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Classic Lucas Ignition/Light Switches

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martyn nutland05/06/2021 07:33:30
133 forum posts
7 photos

Hello All

An electrical query if I may.

I'm sure many of you remember the Lucas ignition/light switches that were fitted to many British cars (and a few motor cycles) in the late 30s and 40s. There was a key slot in the centre and then a short Bakelite toggle that moved an annulus with a window to expose the words 'off', 'side', 'head' beneath (the posh version). Alternatively, there was a tiny arrow on the toggle that pointed to the words engraved on an exposed annulus.

Question is, can I wire just the 'side (light)' function at the back of the switch to act as a simple on/off switch for a magneto.

I know I can buy on/off devices from 'the usual suspects', but it seems a waste when I have several of the Lucas type on the shelf doing nowt. But will it work?

Many thanks for your thoughts and best, as always, from la belle France.


John Olsen05/06/2021 08:15:58
1240 forum posts
94 photos
1 articles

There are probably others more familiar than me with this type of setup, however, here goes. I think usually the way of turning off a magneto is by switching a short across the points. There is of course no battery power going into a mag, so you can't turn that off. On the other hand, the light switch is usually arranged to switch battery power to a bulb. So there will be one connection to the battery live side, and as many output connections as it has functions, eg one for headlight, one for sidelight and so on. So the switch is intended to switch power to a load, while the magneto needs a connection made to ground for off.

So if the switch is being used to switch lights, it probably cannot directly be used to control the magneto as well, however it can be done indirectly. Suppose you want the "side" to turn the ignition on. You provide a relay with a suitable coil, 6V or 12 Volt depending on the battery. You connect a normally closed contact set across the magneto points. When the switch is in the on position, the relay pulls in, the contacts open, and the magneto operates as normal, When the switch is off, the relay drops out, the contacts close, and the sparks stop happening. If it is on a motorcycle, you want a good robust relay as vibration is not good for this sort of thing.

If the switch is not also being used to control anything else, you could also use just the switch on its own, however the "on " position would short the points, stopping things, and the "off" position would allow the engine to run. You wouldn't need a battery for that setup.


Chris Evans 605/06/2021 09:04:44
2050 forum posts

On my pre war magneto powered bikes I just use the decompressor to stop the engine.

Howard Lewis05/06/2021 09:52:39
6005 forum posts
14 photos

The standard "Off", "Side", "Head", switch merely connected the battery feed, to the side lamp circuit, and then to the Headlight circuit.

They came in three forms. In one the ignition switch was merely a knob that controlled the feed to the ignition

A more sophisticated version did the same thing, using a simple "spade" key. When worn the spade could fall out. leaving the engine running (I remember having to stop and retrieve my keys from the road, when they fell through the gap around the handbrake lever!)

The more secure version operated the ignition switch by a key switch. For many years the face of the switch carried the key code. Until clever thieves used a telescope to read the code, went and bought a key and unlocked the car to steal it without any need to "hot wire".

On vehicles fitted with a three brush dynamo,, in the OFF position it left two resistors in the feed to the third brush, minimising the charge rate to the battery.

The "Side" position shorted out on resistor, increasing the charge to compensate for the side and tail lights.

In the "Head" position the second resistor was shorted out, delivering maximum charge to compensate for the electrical load of the headlights.

To stop a magneto equipped engine, you have to short out the mag, as the piklots of rotary engined aircraft did during and after WW1.


SillyOldDuffer05/06/2021 09:57:30
8469 forum posts
1885 photos

From my armchair - I've never owned a magneto ignition engine!

I doubt shorting the magneto output with a Lucas switch of that type or an ordinary relay will satisfy. A magneto produces enough voltage to jump a spark-plug's gap when the fuel-air mix is fully compressed - several thousand volts. Thus an ignition system needs good insulation and any switch or relay in the circuit must have a sufficiently wide gap to avoid flash over.

My guess is the Lucas switch operated something like a solenoid mounted close to the magneto, and the solenoid closed a wide-gapped contact on a big block of Bakelite. John's relay idea, but engineered to short or open a high-voltage circuit.


bernard towers05/06/2021 11:08:56
568 forum posts
109 photos

As the mag is self contained you would have to draw wires from it internally, why would you want to do that?

Speedy Builder505/06/2021 11:53:45
2590 forum posts
207 photos

The "earthing" of the magneto to stop it working is normal done by shorting out the primary windings. The secondary windings produce the high tension. If this wasn't so, magnetos would have to have spark plug (High tension) cable leading to the cutout switch.

Have a look at most "newer" lawn mowers, there is a thin wire which shorts the magneto to stop the spark being produced. "Older" mowers like the Suffolk Punch Etc shorted out the high tension spark plug.

I see no reason why using a lighting switch wouldn't work so long as the internals of the switch permitted it - Ie: that the switch didn't have some form of lamp to indicate that the lights were on.

JA05/06/2021 12:15:52
1345 forum posts
80 photos

I only have knowledge of some post war Lucas motorcycle magnetos.

The K series, used on twins, were rotating coil magnetos and usually had a kill switch. This shorted out the contact breaker and was located either on the end of the magneto or the handle bar.

The SR series, used on singles, tractors dumper truck engines etc, were rotating magnet magnetos. When fitted to motorcycles these did not have a conventional kill switch, the engine was stopped using the valve lifter. However the engine could be stopped by pushing the name plate on the magneto against the body (this is difficult on my bike since the plate is very close to the front of the barrel). Using the mounting screw for the plate I did once wire in a handle bar kill switch to do plug chops.

I doubt if I have answered the question.


Aircraft engine magnetos are generally rotating magnet magnetos and these do have cockpit kill switches.

Edited By JA on 05/06/2021 12:18:17

john halfpenny05/06/2021 12:54:21
232 forum posts
24 photos

My vintage car has a simple lucas switch to earth the lucas mag points, and thereby stop the engine. OE fitment on the low tension 12v side - works perfectly.

old mart05/06/2021 17:52:47
3717 forum posts
233 photos

My RE Constellation, and Intercepter both had Lucas mags with the kill button wire connection in the centre of the end cap. I just used a press to make button on the handlebars. Not all mags had the stop facility, it might have been an extra.

Over the pond, Lucas was referred to as "the prince of darkness", although the worst electrical system I had on a motorcycle was on a PUCH 125 by BOSCH.

Edited By old mart on 05/06/2021 17:56:36

not done it yet05/06/2021 18:12:34
6719 forum posts
20 photos

All(?) mags have provision for shorting the magneto to earth. Some (mostly single and twin magnetos) have a manual spring loaded switch/contact on the mag and others have provision for connecting a wire for remote shorting wire.

That wire only requires shorting to earth potential to effectively stop the magneto.

Don’t connect 12Volts to the magneto.

martyn nutland06/06/2021 11:13:03
133 forum posts
7 photos

Thank you everyone for the comments, observations and advice.

I think what I propose should work without the risk of major explosion or conflagration.

I have a Lucas SR4 magneto which originally had a spring metal hoop on the side that you clearly had to press against the body to short matters out and stop the sparks. The person who rebuilt the unit dispensed with the spring metal (very sensibly, in my view) and has left me with a stud, washer and nut which is obviously intended to take a lead, that, I presume, will go to a switch with a connection to earth. Thus, again I presume, the switch will short the magneto to earth in one direction and leave it live in the other direction.

Another instrument I have is a Bosch ZA4 and that has a thin lead coming out of the contact breaker cover, presumably for the same purpose as the set-up on the Lucas SR4.

The magneto I'm concerned with is a BLIC N4R and although it has no external leads at the moment other than those for the sparking plugs, I clearly need to provide something very similar to that on the Bosch.

That said, if the aforementioned lead is connected to the side light terminal on my Lucas ignition/light device and another lead goes to earth, switching from 'off' to 'side', the original side/tail lamp function, should activate and de-activate the magneto....shouldn't it!?

Thanks again for all the valuable input.


Steviegtr06/06/2021 22:58:49
2421 forum posts
336 photos

Short answer is yes. The usual way i knew about was a horn button on the handlebars that just shorted the single wire to the handlebar.


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