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So I have a mower that runs well. No issues there. It was barely running, but after a cleaned carb and an oil change it fires up on first pull. It's a Briggs & Stratton. I don't have the mower model number on me, but that isn't relevant to my question (I don't think).

I was adjusting the brake lever cable so that the kill switch had a gap (ungrounding it) when the brake lever was pulled. I was testing the magneto ground wire (magneto to kill switch wire) with my multimeter to check for continuity. When I pulled the ground wire from the magneto and used one probe on the magneto end of the wire, and one probe on the other end. I had continuity. Good. And if I moved the second probe across the gap to the contact point for the kill switch, I had no continuity. All good.

Now, if I connected the wire back onto the magneto, still with the brake handle pulled back, and redo the continuity test, except this time using the contact points of the kill switch, there is now continuity (see pics if that doesn't make sense). But there is a gap between the contact points, which means, given my previous tests, the circuit is grounded somewhere between the magneto and the spark plug. When I check for continuity between exposed metal on the mower somewhere and the 'ungrounded' kill switch, I also have continuity. But if that is the case, why does it start? If it is grounded in some other way when the brake lever/handle is pulled back, that should prevent a spark, since there is a more direct route to ground than across the spark plug gap, no?

All tests were obviously done when the mower was not running.

Am I misunderstanding something here, or do I have an issue that somehow does not prevent the mower from starting up?

Again, the mower runs. But I don't understand why, and I would very much like to know. Thanks.

When you pull back on the brake lever you are opening the kill switch, allowing the engine to start. When you release the brake lever you are closing the kill switch, grounding the coil circuit, killing the engine.

Rivets said:

When you pull back on the brake lever you are opening the kill switch, allowing the engine to start. When you release the brake lever you are closing the kill switch, grounding the coil circuit, killing the engine.

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Yeah, I understand that. That's why I'm confused about there being continuity between kill switch contact points when the kill switch is open. That would suggest the coil circuit is grounded whether the kill switch is open or closed, which doesn't make sense given that the mower starts and runs. Maybe have another read through my initial post to see the tests I did with the multimeter.

Very simple, kill wire is shorted to ground between coil and kill switch. You would still have continuity in the wire between the two ends.

Rivets said:

Very simple, kill wire is shorted to ground between coil and kill switch. You would still have continuity in the wire between the two ends.

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Again, I'd encourage you to read through my first post, as I wasn't measuring continuity between the two ends, which of course would have continuity. Here is the relevant bit:

"if I connected the wire back onto the magneto, still with the brake handle pulled back, and redo the continuity test, except this time [ed: I shouldn't have said "this time" as that was what I did previously] using the contact points of the kill switch, there is now continuity (see pics if that doesn't make sense). But there is a gap between the contact points, which means, given my previous tests, the circuit is grounded somewhere between the magneto and the spark plug."

If the kill wire was shorted to ground between coil and kill switch, the mower wouldn't turn on because there would be no spark. It does run. Also, I checked for this in a previous multimeter test.

It appears that the engine brake cable is connected incorrectly as the brake and switch should be releasing not engaging the handle is pulled against the handle bar.

Your reply doesn’t make sense, probably because we are using different terms. I’ll try again. This is a simple circuit, one kill wire and two components, kill switch and magneto. Both the magneto and kill switch are grounded through their bodies to the engine when properly mounted.
1. When the brake bail (lever) is pulled back the current generated by the coil is allowed to flow through the primary circuit of the magneto allowing the engine to run.
2. The kill switch is open at this point, not allowing current to flow through the kill wire.
3. When you release the brake bail the kill switch now closes, we will now have continuity from its terminal and ground.
4. Current will now be allow to flow through the kill wire, through the kill switch to ground. No current will go through the primary circuit in the coil, meaning no spark, stopping the engine.

Do have to add one thing to my previous post. With some of today’s Hall Effect trigger coils it is the opposite. Current is allowed to flow through the primary circuit through the closed kill switch to ground. When the kill switch is opened, current stops flowing, killing the engine.

StarTech said:

It appears that the engine brake cable is connected incorrectly as the brake and switch should be releasing not engaging the handle is pulled against the handle bar.

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Maybe it was worded poorly. That is what is happening. When pulled, brake and switch release, ie switch is open.

Rivets said:

Your reply doesn’t make sense, probably because we are using different terms. I’ll try again. This is a simple circuit, one kill wire and two components, kill switch and magneto. Both the magneto and kill switch are grounded through their bodies to the engine when properly mounted.
1. When the brake bail (lever) is pulled back the current generated by the coil is allowed to flow through the primary circuit of the magneto allowing the engine to run.
2. The kill switch is open at this point, not allowing current to flow through the kill wire.
3. When you release the brake bail the kill switch now closes, we will now have continuity from its terminal and ground.
4. Current will now be allow to flow through the kill wire, through the kill switch to ground. No current will go through the primary circuit in the coil, meaning no spark, stopping the engine.

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Can you tell me which part doesn't make sense? I'm taking what you've written as a given. I understand that's how kill switches work.


My question in a nutshell. With a properly functioning mower, with kill switch open/bail handle pulled back, should there be continuity between the kill wire and ground? This is with the mower off.

I say there shouldn't be. And there isn't when the kill wire is disconnected from the magneto. If there is continuity from kill wire to ground when the kill switch is open, wouldn't that prevent a spark?

You don’t understand what I’m saying. I’m chasing my tail trying to explain how the kill circuit on a small engine works. That being said I’m getting out of this thread and hopefully someone can explain it better than I can.

Rivets said:

You don’t understand what I’m saying. I’m chasing my tail trying to explain how the kill circuit on a small engine works. That being said I’m getting out of this thread and hopefully someone can explain it better than I

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You have explained it. I was already familiar. I'm looking for the answer to a specific question that is not being answered by your explanation.

Grounded said:

You have explained it. I was already familiar. I'm looking for the answer to a specific question that is not being answered by your explanation.

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To test the primary circuit of your coil, one lead of multi meter to coil mount, one lead to primary connector, (ground connector wire unplugged) you should have continuity, Or ground as you call it.

So Grounded, it appears to me that you have a perfectly operating mower with a kill switch that works when you release the brake/bail lever. Is that right? If so, this isn’t a problem, it is you pondering why your kill switch is working. I suggest you google “how does a kill switch work on a lawnmower?” There are some really good videos out there that explain it better than I can. But, if you want the short-cut, not scientific, answer, here’s how I think of it. The kill wire on the magneto is like a lightning rod. Its function is to divert the electricity that the magneto generated from going to the sparkplug. It won’t divert the energy unless there is a clear path for the electrons to follow to ground. That path is via the kill switch.

Grounded said:

Yeah, I understand that. That's why I'm confused about there being continuity between kill switch contact points when the kill switch is open. That would suggest the coil circuit is grounded whether the kill switch is open or closed, which doesn't make sense given that the mower starts and runs. Maybe have another read through my initial post to see the tests I did with the multimeter.

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"the mower starts and runs."
You are looking a gift horse in the mouth.

take an ohm reading of each, some coils have (the kill wire) have a low ohm reading, where kill side (ground) is 0 (ish)

So is it your idea the mower should not run anytime the brake is released?

The Brake kill is to prevent starting when the brake is off just as if you are not seated or the blade is engaged.

Depending on the mower it may have all or only one of these safety switches.

If you ground the coil primary before the contacts or the exciter or block the signal from the primary winding it will not run, If the signal passes through the primary circuit it will run simple as that any path that completes the primary circuit AFTER IT goes through the pulse will induce spark.

A dead kill switch grounds the signal before it reaches the coil winding.
A primary switch kills the direct current before it reaches the points or pulse inducer.

ohm reading contact point, magneto side, to ground at block / ohm reading contact point, ground side, to ground at block / all with bale pulled / different or same?

These ignition coils will read shorted using a continuity checker. This is because the primary winding is 1.5 - 1.7 Ω. When the kill switch is closed then it becomes a dead short
(zero Ω or near zero as the wiring has a little resistance).

Yes, hoping to answer 'Grounded', (Again, the mower runs. But I don't understand why, and I would very much like to know. Thanks.)

This should help a little...

Grounded said:

You have explained it. I was already familiar. I'm looking for the answer to a specific question that is not being answered by your explanation.

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See my question to you & Star Tech post & picture, does that help