Morris Minor Owners Club

Alec wrote:Hello Nick, <BR><BR>yes, across the battery is as good as anywhere.<BR><BR>Alec

<BR>The voltage is 12 at idle but does not alter o&shy;n revs. The battery is not running out I have not had to charge it. Maybe I need a new multimeter as well!<BR>Nick<BR>

Alec wrote:Hello Cam,
I'm sorry, but the polarity of the coil does not alter its configuration, i.e. series or parallel, nor is the current more if it is reversed. From your drawing they are clearly parallel. By the way what is th component after the capacitor?
The temperature of the centre electrode is significant. It is exactly how a thermionic valve is able to operate.

Alec

Alec,

It DOES alter things significantly, which is why the power is down quite a lot when it's reversed incorrectly.

If the 'centre' tap is being earthed then the HT is developed only across the secondary side. If the 'end' tap is being earthed then the HT is developed across the secondary AND primary sides due to the connection.

It's quite awkward to see, but if you have a search on the 'net then I'm sure there will be a clearer explanation.

With regard to the electrode being heated there are a number of differences to that of a thermionic valve. Firstly, only the cathode in the valve is heated to start the electron 'stream'. the valve does not work unless this cathode is hot (same with a cathode ray TV tube). Obviously a spark plug is not using the cathode ray emission principle otherwise the car would never start from cold!! A spark plug will work hot OR cold, a thermionic valve or cathode ray tube won't.

A spark plug gap is short enough for the PD generated to break down the dielectric (air) and cross the gap. The gap across a cathode ray tube and thermionic valve are significantly larger so the thermionic emission principle is employed.

Hello cam,

this could go on for some time, but the primary side of the coil is not a part of the generation of the high voltage for the plugs. The temperature of the spark plug centre electrode is the reason that the coil polarity is so specified. A wrongly polarised ignition system needs a higher voltage to generate the spark, and bearing in mind that the spark voltage requirement increases with combustion pressure, so high revs and a high compression engine may show up the deficiency but a Morris 1000 will probably be less sensitive.

Alec

this could go on for some time, but the primary side of the coil is not a part of the generation of the high voltage for the plugs.

That's right if wired correctly. If the LT side is reversed, then it becomes part of it due to the LT and HT side being connected together internally (bit like a potential divider). This is NOT supposed to happen 'normally' but it seriously degrades the output voltage.

The temperature of the spark plug centre electrode is the reason that the coil polarity is so specified.

I disagree, although it plays a part, the major part is the output of the coil if the LT side is wired incorrectly. This is well known (have a search on the internet) and can be measured.

A wrongly polarised ignition system needs a higher voltage to generate the spark.

Yes, beause the coil is producing less voltage.

A wrongly polarised ignition system needs a higher voltage to generate the spark, and bearing in mind that the spark voltage requirement increases with combustion pressure, so high revs and a high compression engine may show up the deficiency but a Morris 1000 will probably be less sensitive.

Yes, because the coil is producing less HT voltage! For the same HT voltage and atmospheric pressure (and dielectric properties) a spark will be produced either way around at the same intensity. The 'gap' is sufficiently small enough so that the thermionic effect is small. Besides which, I bet the temperature difference between the electrodes is not that much in practical terms anyway. Of course spark erosion is the issue which is why the correct polarity HT is important.

If you do not agree with me then fair enough, and if you like, do a little research into it and for the moment we'll agree to differ.

Hello Cam, <br>my information comes from a book called Tuning Lucas Ignition Systems by Speedsport Motorbooks ISBN 0 85113 06 3 1 which gives a very detailed explanation of the theory. Incidentally the diagram in this book is different to the o­ne you posted in that the common terminal is the feed and the other primary terminal is the return.<br>It also quotes the optimum centre electrode temperature to be in the 500 to 600 degrees C, which to me is significant.<br><br>Going back to the coil itself, the current flow is not coincident, i.e. the primary flows current, then the secondary, so I do not see what effect the reversal of connection would have. I would like to know what references you have found o­n the internet so that I could have a look at them.<br><br>Alec

Just do a general search on google for ignition systems. There's plenty of info out there. Trouble is with the internet (and with scientific papers and books) the information is not always correct! I have read MANY publications where there are serious errors. It's a shame it happens, but the reviewers are often not capable of fully understanding the technical implications of all of the publications!

Yes, 600 degrees C is significant but what is the DIFFERENCE in temperature between the two electrodes?? probably not that much.

Going back to the coil itself, the current flow is not coincident, i.e. the primary flows current, then the secondary, so I do not see what effect the reversal of connection would have.

Well, if the HT and LT are connected together (which they are) and the earth point is not the 'centre' of the two coils (the connection point) but the other LT terminal, then there will be 12v at the centre connection point and the HT will be felt from the output HT connection TO EARTH which now incorporates the LT coil in series with the HT one. I admit it's difficult to see, but easier if you draw the two coils in series with a tap off then it's easier to visualise.

Hello Cam,

I had a quick look at the Kronjaeger site, however I thought that I would carry out a practical test on my wife's Minor.
Normal polarity, coil current at idle 1.65 amps, reversed polarity 1.65 amps, exactly the same, measured with an AVO Mk 8 multimeter. What I did notice was that the coil drew 4 amps with the engine not running, reinforcing someone's comment about burning the coil out by leaving the ignition on.
The theory of ignition coils is confusing as the instant of the points opening creates (induces) a voltage in both the primary and secondary coils by the collapse of the magnetic field. The primary back EMF is absorbed by the capacitor so protecting the points and the secondary discharges across the spark plug gap. The temperature difference would I guess be those I quoted less about 80 degrees? or so of the cylinder head. I don't think that I mentioned that the centre electrode should always be negative for the 'thermionic' effect to work.

Alec

Alec... a coil will always burn out eventually if the ignition is 'on'
without the engine running, at least on a four cylinder engine,
because it will never stop in a position where the points are OPEN
unless there is a dropped valve or lack of compression in a cylinder.
Since the points will be closed then the coil is full short to earth.
Your discussion with cam is very interesting. I must say that in
my opinion having the coil polarity reversed will undoubtedly give
a reduced spark but will not cause the coil to burn out.( I say
that because I came across one Minor which had run for over eight
years with the coil wrongly connected after a change to negative
earth)

Hello Willie,

yes that was my original point, why Nick has had so many failures is hard to diagnose but I'm sure that incorrect polarity is not a factor.

Alec

Alec wrote:Hello Willie,

yes that was my original point, why Nick has had so many failures is hard to diagnose but I'm sure that incorrect polarity is not a factor.

Alec

I've checked the polarity - black/white wire off coil goes to LT on distributor doesn't it?

Thanks Nick

VIBRODOLLY....yes the white/black lead will always go to the LT
contact on the dizzy regardless of the polarity of the car. It is the
other end of that lead which changes over if the polarity is changed
to NEG earth. On an ORIGINAL car the white/black lead would be
connected to the coil terminal marked 'CB' or '+', and the white lead
would go to the 'SW' or '--' terminal. If you are now Negative earth
just swop those two leads over ON THE COIL.

Hello Nick,
yes that is correct and connects to the -ve terminal on the coil in a negative earth car. Should the coil be marked cb and sw the cb terminal goes to the distributor unless the cars system has been altered to negative earth then the sw should be connected to the distributor.

Alec

Alec,

If you are measuring things then the true test I guess is to measure the output of the coil (use a spark-gap type tester).

1. Measure the HT voltage with everything in the the 'normal' position.
2. Measure the HT voltage with the coil polarity reversed. This should be noticably less.

3. With everything connected 'normally' note the intensity and colour of the spark (plug removed from engine.
4. Reverse ONLY the connection to the spark plug (HT lead to plug body and top terminal to engine/earth. Spark intensity/colour should be the same as 3.
5. Reverse coil polarity and note the spark intensity/colour. This should be noticably less intense.

Your discussion with cam is very interesting. I must say that in
my opinion having the coil polarity reversed will undoubtedly give
a reduced spark but will not cause the coil to burn out.( I say
that because I came across one Minor which had run for over eight
years with the coil wrongly connected after a change to negative
earth)

I have just checked my wiring and the coil is wrongly connected for negative earth. It has probably been wrong for over 7 years since the car was rebuilt for the previous owner by a famous specialist in Bath.

Interesting all this - some say on't worry, don't bother, less spark etc.

Having now checked the wiring and changed it round, I have not blown a coil and it's working well!!

So I would say, yes it does matter, get the coil woring correct.

Thanks VERY much for all your advice.

With reference to the unreliability of Ignition Coils. There is an
interesting article by our Technical Advisor in the latest 'Minor
Matters' which suggests that modern replacements are not full
to the top with the oil which cools then down so that they should
be mounted VERTICALLY to ensure that the coil windings are immersed
in the oil. The Minors coil is mounted horizontally so could expose
some of the windings to overheating and thus burn out. Very good
theory, have those with a vertically mounted coil suffered from
burn outs??

I've seen it and read it. It would seem to confirm everything. While I have changed the wires back to the normal position, I have also got an old lucas coil (£2 off ebay). It's working perfectly. I would suggest all do the same.

Thanks

Nick (Vibrodolly)