Kill Button

I'm asked from time to time, and I see posts asking things like: "Can my kill button cause ______?" The quick answer is "possibly". This discussion is pre-MKIII only.

With standard Norton factory wiring, the kill button is a normally closed set of contacts that open when the button is pushed. Power to the coils is thus interrupted. The ignition circuit is not the most robust. From the battery negative there is a wire to the master switch and from there a wire to the jumble of connections under the tank from there to the handlebar console with the kill button from there back to the jumble of connections under the tank from there to the ballast resistor and finally from there to the coils (If I didn't miss any, 6 spade connectors, 8 bullets, 4 bullet connectors, and the kill button contacts).

Spade and bullet connectors connected one time and left untouched can maintain a good connection for a very long time even if the outside is corroded as the initial metal-to-metal initial contact usually causes a "gas tight seal". However, disconnect and reconnect just once with a little corrosion and the connection won't last. The kill button contacts are open to the air and have a tiny contact point. Resistance will build up in those contacts over time.

Any connection or contact that corrodes becomes a resistor and therefore, there is a voltage drop across the connection. See: https://www.gregmarsh.com/MC/uvd.pdf for a little more depth.

A points ignition will seem to work at 10 volts to the ballast resistor. The spark is lower voltage than normal but there still is one. Some electronic ignitions will also work. For instance, Tri-Spark specifies that the minimum is 8 volts for it to do its job. However, what they don't say is that at 8 volts the spark will likely be too weak for the engine to run. Best I can tell Pazon requires 10 volts minimum. One source says that Boyer requires 11 volts (I have no personal experience with Pazon or Boyer).

So, "My bike was running perfectly and now I have no spark. Could my kill switch cause this?" Yes, it could. The quickest test with a fully charged battery is to run a wire from the battery negative (positive ground) to the ignition's power lead if an electronic ignition or to the White/Blue wire in the jumble of connections under the tank if points – this will eliminate the entire ignition circuit. If it then works, try disconnecting the White and White/Yellow wires that come from the handlebar switch from the jumble under the tank and replace with a jumper where they came from in the jumble. If it then works, the kill button is the cause; otherwise, it's one or more of the connections in the circuit.

My kill switch became intermittent within a few years of the bike being new. Interrupting ignition current will generate a spark just like the points do. The switch contacts get burned and lose continuity. Maybe they should have put a condenser across the kill switch? The spring is just a bent metal arm (phosphor bronze?), not strong enough to do the job. If you're on the roadside and suspect the switch integrity, sometimes "snapping" the button a few times will remake the contact.

I bypassed mine more than 40 years ago. I intend to reinstate it, with a relay doing the coil switching. And a capacitor across the relay. I also think it's a good idea to mount a small "pilot light" on the coil bracket for a visual indication that the coils are powered.

when I rewired my bike (no more bullets) I installed a REAL switch at the handlebar, and also a switch on the side stand which operates a buzzer.
The buzzer tells me:
1. that the side stand is out.
2. that I have current ( no flat battery)
3. That the Pazon gets current (90% sure..)
4. occasionally reminds me that I forgot to switch off the ignition.

I would not start a bike without a functional kill switch.

ludwig said:

I would not start a bike without a functional kill switch.

Click to expand...

marshg246 said:

A points ignition will seem to work at 10 volts to the ballast resistor. The spark is lower voltage than normal but there still is one. Some electronic ignitions will also work. For instance, Tri-Spark specifies that the minimum is 8 volts for it to do its job. However, what they don't say is that at 8 volts the spark will likely be too weak for the

Click to expand...

I like both responses, but see each with a common issue. What do you use a kill switch for when the master switch is more effective? kill switches, typically are employed to shut down a runaway engine or to keep a fuel leak from toasting you and the motorcycle; assuming you notice and have the time and brass to use the kill switch.

The common issue is that a push type kill switch needs to be pushed and held down which pretty much makes your right hand a noncombatant in the emergency you find yourself dealing with.

As Greg points out corrosion increases resistance (E=IR), dielectric grease used after a good cleaning and a bit of pressure on the female part of the connection can also eliminate (greatly reduce) spurious resistance.

However, if your wiring harness is factory original all bets are off...

Best

Our antiques have single pull throttles that can stick.
They can stick for several reasons.
A thumb operated kill switch sure comes handy during such an issue.

Modern day throttles are push pull to forceably close the throttle.

If kill switch can be corroded from actual use on regular basis, it the ign switch also likely to suffer contact corrosion if it used instead?

Tornado said:

If kill switch can be corroded from actual use on regular basis, it the ign switch also likely to suffer contact corrosion if it used instead?

Click to expand...

Not as much. The kill contact patch is tiny and more open to the air. The master switch is more enclosed, and the contacts are more robust. Also, the master switch contacts rub while being turned - the kill is more like points. Besides, you have to turn the bike off with the master switch so there's no way to reduce it's use.

MichaelB said:

Our antiques have single pull throttles that can stick.
They can stick for several reasons.
A thumb operated kill switch sure comes handy during such an issue.

Click to expand...

Assuming that none of those reasons can lead to a life threatening or an engine meltdown scenario(s). If you were referring to a toggle type, then I misunderstood, and apologize for second guessing you.

Like the horn you (most riders) have to think about using either or both, time that may be useful doing something else. The bikes I build have toggle kill switches; fortunately I've not needed to do a hot-kill, but I'm glade to have that option so I can turn the fire hose onto something else, or clean up for a meeting with my Maker.

Best.

Not sure who's for what and I'm just being a devils advocate (or a-hole)

If you've ever been on fire with gasoline, and you take time to think stopping the engine on your bike from running, the kill button is not your friend. A toggle kill - I'll buy a case for that. The master switch, maybe a case for that. However, I promise you, you won't stand there holding the kill button until the engine stops!

I'm also confident that you will be thinking about getting the fire out and not your bike. Yes, I have been on fire with gasoline - 60 years ago and I remember every millisecond of the experience vividly! I remember rolling on the ground trying to put it out, I remember the old drunk yelling to keep rolling, remember seeing a doghouse next door with an Army blanket on the ground, I ran for the blanket and wrapped up in it - finally got the fire out. Almost worse was getting to the hospital where they had a big tub of ice water waiting for me, the giant blisters forming and breaking, the stink of my hair mostly gone, the pain.

Seen a fair number of bike crash video's where the modern machine is laying on its side, engine still running, rider over ina ditch someplace. Bystander/other rider runs up to hit the kill switch to shut it down. So I guess this is why they are standard fair on bikes now (legal requirement?).

Tornado said:

Seen a fair number of bike crash video's where the modern machine is laying on its side, engine still running, rider over ina ditch someplace. Bystander/other rider runs up to hit the kill switch to shut it down. So I guess this is why they are standard fair on bikes now (legal requirement?).

Click to expand...

Fair enough, but I wouldn't even know where to look for a button or switch on a modern bike, but a key would probably be easy to find. A vintage Norton or Triumph on their right side would make the key very easy to find. On the left, nearly impossible in your scenario so the kill would be required and maybe findable if on the right side.

marshg246 said:

Fair enough, but I wouldn't even know where to look for a button or switch on a modern bike, but a key would probably be easy to find. A vintage Norton or Triumph on their right side would make the key very easy to find. On the left, nearly impossible in your scenario so the kill would be required and maybe findable if on the right side.

Click to expand...

kill switch is almost always on right switchgear. Quite often it is a red rocker.

marshg246 said:

Fair enough, but I wouldn't even know where to look for a button or switch on a modern bike,

Click to expand...

Possibly useless trivia but "modern" would be from approximately 1975 production due to the requirement for a toggled engine stop switch on the right-hand control that was part of US Federal Motor Vehicle Safety Standards No. 123 (along with the change to R/H rear brake, L/H gearchange, identified switches, warning lamps, etc., etc.) introduced at that time, therefore, why the 850 Mk3 has the sliding ("Off-RUN-OFF"- ENGINE STOP",) kill switch (and Triumphs then had the red OFF-RUN 'flag' toggle) the previous momentary push to kill button no longer allowed.

Learn something every day - MKIII and T160 are too new for me so you can imagine what I think of bikes a few years old. Of course, a bike down on its right side may well hide the kill and we're back to the key.

A toggle does make a lot more sense to me than a momentary contact push button.

marshg246 said:

A toggle does make a lot more sense to me than a momentary contact push button.

Click to expand...

You can do your racing gearchanges with the kill button, keeping the throttle fully open.

marshg246 said:

Learn something every day - MKIII and T160 are too new for me so you can imagine what I think of bikes a few years old. Of course, a bike down on its right side may well hide the kill and we're back to the key.

Click to expand...

Well, at least if you happen to encounter one of those new-fangled Mk3s or Triumph twins/T160s down on its left side (has to be a 50-50 chance) and revving itself into oblivion then hopefully you will know what action to take to cut the engine.

Here's the Triumph switch so anyone should be left in no doubt about what to do especially if the no-expense-spared stickers have fallen off although (more trivia) the earliest T160 kill switches operated in the opposite direction, so down was ON (in the traditional UK switch operation direction) but apparently contrary to US FMVSS 123 which stated the advanced positions of switches should be selected by moving the switch toggle in the 'upward' direction.

RoadScholar said:

What do you use a kill switch for when the master switch is more effective?

Click to expand...

Using the master switch requires removing one hand from the handlebars. In the case of pre-MKIII Commandos that means letting go of the clutch. I like to shut the motor off while in first gear at a stop, so that the bike won't move when I get off. I use the kill button all the time.

Doing a plug chop is another time where the kill button comes in handy. Yes, I wish it had been a toggle instead of a button. Thanks, Mr. Lucas.

For a few years I ran a keyless ign setup on my modern bonneville. Had a remote fob to carry in a pocket. To start bike, just had to walk up to within a yard or two, running lights flash. Clutch in would then power on ign, starter button then to fire it up. To shut it down, kill switch toggled to off. So the switch got used a lot.

There was some blurb ib the Triumph shop manual about not using kill switch regularly to protect some unstated aspect of the electrical system. Perhaps had something to do with everything except ign circuit still powered up once kill toggled. If left in that state for prolonged period, battery drains from headlight.

Have also heard of people using kill switch on modern bikes to shut off engine at traffic lights or in traffic jams to save fuel. Bike is ready to restart with a kick of the electric leg.

Lack of a kill button killed my 650ss back in the 90s when it still lived in Ontario.
The story is that the new owner stored it over winter in a damp area. Corrosion in the throttle cable caused the slides to stick wide open on starting. While the owner was fumbling for the key switch the revs went to the moon causing a connecting rod to break , the jagged end exiting thru a crankcase half.
The bike became an insurance write off, went to a Breaker's yard and was spotted by none other than Herb Becker.
Herb bought the wreck and donated it to the Ontario Vintage group for a resto project. Eventually it was completed then raffled off to an non riding winner. He kept it for a few years then sold it to me.
So in this case, it worked out well for me that the bike did not have a kill switch, although it does have one now!

Glen

MichaelB said:

Our antiques have single pull throttles that can stick.
They can stick for several reasons.
A thumb operated kill switch sure comes handy during such an issue.

Modern day throttles are push pull to forceably close the throttle.

Click to expand...

And even those modern bikes with push pull throttles have a good thumb operated rocker type kill switch.
That is one of the benefits of using modern switchgear. I used all clean second hand GSXR stuff on the Egli and on the 920. It's cheap to buy on eBay, there is scads of the stuff.
Designed and made in Japan, the switches have a failure rate hovering around zero.
Not traditional looking of course, so not to everyone's liking.

Glen