail. Whenever I thought I did it was something else. You may be dealing with some specific design weakness that is not typical of all ignitions.

In general I don't think adding coil gap is a benefit. A smaller gap will give higher core flux, but magnetic systems are complex and non-linear so other effects can come into play. So for example if the core saturates a smaller gap will not increase flux. It may be that increasing the gap reduces the current and takes load off of marginal parts, helping them to last longer.

 

[aokpops]

Not sure how many coils I put on a Husqvarna 50 an 51 bought OEM after market seen no difference . Seen them fire blue an still not run ,got a generator that will eat coils to showed the guy I bought it from ran a half hour .Was getting blamed there something wrong with your engine said maybe the magnet is bad