Poring over some old Lucas publications recently, I came across a few things which I thought might interest some forumists.

1) Spark polarity.
Coils are normally wound to give a positive earth spark - that is the spark plug insulated electrode is negative with respect to the block; this usually refered to as a negative spark.
This has the advantage of the same sparking efficiency but at roughly a 10% lower HT voltage required to break down the plug gap, thereby reducing the "strain" on the HT lead insulation and that of the distributor cap and rotor arm.
A further advantage is little or no wear at the rotor arm, instead the wear is shared between the 4 brass electrodes in the distributor cap.
This is an excellent indicator of whether you have a negative spark(GOOD) or a positive spark (BAD). Clean the burn marks from rotor arm and distributor cap segments, then after running the engine for a hundred miles or so inspect both; if the burning is most evident on the rotor arm you have a positive spark.

How can this be? The coil "CB" terminal is connected to the distributor and the "SW" terminal goes to the ignition switch as it should be.

Up until mid 1935 all A 7s were wired with the battery (-)negative connected to the chassis. In mid 1935 the British motor industry generally, and Austin were no exception, changed over to the battery(+) positive being connected to the chassis. Consequentially, all coils made after this date (until, I think, sometime in the mid '60s when the industry reverted to negative earth) unless specially labeled "for negative earth only", were wound for positive earth installations. If an earlier, negative earth car has been fitted with a later coil designed for positive earth the spark polarity will be wrong. My 1929 Chummy was such a car having been fitted, at some stage of it's life, with a 1938 coil.
The solution is simple, just swap over the two wires on the coil such that the "CB" terminal is now connected to the ignition switch, and the "SW" terminal is connected to the distributor.
In theory there is a minor disadvantage in doing this as the additive auto transformer effect within the coil is lost but the advantages of negative spark easily outweigh this.


I'll add further Ignition snippets to this thread as the mood takes me.

Ian Mc.

Location: Shropshire

Great stuff Ian, keep it coming please!

This one is more of an anecdote than a snippet (!!) but, it may strike a chord with someone.

2) The Dilemma of the DJ4 Distributor.

Some years ago, when I first acquired my current 1929 Chummy, I could see that the rotor arm on the fitted DJ4 distributor had seen better days but I noticed that it was only eroded ( due to spark action ) at its leading edge.(I now know that the excessive erosion was due to “positive spark” – but I didn’t then!)

I had a spare Distributor cap to hand ( identical to the one fitted to the DK4 distributor that my previous car, an ARR Ruby, had ) and I bought a new rotor arm; I fitted both new parts, the engine started and ran OK and I felt fairly pleased with myself.

About 100 miles or so later I decided that as the condenser looked as if it could have been the original fitment for the distributor I would replace that too as part of my preventative maintenance. Eventually the condenser that I had ordered arrived and I was fitting it when I noticed that the new rotor arm and the new distributor cap were showing signs (the grey-black markings) that the spark was occurring as the leading edge of the rotor arm brass segment was just arriving in line with the leading edge of the HT segments of the distributor cap, just as the wear on the old components had indicated.

So, I had a serious think about how this situation could be, as the ideal design of distributor would allow for the spark to occur when the rotor arm brass segment and the distributor cap HT brass segments are in complete alignment.

The cam controlling the points opening and the rotor arm locating “D” sector are machined from the same solid steel bar so are decided at the design stage of the distributor.
If the rotor arm is manufactured to design specification and, if the points are manufactured to the design specification and gapped correctly the spark should, as previously stated, occur when the rotor arm and the HT segments in the distributor cap are in complete alignment.
What was wrong?

The rotor arm that every supplier of Austin 7 parts will sell you for the DJ4 distributor fitted to an Austin 7 is a Lucas 401122 or pattern equivalent thereof.
The points similarly supplied are the ST361 (Lucas 400833) for which there appears to be no alternative available today that will fit the DJ4.
Again, so what was going wrong?

Having made a few relevant measurements, and done a few calculations, I came to the conclusion that the points were opening approximately 40 distributor degrees before the rotor arm was ideally placed to distribute the spark. This difference could not possibly be compensated for by changing the gap setting of the points.
Yet again, what was wrong?

The 401122 rotor arm design and orientation is exactly what I would expect to find on a manual advance distributor. I have no documentary evidence to support my opinion but, I believe that the Lucas 401122 rotor arm is as specified for the DJ4 fitted to Austin 7s originally. If we accept this, a possible conclusion is that the ST361 points available today are not identical to the points originally designed for the DJ4 fitted to Austin 7s.
It seems unlikely, however, that any “pattern” points would be sufficiently different from the original to cause this considerable error.
`So, once again, what was the cause of the problem?

After much head scratching I had a thought. The dwell angle is approximately 45 distributor degrees; this 45 degrees different location is exactly where the points would be opening if the distributor shaft was rotating clockwise (as viewed from the drive end) instead of the (correct) anti-clockwise rotation.
Hurriedly cleaning the muck off the distributor information plate and using a very small inspection mirror I discovered that the distributor plate showed the correct part number for the Austin 7 and showed an arrow indicating the correct ( anticlockwise viewed from the drive end) rotation. So all should be well but, it wasn’t!

By a stroke of good fortune, whilst browsing the Web, I came across the Lucas 400823 rotor arm. This fits the DJ4 distributor, is designed for anticlockwise distributor rotation (as viewed from the drive end) which the Austin 7 has, and it has a “hook” extended brass segment that will take care of the misalignment. Presumably it was manufactured for the DJ4A distributor with auto advance (never fitted to A7s)

The alternative 400823 rotor arm showed the same point of spark erosion but this was now accommodated by the extended brass segment.
I had cured the symptom not discovered the cause but, despite this, the problem was solved.
There is precious little spare spark energy in a 6 volt coil ignition system, I believe in trying to make it as efficient as possible without straying too far from originality. I believe that this rotor arm, the Lucas 400823, does that in these circumstances.

Many months later I had the opportunity of comparing some close-up photographs of a DJ4 on an Austin 7 that didn’t have my problem, when compared with my own distributor the true cause of the trouble was revealed. The rotor arm locating “D” at the top of the shaft was machined at a slightly different angle to the square cam from that on my distributor.
Despite the information on the distributor plate, my distributor, no doubt to overcome wear issues at some stage in its long life, had obviously had the cam shaft changed.
The “new” shaft was designed to be used in a DJ4 distributor with clockwise rotation!

Eureka!!!! The mystery was solved at last

However, having already overcome the problems caused (by fitting the non-standard rotor arm) I decided to leave the incorrect shaft in situ. As the square cam is symmetrical it makes no difference to performance which way it rotates.

For anyone interested, the only source of the 400823 rotor arm that I have been able to find is:
http://www.vintagemotorspares.com/

It is his part number 136 and costs £ 6.92 plus P&P

Location: Shropshire

Bl**dy marvellous and not an argument in sight!

Wait a while, Ruairidh, Tony is probably still snoring!

Ian Mc.

Location: Shropshire

I can add a bit more. Ian's Chummy used to have a rotor arm with the hook as he describes. I know that for a fact because that rotor arm is in a draw in my garage where I put it all those years ago when I replaced it with the incorrect rotor arm that Ian also describes. In my defence, it is pretty well beat up and has seen much better days.

Oops

Steve

PS: I'm pretty sure I know who it was who replaced the shaft in the distributor as well but that, at least, wasn't me!!

Location: Nr. Selby, North Yorkshire

Steve, I know you too well to suspect you of any mis-engineering, the thought that it might be you never crossed my mind.

You know what I'm like with electrics, I had to make it functionally efficient if un-original!!

Hope you and Ange had a great Christmas, and I wish you a happy and healthy 2014.

Ian Mc.

Location: Shropshire

Location: On the very edge of Europe...

Ian, re: negative spark vs. positive spark..... does it follow that the opposite is true with a positive earth car? In other words will the polarity be wrong if a modern (presumably negative earth) coil is used with a positive earth system?

Location: Derby

Hi Sandy, I hope all is well down there.

What I should have said, of course, is "never an original fitment on Austin 7s". I guess there isn't much that hasn't been fitted to some seven somewhere over the years.

Save me a piece of "stargazie pie"

Ian Mc.

Location: Shropshire

Location: Shropshire

Thank you Ian that is clear.

Location: Derby

Not really Austin related, but Citroen 2CV spark plugs are connected in series and spark twice as often as needed (so no distributor is required), so one is positive spark and the other is negative spark. It is recommended that you change round the plugs at 3000 miles then replace them at 6000 miles.

Location: Isle of Wight: the Wight place to weld rusty heaps

I have encountered sparks part off the rotor on other make cars with vac advance. Repeat dressing of points and worn rubbing contact also shifts things slightly. After any major rehash of 1950s/60s cars I always turn motor and note exact position of ign. Note that spark continues for several crankshaft degrees, and half same on distributor. Severe wear of rotor and/or studs is the obvious symptom.

Location: Auckland

Location: On the very edge of Europe - where it's a gale and wet, wet, wet.

Location: Shropshire

Location: Shropshire

I have owned several 2CVs over the years and can vouch that the coil does cause starting problems if hot. On my last European Tour in a "sports" model I got so fed up that I modified the coil holder so that I could remove the coil and swap it with a cool one in a matter of seconds.

Hi
Ive recently put a new 6v coil on my 32 RN phaps done under 100 miles since fitting it,and the rotor is burnt?I connnected CB to contacts and SW to igntion as the original coil was fitted this way round, do you suggest i swap them around for a stronger spark?
Thanks and a Happy New Year

Location: Shropshire

Ian,

Your information on the Austin ignition system caused me to read much more than I ever wanted to about the automobile coil.

Firstly- there are some very confused (and confusing) people contributing to the internet

Secondly - with the early negative earth Austin 7 system - surely no matter which way the coil is connected to the electric wiring, the earthed plug body (and side electrode) must always be negative polarity. Presumably the centre electrode is positive polarity. From reading it seems that the current flow is always from the positive battery terminal to the earthed components.

Also I understand that the electron flow* is from the negative battery terminal to the positive, so the talk of helping by sending electrons from the plug centre electrode doesn't seem to apply either.

*Although I read that it isn't a flow really, strangely more a jiggle- imagine if steam worked this way- or even petrol, things just wouldn't be the same.

I am confused as to how changing the feed to the +ve and -ve connections on the coil will alter the flow of current to the plug if the battery connections aren't changed.

Enough - back to simple mechanical things which don't make my head hurt.

Tony.

Location: Malvern, Melbourne.

Oh Tony, I'm sorry to make your head hurt - are you sure it isn't too much Fosters over Christmas??

You need to understand that the voltage produced by the secondary (HT) winding of the coil is there by virtue of mutual inductance from the primary (LT) winding. Reversing the current flow in the primary winding will reverse the polarity at the "hot" end of the secondary winding; all regardless of whether the electrics generally are positive earth or negative earth, Simples!
If your head still hurts, just trust me on this. I promise not to question your knowledge and understanding of bearings.


Ian Mc.

Location: Shropshire

I thought it might be something like that, but in all the dross on the internet there was no explanation that the polarity of the secondary coil system depended on the primary coil feed direction.
I wondered how this secondary coil polarity affected the primary circuit at the point of connection between the two ?

Tony.

Location: Malvern, Melbourne.

Read Faraday's Laws of electromagnetic induction

Location: scarborough Western Australia

The most widespread version of Faraday's law states:

The induced electromotive force in any closed circuit is equal to the negative of the time rate of change of the magnetic flux through the circuit.

Tony.

Location: Malvern, Melbourne.

Location: Shropshire

Ian, how might I check for a negative spark by using test equipment?

Location: Derby

Ian,

Thank you- now I see why some coil pictures showed the primary/secondary connection at the positive terminal, others at the negative terminal.

I must say that thinking about Faradys experiments with a moving magnet inside a coil producing a reversing current flow had me slightly confused when relating it to a collapsing field in a fixed system, but now I believe I understand

I think that the various internet stories on coils would benefit from your explanation, particularly with reference to the +ve vs -ve battery connections and the appropriate coil

With reference to checking polarity, there are a few ways mentioned - one using a pencil and spark??? Maybe you have something easier.

Tony (whose head has stopped hurting- maybe the cricket?).

Location: Malvern, Melbourne.

In my earlier life as a motor vehicle lecturer in various technical colleges, spark plug negative polarity was part of our ignition studies. The system was first introduced in the early 1960s, and did indeed reduce the voltage required from the coil to fire the spark. Prior to the introduction of this system, the spark jumped from the earth electrode to the centre electrode. This is the opposite of what was generally supposed. The change in spark plug polarity was effected by reversing the internal connections of the secondary windings inside the coil. It cannot be achieved by reversing the external primary ignition coil leads. Prior to the introduction of this system, when checking the spark plug gaps you could see where the earth electrode was eroding away in service. With the new system, the earth electrode remained square whilst the centre electrode became rounded by spark erosion. I am not aware that 6 volt coils as supplied as replacements for original equipment were ever changed to the modern system. So if your spark plug earth electrode is till eroding away, you are stuck with it.

Location: s.wales

Location: Shropshire

Thanks Ian and Tony. I will try this on my '26 Dodge Brothers tourer which has a 6 volt positive earth system. If I have got a positive spark it might benefit me to change it.

Location: Derby

Location: Shropshire

Ian

My Father did an article for the Devon club news letter about this. He spent many happy hours making a test rig using a pencil to show the blue flash denoting polarity and many more happy (maybe!) hours trying to photograph it!
His findings also support your and Lucas's view on changing polarity.

Stuart.

Location: Devon

Location: Shropshire

Readers, trust Ian, he has it sorted.
Some claim that simply reversing battery is nbg but, as Ian mentions, some catalogues seem to condone this. The auto transformer effect will be lost connected the unintended way, but the ratio is hundreds of pri turns to many thousands of secondary, so the % voltage reduction is small, and, as per Ian, more than compensated if proper neg spark achieved.
Beware pulling long sparks, or gaps so large that none, as voltages are then very high and stress rotor, cap, coil etc. In moderns, the practice stuffs the electronics, a severe calamity, and also delivers lethal voltage. Even a Seven will deliver a wallop dangerous to aged hearts, more so if an electronic conversion. Dont forget the middle of the pencil is graphite!
Rotors firing beyond the end also stress components.
My interest in cars and electricity began about age 3 watching my father pull long sparks from our/my Seven in the dark!
Don't try for long sparks, but the corona etc assoc normal running in the dark can be fascinating. A whole new topic!! Even more open to speculation and uninformed comment!!

Location: Auckland

You can check spark polarity using an analog (moving needle) volt meter. Hook up a voltmeter with the negative lead to the plug terminal and the positive lead to the block. Set the meter on the highest volt range. Crank the engine over (no need to start it), and you should see an upward swing of the voltmeter needle (don't be concerned with taking a reading). If the needle swings down off the scale, your coil is hooked up wrong. To correct it reverse coil primary leads. Do not worry about the coil markings, but make note of them for future reference.

Reference http://mgaguru.com/mgtech/ignition/ig104.htm

Location: Scarborough Western Australia

Tony

Any law of physics cannot have a version they are proven scientific understanding. This applies to both of Faraday's laws of induction, what you need to appreciate is the second law.

Faraday's Laws of electromagnetic induction
Faraday's First Law
Any change in the magnetic field of a coil of wire will cause an emf to be induced in the coil. This emf induced is called induced emf and if the conductor circuit is closed, the current will also circulate through the circuit and this current is called induced current.
Method to change magnetic field:
1. by moving a magnet toward or away from the coil
2. by moving the coil into or out of the magnetic field.
3. by changing area of a coil placed in the magnetic field
4. by rotating the coil relative to the magnet.

Faraday's Second Law
States, the magnitude of emf induced in the coil is equal to the rate of change of flux linkages with the coil. The flux linkages of the coil is the product of number of turns in the coil and flux associated with the coil.

Source reference: http://www.electrical4u.com/faraday-law-of-electromagnetic-induction/

For another explanation
http://en.wikipedia.org/wiki/Faraday%27s_law_of_induction


In conclusion for the purpose of this conversation in any coil you will will "generate" one volt per turn of wire ie primary 200volts 200 turns secondary 18Kv 18000 turns.

Location: Scarborough Western Australia

Sorry fellahs it is fatally easy to enter confusing statements. I mentioned reversing battery but meant battery connections to coil ie reverse coil terminals.
I know the analog meter method is often used to check spark polarity but without some well insulated resistor voltage divider the prospect of very high voltage in meter is worrying.
Someone mentioned an oscilloscope. For the curious the pri coil connection produces the most incredibly busy and lively trace.

Location: Auckland

Hi the coil is brand new,the rotor is more burnt than the brass segments and the positive terminal on the battery goes straight to the starter motor,so does that mean negative or positve earth?
Thanks alot

EV Your electrical system is negative earth.

From your description I would say that you have "positive spark" which you could check with the "pencil point in the spark test" as described earlier in this thread.
If you wish to change your spark polarity, just swap over the 2 LT leads on the coil terminals.

Ian Mc.

Location: Shropshire

For what it may be worth, old Lucas catalogues from 1980 list exactly the same replacement coils, with + and -, for models which span pre and post 1965 when battery earth polarity was generally changed (ie Minx range).
But a footnote that coils designated CB and IGN for +ve earth only!!

Location: Auckland

Gents. This all makes my head hurt.

Just to make it clear in my mind

If car is -ve earth. Post marked -ve on the coil goes to dizzy?

If car is +ve earth. +ve post to dizzy?

My daughters RP is currently U/S. Not spent too long on it yet but I believe +ve earth. But -ve post goes to dizzy.

It has required lots of cranking to start in the time ive had it. And ran poorly till warm.

Interestingly there were a pile of burnt out coils in dads shed. If they are wired up **** about face do they burn out?

Hedd,

YES

YES

NO but, what does knacker coils is excessive plug gaps, worn rotor arms and dizzie caps, and dodgy HT leads and connections. All of which require the voltage of the coil secondary to build up above its design value before the spark will occur, thus stressing the coil.

Ian Mc.

Location: Shropshire

Ok.

Not used to High compression heads.

Running a willie Bosch clone dizzy and champion plugs what is recommended for plug and spark gaps?

Cap and rotor do look a bit dirty. Leads are not that old.

I had feeble spark on saturday with a flat battery. But after a night charge I currently have no spark despite swapping the condensor. Gave up when the rain came. Will set too with a multimeter next time Ive got a minuite.

Im certain the coil is wired up **** about face.

This was dads car, I dont know how old bits are like I do with mine.

To the best of my knowledge AMC never varied from their recommendation for plug points to be set at 18 to 20 thou (though I tend to set mine a little wider at 22 to 25 thou without a problem - so far!).
Bosch recommend ( I believe ) that points on their 009 distributor be gapped at 18 thou. (see page 13 of PWA7C mag. for MAY 2016)

Ian Mc.

Location: Shropshire

When this topic first appeared, the comment about auto transformer effect set me a pondering. When the points break a high voltage appears across the pri; if + earth this is – at the points end, if –ve earth, + at the points end. The secondary if connected in the same direction experiences an induced voltage of the same polarity. I have drawn every conceivable combination of direction of connection of secondary and leads etc but I can only get a –ve spark in series additive with the pri and battery (and hence earth) with + earth battery. (Points in effect tapped off part way up the coil)
Maybe I have got it wrong somewhere (again, according several) but perhaps this was the reason for adoption of + earth, not reduced corrosion, the usual somewhat curious “explanation”. It is a few hundred volts cf several thousand, so not vital. I did not consider the lesser earth path thru the condenser.

Anyone who enjoys puzzles can try diagrams for themselves.

It is all related to Lenzs law which, as Tony notes above, the internet seems to have made incredibly complex since it was taught to my father on a trades course in the 3rd form, somewhat before. You would think someone could provide Wikipedia with understandable examples?

In my Seven when in everyday use I could detect reduced harshness with smaller plug gaps. I reckon a smaller spark effectively retards the ignition process slightly.

Location: Auckland, NZ