Schematic for small engine ignition coil

[Guineafowl]

TL;DR

For interest, and to help in testing, I’d like to find a schematic for small engine ignition coils - the type with two magnets on the flywheel passing by the core.

As above. As I understand it, there is a charge coil and a trigger coil, and some circuitry potted in together.

 

[jack action]

How does the following link NOT answer your question?

https://www.google.com/search?q=schematic+for+small+engine+ignition+coils&udm=2

 

[Baluncore]

Google: magneto ignition system diagram

 

[Guineafowl]

How does the following link NOT answer your question?

https://www.google.com/search?q=schematic+for+small+engine+ignition+coils&udm=2

Slightly brusque, I assume from too many posters not bothering to research first. I didn’t put enough in the OP, sorry.

I meant to say I’d already searched, and found much the same results. Almost all of those depict external points or CDI units. The closest one features a third ‘salient’ pole for the trigger coil.

The type I’m interested in is like this, just two poles, a ‘kill’ terminal, and is set against two magnets on the flywheel. These seem to be more common on small garden stuff, but don’t seem to feature in explanations:

To diagnose a dud coil properly, rather than be a parts thrower, I do the following:

1. Remove spark plug and clip a ground lead on, so as not to accidentally stress the coil with an open circuit state.

2. Remove the kill lead and check for spark, check again with known good plug.

3. Check coil-flywheel gap and coil ground connection. Check for spark again.

A few basic multimeter (or even Megger) tests would be useful to know, and for that I need to know what’s inside. With the proliferation of cheaply-made parts, sometimes in genuine boxes, I see quite a few dead/new parts.

 

[sophiecentaur]

rather than be a parts thrower

A good philosophy and I did manage to repair an intermittent fault on an old Avenger. The coil went open circuit when hot. Luckily the break was right at the top of the unit and it soldered back OK. But the circuit was 'transitional' (get-attable) with a mix of electronic and mechanical bits and very expensive iirc.

Have you checked the price of a replacement? I'd bet there aren't many different models. How much is your time worth? (I know: nothing and it's for entertainment and that's a priceless commodity)

A few basic multimeter (or even Megger) tests would be useful to know,

I imagine that you can't easily get inside so continuity and insulation (but do you have the volts to test that with?).

 

[Guineafowl]

Have you checked the price of a replacement?

Cheap enough, about £25/£60 for aftermarket/genuine. Expensive, however, if it’s not at fault, and also takes a few days to arrive.

I imagine that you can't easily get inside so continuity and insulation (but do you have the volts to test that with?).

That’s right - potted in resin. I can do up 1kV DC and 5kV AC megger tests.

 

[Lnewqban]

The coil went open circuit when hot.

@Guineafowl
That is a common occurrence.
If sparks quit only when hot, it should be the coil.
Is that this case?

This link may help:
http://www.dansmc.com/mc_repaircourse.htm

I would also check the plug cap internal connection to the high voltage wire (carbon builds up in there), as well as the kill switch and any other safety switch that connects the CDI to ground in order to kill the sparks.

 

[jack action]

By searching for breakerless flywheel magneto ignition system I found the following:

• https://www.freepatentsonline.com/3828753.pdf
• https://www.freepatentsonline.com/RE31230.pdf
• https://www.freepatentsonline.com/4188930.pdf
• https://www.gardentractortalk.com/attachments/kohler-electronic-ignition-pdf.186464/

Is this helpful?

 

[sophiecentaur]

it should be the coil.
Is that this case?

After about 20miles the coil was warm enough to expand and pull off the (thankfully visible) connection. Knowing what I was looking for, I located the loose bit with a small screwdriver. I was hard up at the time so it was a relief.
Funny story about it: I used to do a car share and I knew the fault would recover with time (faith). Sure enough,( before I'd actually found the fault) the engine stopped and I apologised but told the passengers it would be OK soon. They laughed like drains but, sure enough it restarted after a while and got us the remaining 20 miles

 

[Guineafowl]

The coil is dead from cold.

@jack action I can’t see, from those links, an equivalent circuit, unless I’m missing something. I’m looking for two magnetic poles and three connections, kill, earth and spark. The positions of the poles are critical for defining charge and trigger, for ignition timing, and I think there’s some significance in the presence of two magnets, not one, on the flywheel.

Having fitted an aftermarket coil, I have spark, but no run. An occasional spit out of the exhaust or carb is all I can get, even with starter spray.

I have checked:
- flywheel was still keyed in the right place.
- No airflow blockage in exhaust or carb side. It’s getting fuel, as the plug gets wet.
- Compression reasonable, by feel.

So I have spark, fuel, compression, timing and airflow. But it won’t run.

Considering the starter fluid test as diagnostic for a no-spark condition, the only thing I can conclude is that the new coil is able to spark in free air, but not under compression. Genuine coil ordered.

 

[sophiecentaur]

Considering the starter fluid test as diagnostic for a no-spark condition, the only thing I can conclude is that the new coil is able to spark in free air, but not under compression. Genuine coil ordered.

There are forums about pretty well everything and Lawnmowers are popular. It may be worth while dropping a pebble into one of those pools and see what comes back. It could be a well known prob.

On the topic of timing, is it possible that the location of the crankshaft vs the timing detection has come loose? I had this on an old 2stroke motorcycle and it only became obvious when I tightened a screw and the cam fell off. Can you detect any slop?

 

[Baluncore]

There may be a coil fault, but how do you know your fuel is good. It may be old and stale, or contain water that wets the plug.

 

[Guineafowl]

I’ve checked the flywheel is keyed onto the crank correctly. There’s no other scope for timing adjustment.

I did put in some fresh fuel, but the starter spray test (with a cleaned, regapped plug) should have bypassed the fuel side of things.

 

[Baluncore]

I did put in some fresh fuel, but the starter spray test (with a cleaned, regapped plug) should have bypassed the fuel side of things.

Replace the coil whenever.

Too much time is wasted with stale and dirty fuel. Drain the fuel, and all the water from the fuel system, clean all the filters, then refill the tank and prime the carburettor with fresh fuel. That is the first thing to do with any small engine that has been sitting, or that does not start reliably.

 

[Averagesupernova]

I have yet to have an engine not start by giving the carburetor a squirt of fresh gasoline providing the ignition is working and timed properly. If it doesn't start then it is ignition related. Not sure what you're using for starter fluid but I've found for gasoline engines that a small squirt of gasoline in the carb is the best.

 

[Baluncore]

I have yet to have an engine not start by giving the carburetor a squirt of fresh gasoline providing the ignition is working and timed properly.

It may fire and run for a few seconds, but if the carburettor or fuel tank contains water, it will not continue to run. Water in the fuel will break the pull-cord, before they ask me to fix it.

 

[jack action]

but if the carburettor or fuel tank contains water, it will not continue to run.

Or if the carburetor orifices are plugged from varnish build-up.

 

[Averagesupernova]

It may fire and run for a few seconds, but if the carburettor or fuel tank contains water, it will not continue to run. Water in the fuel will break the pull-cord, before they ask me to fix it.

Yes but it proves or disproves the ignition which is what is in question. I could clean the fuel system and put in new and still have the same problem. Pick one system and do it first. Squirt some gasoline in and try it is quick and easy.
-
Also, I have been in the situation where the engine won't start and I pull the spark plug and found moisture droplets on the plug. It's easy to tell. If that's the case, sure drain the fuel, etc.

 

[Baluncore]

Once the ignition fails, the tool sits around until it has breathed in more moist air, condensed that water, and breathed out the volatile fuel components.

If you worked for any time at fixing other people's small engines, the first thing you would do is to check the fuel system. You would do that, even if you suspected the ignition to be faulty. Not checking and changing the fuel is certainly a false economy.

 

[Averagesupernova]

If you worked for any time at fixing other people's small engines, the first thing you would do is to check the fuel system.

Maybe. I have not and likely will not work on other people's engines for pay. But I know draining and cleaning a fuel system is a guaranteed income for small engine shops. The reason it's done is it is a bit of a guarantee that the unit won't come back that afternoon because it has contaminated fuel. Or at least won't have it until contaminated fuel is poured in from a carelessly placed jug. All shops work that way. It's not worth taking something in unless a certain minimum amount of work is done. I don't think the op is running a commercial business for small engine repair.

 

[Averagesupernova]

Typical lawnmower or small engine shop's spring special:
Drain and replace oil, drain and replace fuel, sharpen or replace blades. Most homeowners utilizing this service don't even bother trying to start the mower or bother looking at the blades. I guess it's a good thing otherwise small engine shops would have a harder time staying in business.

 

[Lnewqban]

The coil is dead from cold.

These coils are only transformers.
They have two winding copper coils (one in-lead and one plug- lead), both wrapping a common U-shape iron nucleus and each having a very different number of turns.
The kill switch-lead is only a by-pass of the CDI (connection directly to ground).

Please, see:
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/igcoil.html

https://www.denso-am.eu/news/deneur21_04_ignition-coil-basic-principles

https://en.wikipedia.org/wiki/Ignition_coil#Design

https://assets.denso-am.eu/production/news/2021/fig6_446x572.jpg

As your device lacks a battery, the input voltage comes from those rotating magnets inducing the primary winding current (low voltage).
Then, the secondary (high voltage) winding induces the desired spark only when that circuit is suddenly opened (pick-up coil and CDI).

 

[Lnewqban]

Having fitted an aftermarket coil, I have spark, but no run.

Does that new coil have its own high voltage wire and spark plug cap?
If you had to use the old one, did you check the wire to cap connection for corrosion?

So I have spark, fuel, compression, timing and airflow. But it won’t run.

Is this a two-stroke or a four-stroke engine?

Please, see:
https://www.briggsandstratton.com/e...rowse/ignition-system-theory-and-testing.html

 

[Lnewqban]

Considering the starter fluid test as diagnostic for a no-spark condition, the only thing I can conclude is that the new coil is able to spark in free air, but not under compression. Genuine coil ordered.

It could very well be.
Nevertheless, the ignition coil may not be the weakest link in this case.

In order to produce a healthy spark under compression, we need a strong magnetic field and good electrical connections.

I have seen magnets that have detached from the flywheel due to failing glue, getting tangentially re-located (all packed together).
I have seen magnets that have lost good portion of their originally magnetism.

I have also seen situations in which the recommended magnetic gaps for flywheel and pick-up coils are unable to stay put during operation of the engine (vibrations, loose bolts, heat).

 

[Averagesupernova]

An ignition armature must be set at a precise distance from the flywheel. Your engine repair manual will provide the proper gap for your engine. Common armature gap ranges are .006 - .010" and .010 - .014". Armatures are often packaged with a shim to assist in setting the gap. Index cards of the proper thickness also work well.

The above is from the Briggs and Stratton link provided by @Lnewqban . I had forgotten about this. The proper distance between the flywheel and the stator is important. Also, sand the entire circumference of the flywheel. Get it cleaned up so no rust can bridge this gap. An improper gap may get you spark but it might not get you spark under compression. I have typically used a piece of paper. Hasn't failed me yet and I've done a lot of them. They refer to it as an armature but I think that is incorrect terminology. At any rate, I think we know which piece has to fit where with a paper thin clearance.

 

[Baluncore]

@Guineafowl
Why have you not identified the equipment, along with the make and model of the engine?

 

[Guineafowl]

I know the principles of magneto ignition systems, which are the subject of the links posted, but all of these involve an external trigger. The type I’m dealing with, pictured at the very top, is self-contained and has been fitted to every chainsaw, strimmer (weed whacker?), mower etc that I’ve ever worked on.

The original question was about the internals of this, very common, type of ignition coil, and whether there’s any chance of electrically testing same to confirm dead.

On the diagnostics side, here are some more details:

- It’s a Stihl HS45 2-stroke hedge trimmer. Presented as running this season, but now not.
- I did the checks (post #4) to diagnose a no spark due to faulty coil.
- Once the new coil was fitted, I had spark but no run, so tried starter fluid (easy start).

Even with no carb attached, a sparking engine will fire briefly on easy start. This wouldn’t, and I’d consider that diagnostic for a no spark condition.

In some desperation, I have drained and replaced all the fuel, had the carb apart and blown the various orifices out, and checked the H/L settings. No go. Not surprising because of the easy start test.

 

[Guineafowl]

^ Just after I posted that, the new, genuine Stihl coil arrived.

Fitted, and the engine started second pull.

So, to close this out, what must have happened is:
- Original coil failed.
- Aftermarket coil sparked in free air, but not in cylinder. I’ve heard of this, but never seen it.

Thanks for the advice, and if anyone knows of a schematic for this particular type of ignition module, I’d still be interested, as this certainly won’t be the last faulty small engine I’ll see.

 

[Averagesupernova]

...and if anyone knows of a schematic for this particular type of ignition module...

It seems you took a bit of heat for this question due to all the resources on the Web. But, none of those resources get to the nitty gritty of what is happening. Yes, we all know how the old points style work and we know the solid state version has to do something similar. But, the question I have is how is the trigger activated as precise as it needs to be. And I'm sure this is part of your question too. With a module that is all in one and relatively easy to replace it is expected that the average joe who replaced it doesn't ask questions. Glad you got it working.

 

[Guineafowl]

Yes, the links were well-meaning but didn’t capture this type of coil (strictly: ignition module). It’s certainly not a plain transformer, from its function and from the potted form, which looks like it contains a PCB, or at least other components.

I have noticed a certain level of grumpiness on here, not just directed at me. Presumably related to these turbulent times (as pointed out by another member), crackpots, non-Googlers and ‘do my homework for me’ types. On forums, I try to adopt the mantra: what would Jim Hardy do? His thoughtful and encouraging replies (and good-natured scolding, sometimes!) are a model for us all, I think.

Back to the coil, this is a sketch of the flywheel at TDC, roughly gauged with a screwdriver down the bore. The letters ‘N’ and ‘S’ are cast into the aluminium over the magnets, so I assume the field is as drawn. Since rotation is anti-clockwise, it looks like the spark will occur after TDC.

To be clear, there are no other components, points, battery, CDI box, etc., other than the spark plug itself. The spade terminal is simply switched to chassis to kill the spark. Chassis ground is from the core mounts.