Coil replacement

[LEM]

My old coil does not have plus or minus marked on the terminals just indicates distributor terminal. My question is this the positive or negative terminal?

[Patrick L.]

It would depend on the coil. But my money is on it being - .


You can use the lead pencil trick to determine the polarity, or, the 'A' vendors have a little gizmo for doing that.

[ursus]

Positive ground and 6-volt systems went away about the same time back in the mid 1950's. Was Ford the only positive ground system at that point in time?

[katy]

Originally Posted by ursus

Positive ground and 6-volt systems went away about the same time back in the mid 1950's. Was Ford the only positive ground system at that point in time?

Other users of positive ground were: Chrysler products, Hudson, Jaguar, MG, Nash, Packard, and Studebaker, maybe more.

[old ugly]

help me. i need a tech lesson here.

why does it matter which direction the current flows through the ignition coil?

[Patrick L.]

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Originally Posted by old ugly

help me. i need a tech lesson here.

why does it matter which direction the current flows through the ignition coil?

Its how the coil/transformer is wired internally. There is about a 30% difference in voltage.

[updraught]

No electrical expert, but there are several references to this on the interweb.
https://mgaguru.com/mgtech/ignition/ig104.htm

[alexiskai]

Originally Posted by Patrick L.

Its how the coil/transformer is wired internally. There is about a 30% difference in voltage.

This isn't quite true, the voltage output from the coil will be the same magnitude either way, but its direction will be different.

The secondary winding in the coil has one end leading to the high-tension terminal that goes to the plugs. The other end connects to one of the low-voltage terminals. That's how it makes a circuit. If the coil is wired up in such a way that the reversal of the magnetic field (caused by the opening of the points) creates a voltage in the secondary winding that pushes electrons toward the spark plugs, so that the current in the plug flows from the center electrode to the side electrode, the resistance of that air gap is relatively lower.

If the coil is wired up the other way, so that the voltage in the secondary winding pushes electrons back toward the battery, thus in effect "pulling" electrons from the ground electrode across the gap to the center electrode, the resistance of the air gap is relatively higher.

If the resistance in a circuit goes up, and the voltage stays the same, the current goes down. Since the coil is a transformer that always generates the same voltage (assuming constant performance of the points, condenser, battery, etc.) if the resistance of the spark plug air gap goes up, the current crossing that gap, i.e. the strength of the spark, goes down.

As the engine runs faster and hotter, the resistance of the air gap goes up independently of the coil, because the air between the electrodes is being dispersed by turbulence as the plasma arc tries to form. So the practical effect of the coil wiring not matching the plug is that, at any given rpm and load, the strength of the spark will be weaker, and at some point the spark will simply stop.

So it would be more accurate to say that, if the polarity of current in the secondary winding doesn't align with the spark plug, the spark is 30% (or some number around there) weaker. It may also be shorter-duration; I'm not sure, this is roughly where my electrical understanding runs out.

[duke36]

Is this a new coil, oil or epoxy filled? Polarity is important as per post 2,7.

[alexiskai]

With a coil of unknown provenance, I always recommend just installing it and testing with an analog voltmeter to see. You're never going to get 100% certainty trying to deduce its polarity from external indicators.

[katy]

Originally Posted by alexiskai

This isn't quite true, the voltage output from the coil will be the same magnitude either way, but its direction will be different.

The secondary winding in the coil has one end leading to the high-tension terminal that goes to the plugs. The other end connects to one of the low-voltage terminals. That's how it makes a circuit. If the coil is wired up in such a way that the reversal of the magnetic field (caused by the opening of the points) creates a voltage in the secondary winding that pushes electrons toward the spark plugs, so that the current in the plug flows from the center electrode to the side electrode, the resistance of that air gap is relatively lower.

If the coil is wired up the other way, so that the voltage in the secondary winding pushes electrons back toward the battery, thus in effect "pulling" electrons from the ground electrode across the gap to the center electrode, the resistance of the air gap is relatively higher.

If the resistance in a circuit goes up, and the voltage stays the same, the current goes down. Since the coil is a transformer that always generates the same voltage (assuming constant performance of the points, condenser, battery, etc.) if the resistance of the spark plug air gap goes up, the current crossing that gap, i.e. the strength of the spark, goes down.

As the engine runs faster and hotter, the resistance of the air gap goes up independently of the coil, because the air between the electrodes is being dispersed by turbulence as the plasma arc tries to form. So the practical effect of the coil wiring not matching the plug is that, at any given rpm and load, the strength of the spark will be weaker, and at some point the spark will simply stop.

So it would be more accurate to say that, if the polarity of current in the secondary winding doesn't align with the spark plug, the spark is 30% (or some number around there) weaker. It may also be shorter-duration; I'm not sure, this is roughly where my electrical understanding runs out.

So, how does some of the modern systems work, where they use one coil to fire 2 different cylinders?

I kinda like the modern systems where they use one coil for each cylinder, mounded on top of the spark plug.

[old ugly]

so when the primary coil collapses around the secondary coil, and when the coil is hooked up backwards, the electrons are traveling the wrong direction which pulls the spark at the plug backwards from the block to the electrode. effectively reducing the efficiency of the spark.
did i get that rite? lol
i don't think that MGA picture is accurate.

[alexiskai]

Originally Posted by katy

So, how does some of the modern systems work, where they use one coil to fire 2 different cylinders?

They work like you'd expect, one of the plugs has a spark that goes the "correct" way and the other has a spark that flows "backwards." If the coil produces sufficient voltage to overcome the resistance either way, the engine will still run fine. The only catch is that the wear on the plugs will be uneven – half will erode at the ground terminal and half at the center. So waste-spark systems often use plugs where both terminals are specially treated or use a special alloy to control erosion.

[Patrick L.]

Also a waste-spark fires on compression as well as exhaust stroke.


Thats how to fire 2 cylinders with one coil.

[old ugly]

a few perplexing questions.

a coil that has a + and -

is this coil hooked up the same on a positive ground car as it is on a negative ground car?

is the coil wired different inside so that when i hook it up to a negative ground car the electrons don't travel backwards ?

if i use a coil with a + and - that came off of a 6 volt negative ground car on my positive ground model A will it have 30%less efficiency?

last question for now. how do we know these coils are wired different or how do we know what application they are for?

[Patrick L.]

Originally Posted by old ugly

a few perplexing questions.

a coil that has a + and -

is this coil hooked up the same on a positive ground car as it is on a negative ground car?

is the coil wired different inside so that when i hook it up to a negative ground car the electrons don't travel backwards ?

if i use a coil with a + and - that came off of a 6 volt negative ground car on my positive ground model A will it have 30%less efficiency?

last question for now. how do we know these coils are wired different or how do we know what application they are for?

A coil is connected same abattery s the battery. If pos battery grd then pos coil terminal goes to grd [points].

[shew01]

Originally Posted by Patrick L.

the 'A' vendors have a little gizmo for doing that.

https://www.brattons.com/IGNITION-SP...uctinfo/17140/


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[JoeCB]

Good explanation, Thanks Alex. While on the subject of electron travel direction, can someone share the "pencil lead/ spark" trick to tell which way the little critters are traveling.

Joe B

[Patrick L.]

Originally Posted by JoeCB

Good explanation, Thanks Alex. While on the subject of electron travel direction, can someone share the "pencil lead/ spark" trick to tell which way the little critters are traveling.

Joe B

https://www.chicagolandmgclub.com/te...neral/574.html

[LEM]

Wow that’s a lot of information for a simple question that I asked but it’s all good knowledge. I hooked up my black wire to the negative side of the new coil. I then use the lead pencil test and according to what I saw the coil is wired correctly.

[Tom Endy]

Originally Posted by shew01

https://www.brattons.com/IGNITION-SP...uctinfo/17140/


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I bought one of these many years ago when they were first offered. At a club event we tested about 20 cars and found about half had the polarity backwards and the owners were not aware of it.

Back in high school around 1950 my auto shop teacher showed the class how to determine the polarity by lifting a plug wire and creating a spark and then inserting the tip of a lead pencil into the spark. The side it flashed on told the polarity. I forget which side. Then he would hand the pencil to the class clown and have him do it, resulting in the kid getting shocked.

Tom Endy

[shew01]

Originally Posted by Tom Endy

Then he would hand the pencil to the class clown and have him do it, resulting in the kid getting shocked.

Lol. I love it.


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[katy]

Originally Posted by Tom Endy

Then he would hand the pencil to the class clown and have him do it, resulting in the kid getting shocked.

That's a mean trick.

[Patrick L.]

Originally Posted by katy

That's a mean trick.

Heck, most folks get hit with coil voltage from time to time. Keeps the heart going. Its not as bad as getting hit with a charged condenser.

[alexiskai]

Originally Posted by Patrick L.

Heck, most folks get hit with coil voltage from time to time.

It's an effective error correction tool.

[rotorwrench]

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Most of the coils I've found with DIS and IGN on them were for positive ground systems so there would be no confusion about connecting it up. I've used a pencil to test but have had the Nu-Rex polarity tester for some time. The Nu-Rex is not a new thing. This type of tester has been around for 80 years or more.

On coils marked with + and - this at least shows the polarity for a negative ground system which became the norm after the change over in 1956. There was no need to use DIS and IGN with all domestic systems using negative ground. A person may even run into coils that have no marking at all. More modern coils generally have power applied to the + terminal and the - terminal is connected to the ground breaker points so there is no confusion here. The negative terminal is the one that has the dual connection of both primary and secondary coils since that is the ground side for both coils on negative ground. The secondary coil needs a circuit to function but it will only function on either pulsating direct current or AC current to step up the voltage of the primary. The stepped up voltage goes both ways but is caught by the condenser on the breaker side so the other side has to be the high tension exit when properly connected.

It's all about polarity with inductor coils. They don't care how they are wired since they will function either way but only one way is polarity correct for the way the battery is connected in the system. The primary coil is what makes an electro magnet out of the steel coil core. It can be a magnet either way but only one way is efficient for a spark at the plug since it is important which direction that spark flows between high tension release and ground. The high tension coil or secondary has a ground provided for it either way but the direction the high tension goes is fully dependent on the POLARITY of the magnetic field set up in the primary coil and core before that field collapses as the points open.