Ignition coil connections?

[rvwp]

Hi Guys,

Recently purchased a new ignition coil from Moss Pt No GCL110 for my TR4A.

When connecting I cannot see any marking which identify positive and negative connections. In the past coils have always been marked positive and negative.

Can I assume it does not matter, or do I need to use a multimeter to check which way around to connect it? What reading should I expect?

Thanks

Rich

[stuart]

3 minutes ago, rvwp said:

Hi Guys,

Recently purchased a new ignition coil from Moss Pt No GCL110 for my TR4A.

When connecting I cannot see any marking which identify positive and negative connections. In the past coils have always been marked positive and negative.

Can I assume it does not matter, or do I need to use a multimeter to check which way around to connect it? What reading should I expect?

Thanks

Rich

There should be some sort of marking on it as yes it does make a difference which way round theyre connected. I would ask them why theyre not marked.

Stuart.

[trchris]

Hi Rich

some coils have sw (switch) and cb (contact breaker) markings instead of  + or -  as Stuart says there should be some sort of markings

Chris

[ianc]

Makes a big difference if connected the wrong way - spark would be very weak.

As it's for your TR6, check with Moss that any markings (if present) are for negative earth car.  If not marked, I suggest that Moss should provide a correctly marked replacement.

Ian Cornish

[RobH]

32 minutes ago, ianc said:

Makes a big difference if connected the wrong way - spark would be very weak.

I think that is overstating the problem a bit. Polarity only makes a small difference in most engines and should be hardly noticeable if the ignition system is otherwise in good order. The effect is to increase the voltage necessary to cause break-down of the spark-plug gap but that is not of great importance unless the system is very marginal, since there should be plenty of voltage in hand.  Once the gap is broken down the spark energy is not affected.  There may be some additional hesitation during engine warm-up but once up to temperature there should be little effect. 

There are many other factors which influence the breakdown voltage - such as mixture temperature, air-fuel ratio, plug electrode geometry and compression pressure so this is just another contributory factor. The effect of incorrect spark polarity may be more noticeable in a high-compression engine running a lean mixture and with 'cold' plugs as all these require additional voltage from the coil. 

[ianc]

Rob - I've never tried connecting the coil the wrong way about, but I am basing my statement on this information which I collected some years ago:

According to Tony Tranter in Haynes "Automobile Electrical Manual", ignition systems are designed to have negative polarity at the centre electrode and if reversed can be up to 40% less efficient.

Coils for both Positive and Negative earth vehicles were produced, but who knows if anyone has changed a coil for the wrong type in the history of older vehicles, so it is wise to check polarity of the spark (see below for testing polarity using a lead pencil).

With early coils, swapping the CB and SW leads will switch polarity of the spark.  Later coils use + and - but the requirements are the same.

To test the spark direction with a lead pencil, interpose the writing tip of a lead pencil between the spark plug top stud and the HT lead, leaving a small gap either side of the pencil tip.  If the polarity is correct, a flare between the pencil and the spark plug will be seen (if the flare is between the pencil tip and the HT lead, then the polarity is reversed).

Ian Cornish

[RobH]

1 minute ago, ianc said:

According to Tony Tranter in Haynes "Automobile Electrical Manual", ignition systems are designed to have negative polarity at the centre electrode and if reversed can be up to 40% less efficient.

No he has that description a bit wrong Ian, it is not really efficiency.

What he is talking about is as I described, the breakdown voltage at the spark gap. That does indeed increase with inverse polarity.  However,  it does not materially affect the overall efficiency of the system. Once the plug fires, the spark has the same energy whichever way round the current  flows, since that is determined by the energy stored in the coil. The extra time taken to reach the higher voltage is not of any consequence either as it will be in the order of microseconds.

The mechanism involved is the generation of electrons at the electrodes. A hot surface will more readily provide free electrons - think of the old thermionic valves where the cathode is heated to ensure this. The centre electrode of a spark plug is shrouded by the ceramic nose and runs hotter than the earth electrode, which can dissipate heat to the cylinder head by conduction. It follows that there are more free electrons available at the centre which makes it easier to start the spark from that electrode than it is from the earth electrode. That is why the centre electrode should always be negative in the system. 

The polarity will only make a noticeable difference where the ignition system is marginal and cannot reliably supply the extra voltage required to initiate the breakdown. Also the higher peak voltages in the ignition system mean higher electrical stress and may lead to premature failure of the coil insulation. 

 

[tthomson]

The coil on a TR, like other vehicles of the same age, is an autotransformer, with the CB contact common to both the Low voltage primary coil and the High Voltage output coil. The other end of the Low Voltage coil is not intended to be connected to the contact breaker, otherwise the secondary voltage is developed across the two coils in series. I do not know if the two coils are in a series aiding for for the induction, but I if not it diminishes the output.

 

TT

[RobH]

True TT,  but the coil turns-ratio is typically 100:1 so that means the HT can only be changed by a factor of 1% by incorrect connection.  I believe that is negligible when compared with the other variables which can increase the breakdown voltage at the spark gap by 20-40%.

[Z320]

I knew this yaers ago and try to remenber....

the spin of the coil and connection of ( + , ignition, german / Bosch "15") and (braker, ground, german / Bosch "1" )

causes a magnetic north-south field the way it is better for the spark / the spark plug.

If you change (15) and (1) on the coil from correct to wrong way you swap the field 180°

and the resulting spark is "the more unfortunable" way on the spark plug.

This causes a visible difference of the intensity of the spark.

Edited August 30, 2020 by Z320

[RogerH]

9 hours ago, Z320 said:

I knew this yaers ago and try to remenber....

the spin of the coil and connection of ( + , ignition, german / Bosch "15") and (braker, ground, german / Bosch "1" )

causes a magnetic north-south field the way it is better for the spark / the spark plug.

If you change (15) and (1) on the coil from correct to wrong way you swap the field 180°

and the resulting spark is "the more unfortunable" way on the spark plug.

This causes a visible difference of the intensity of the spark.

witch craft !!!

 

Roger

[Z320]

Shure you know "Catweazle", oh I loved that when I was a child!

But it's only physics: spark plugs work better with the spark "jumping" from the hotter eletrode zu the colder one.

 

[roger murray-evans]

I've found many instances of 'incorrect' fitting of coils, esp. to pos earth cars

where the coil is marked cb/sw and presumably the maker of said coils is

oblivious to the fact that not all cars are neg.earth.

In my personal experience it hasn't made the slightest discernible difference

which way around it's wired.I do though, now I've picked up a bit of knowledge ,

try and and connect up the 'correct' way.

Roger M-E

 

[Drewmotty]

Another stumbling block is the fact that many early coils are marked cb/sw or +/- but are for positive earth cars. 

[ianc]

I have been mulling over the matter of polarity of the feed to the spark plug, and these are my thoughts:

The plug is designed to fire from the centre tip to the surrounding (outer steel) ring - the body of the plug.

To gain the maximum amount of charge on an object, one needs a small tip, think of a lightning conductor. A sparking plug's tip cannot be made too small or it would burn away rapidly.

If the coil were reversed, one would be trying to fire from the outer steel ring onto the tip. There would be no consistent point of discharge on the ring, so the timing could be slightly different at each firing and ignition would be inconsistent (I realise that we are talking of miniscule fractions of a second, but even so there would be inconsistency).

Inconsistency would reduce the efficiency of the engine, which would imply reduced power (perhaps not noticeable most of the time) and increased fuel consumption. 

Then, I had a look at a useful website called MGA guru (it is American, as you can tell, and when he writes end electrode he means the threaded metal case of the plug which screws into the cylinder head): 

http://mgaguru.com/mgtech/ignition/ig104.htm  in which is the following

Why then do we worry about coil polarity?  Because the spark plugs do care which way the electrons are flowing in the high tension circuit.  The spark plug has a thermally insulated center electrode (surrounded by ceramic).  With engine running, the center electrode runs substantially hotter than the exposed end electrode.  Design of the ceramic insulator determines how hot the center electrode will run, leading to the designation of hotter or colder spark plugs.  As electrons go, they love to jump away from a hot surface and fly toward a colder surface, so it is easier to drive them from hot to cold rather than from cold to hot.  End result is a difference of 15 to 30 percent in voltage required to make spark "initially" jump the gap on the plug depending on which way it is going.  So the spark plug prefers to see a voltage potential that is negative on the center electrode and positive on the end electrode for the very first hop of the spark. 

You can check the polarity on your own car using a lead pencil (i.e. one with graphite core, graphite being a conductor):

Interpose the writing tip of a lead pencil between the spark plug top stud and the HT lead, leaving a small gap either side of the pencil tip.  If the polarity is correct, a flare between the pencil and the spark plug will be seen (if the flare is between the pencil tip and the HT lead, then the polarity is reversed).

Ian Cornish

[RogerH]

10 minutes ago, ianc said:

I have been mulling over the matter of polarity of the feed to the spark plug, and these are my thoughts:

The plug is designed to fire from the centre tip to the surrounding (outer steel) ring - the body of the plug.

 

If the coil were reversed, one would be trying to fire from the outer steel ring onto the tip. There would be no consistent point of discharge on the ring, so the timing could be slightly different at each firing and ignition would be inconsistent (I realise that we are talking of miniscule fractions of a second, but even so there would be inconsistency).

 

Ian Cornish

Hi Ian,

 the outer ring of the plug is much further away from the centre tip than the end electrode.

So you would not get sparks jumping across to the outer ring.

Assuming I am understanding your point.

 

Roger

[ianc]

Correct, Roger.  I should have had a good look at a plug before typing!

The spark should be jumping from the tip of  the central rod to the curved rod which is connected to the outer ring and hangs over the central rod some 20 to 35 thou distant.

However, there will be a greater concentration of charge on the tip of the central rod, and that the point from which the ignition spark was designed to be initiated.

Ian Cornish

[RogerH]

Upon reading RobH's post again (and again.......) two things happen -

The electrons move from Neg to pos.

The curved earth electrode gets hotter than other areas of the plug (more exposed and less cooling)

Electrons are released more freely from hot  things

So the spark goes from the earth electrode to the positive centre electrode.

 

Roger

[RobH]

No Roger I obviously didn't write it clearly enough, sorry. It's t'other way round - the central electrode gets hotter. The earth electrode can conduct and radiate heat away but the centre one has a ceramic shroud which keeps it hotter.  The earth electrode should be the positive one which attracts electrons from the centre one. 

Electron flow is from the centre electrode to earth (but if you are talking conventional current flow, it  is from the earth electrode to the centre). Confusing isn't it. 

[RogerH]

aaarrrrggghhhhhh!!!

[Charlie D]

16 hours ago, ianc said:

...To gain the maximum amount of charge on an object, one needs a small tip, think of a lightning conductor...

I remember that fact when I worked at EMI on x-Ray machines. The tubes ran at 150,000 volts and we were told if ever we had to take the covers off the EHT generator we must not point at anything inside, because there was more possibility of getting electrocuted if we did that.

It terrified us so much that one of our team, who had a long pointed nose, refused to go near the thing.

 

Charlie.