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Ignition Coil Low Tension Connections

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MGTF1250Dave

MGTF1250Dave

Jedi Knight
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Aloha All,

I have been pondering the low voltage circuitry of my ignition system. In my experience ignition coils come marked either + and - or CB and SW. In the later case, the wire from the ignition switch (white) would connect to the SW terminal and the wire to the distributor (white with black tracer) would be attached to the CB terminal. If a coil is marked + and -, and the car is positive earth, the ignition switch wire connects to - and the distributor wire connects to + as it is the path to ground.

A. I have also read and been told that if the car is converted to negative earth, the switch wire should be moved to the + terminal and the distributor wire should be changed to the - post. Is this correct?

B. I've never dissected an ignition coil, but this is what has me questioning distributor wiring described in A. The voltage input from the ignition switch is the source of power for both the primary and secondary coil, the path to ground for the primary coil is through the distributor via the CB terminal and the secondary coil discharges to ground at the spark plug. It would seem to me that internally, the coil terminal SW connects directly to both primary and secondary coils and the CB terminal connects only to the primary coil. Does this sound reasonable?

C. I have never seen an ignition coil labeled as either positive or negative ground. If my assumption in B is correct, it would seem to me for best ignition performance the switch wire needs to be connected to the common feed to both internal coils and the distributor wire needs to be connected to the ground for the primary internal coil. With coils labeled only with + and -, how do you determine this? Or, does it make any difference?
 
I had always suspected that the CB/SW coils date from when all British Cars were positive ground and when the conversion to negative ground began it became necessary to label the connections + & - to avoid having to sell coils marked +ground and -ground.

As for the electrical question you pose -- no idea.
 
The only thing I have ever read about this is that the coil will work either way, BUT, if the polarity is backwards, the plug fires in the wrong direction and you effectively lose a couple of thousand volts. If the coil is capable of putting out enough voltage, it shouldn't affect anything. I know on my Miata, it has a waste spark system where two plugs fire at once. The juice comes out of one end of the coil, jumps across the plug, travels to the other plug hooked to that coil, jumps across that gap, up the ignition wire and back to the coil. There are two coil packs for the four cylinders. Apparently, the Miata engineers weren't too concerned about plugs firing backwards. Then, of course, those coil packs throw out a lot more voltage.
 
Am ignition coil is a simple step-up transformer consisting of two separate coils -- primary and secondary -- and an iron core.
the ratio of windings between the coils determines the amount of voltage change. Assuming a 12 volt source and desire for a 12,000 volt spark, the ratio would be 1 : 1000, or 100 turns in the primary (for instance) and 100,000 turns on the secondary.

In the case of a car's ignition coil, power is applied to the primary at a terminal and the other end of the primary connects to ground intermittently through the points. The alternating current (off - on - off - on) through the primary induces (an actual electrical term) a much higher voltage in the secondary which discharges to ground through the spark plug every time the points open. One end of the secondary is connected to the center distributor lead. There is no need to connect the other end of the secondary to anything.
 
Here's the article on coil polarity and how things are affected. It says that the coil polarity itself makes no difference, but the difference occurs at the polarity of the plugs. Therefore, you want to get the polarity on the coil correct so it will be correct at the plugs.
 
In my limited experience with this... having the coil polarity incorrect will work but the points will suffer, requiring dressing in as few as a few hundred miles.
 
Aloha Art,

Mahalo (thanks) for the link to that article. It was really very helpful. The test to determine coil polarity in relation to the spark plugs seems the best way to make sure the coil low tension circuit is wired for optimum spark.
 
Moseso said:
Am ignition coil is a simple step-up transformer consisting of two separate coils -- primary and secondary -- and an iron core.
the ratio of windings between the coils determines the amount of voltage change. Assuming a 12 volt source and desire for a 12,000 volt spark, the ratio would be 1 : 1000, or 100 turns in the primary (for instance) and 100,000 turns on the secondary.
Click to expand...
Actually doesn't work that way; which is why most ignition coils only have a turns ratio of approximately 100:1. "High Performance" coils typically have even lower ratios.

Instead, the current through the primary (while the points are closed) builds up a magnetic field in the core, which is then discharged much quicker than it builds up (hence the higher voltage) when the points open. The coil does act like a transfomer in that the primary and secondary circuits both get a voltage pulse, and the relative voltages are determined by the turns ratio; but the output voltage is determined by the strength of the magnetic field, how fast it collapses, and the number of turns. For this reason the primary voltage also rises to many hundreds of volts. In fact, the most important role played by the condensor is to limit how fast the primary voltage rises, so it doesn't jump the slowly opening points.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:]There is no need to connect the other end of the secondary to anything. [/QUOTE]Again, not true. The other end of the secondary is internally connected to the primary, so the spark current (which is quite small) actually flows through the condensor (and possibly the primary winding) as well.

Check out this article for a more complete explanation (written by Dan Masters)
https://www.triumphexperience.com/article/understand-ignition.html
 
MGTF1250Dave said:
A. I have also read and been told that if the car is converted to negative earth, the switch wire should be moved to the + terminal and the distributor wire should be changed to the - post. Is this correct?
Click to expand...
Just to echo what others said, the answer is Yes. It doesn't make a huge difference (in my experience), the engine will still start and run with the coil (and spark) polarity backwards, but it will start just a little bit easier under adverse conditions (ie cold) with it correct.
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]B. I've never dissected an ignition coil, but this is what has me questioning distributor wiring described in A. The voltage input from the ignition switch is the source of power for both the primary and secondary coil, the path to ground for the primary coil is through the distributor via the CB terminal and the secondary coil discharges to ground at the spark plug. It would seem to me that internally, the coil terminal SW connects directly to both primary and secondary coils and the CB terminal connects only to the primary coil. Does this sound reasonable?[/QUOTE]The secondary coil definitely connects back to one side of the primary, but I don't know which side.
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]C. I have never seen an ignition coil labeled as either positive or negative ground. If my assumption in B is correct, it would seem to me for best ignition performance the switch wire needs to be connected to the common feed to both internal coils and the distributor wire needs to be connected to the ground for the primary internal coil. With coils labeled only with + and -, how do you determine this? Or, does it make any difference? [/QUOTE]My belief is that it doesn't make enough difference to worry about. I've run an original Lucas coil (CB/SW markings) on negative ground and it worked fine; I've also run a modern coil (from K-mart, +/- markings) on positive ground and it worked fine.

The difference in voltage before the spark starts would only be 1% or less anyway; likely less than the normal unit-to-unit variation in coils & condensors. And once the spark begins, there would be no difference at all.
 
Aloha Geo,

I suspect you are correct about the low tension connection markings.
 
Aloha Randall,

Thanks for the input, I appreciate your thorough answers. The link you provided to the Triumph Experience Library and the one from Art to Chicagoland MG Club Tech Tips are great references on inductive coil ignition systems in our LBCs.
 
You're quite welcome, Dave, even if you do have one foot on the dark side :jester:
 
Aloha Randall,

It would seem to me that the most important thing regarding coil polarity is to make the connections so the spark plug arches from the center electrode to the ground electrode attached to the threaded body. The Chicagoland article describes how to test for that.
 
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