Coil Hookup Backwards

Bob Bidonde

Bob Bidonde

Well-known member
I am looking for a technical lesson from an electrical engineer on the Barn. It has been discussed that if the wire connections to the ignition coil are reversed, the coil's output is reduced, so engine performance suffers. Regarding the picture of the coil below, please explain what goes on in the coil to affect its output.
According to the factory wiring diagram, the hookup has a Black Wire from the Terminal Box (Battery power in) going to the Left Terminal A, and a Red Wire (power out) going from the Right Terminal A to the Points via the Ignition Switch. In this case, Coil A-A induces a hi-voltage in Coil B-B.
What happens when the Black Wire (Battery power in) is connected to the Right Terminal A? I am not sure I understand why the Coil's output is degraded.
 

Attachments

  • Coil .jpg
    Coil .jpg
    21.7 KB · Views: 20
Coil output is the same either way. Degradation occurs because the electrons jump across the spark plug gap easier in one direction vs. the other. (But I don't recall which.)
 
I believe Ford changed the coil wire connections in mid to late 29. So, there are two official ways to hookup the coil primary wiring correctly, depending on your vehicles year. Ford used slant pole coils for their first coils in 28 and into 29. Our local club has a coil tester we can use to check the polarity that provides the best spark at the plugs. I attached a picture of the tester, which has two lights on it to show whether the polarity is correct or reversed. The second picture shows the correct wire connections for 28 and into 29. I used the coil polarity device to determine the proper wire connection. I don't know why it makes a difference the way the wires are connected, but it does, even though the engine will run with the wires connected either way.
Rusty Nelson
 

Attachments

  • Coil polarity tester.JPG
    Coil polarity tester.JPG
    80.6 KB · Views: 43
  • Slant pole correct phaeton primary connection.jpg
    Slant pole correct phaeton primary connection.jpg
    49.6 KB · Views: 56
The change Ford made to the coil hook up did not change the polarity, what it did was include the ammeter in the circuit so you could see the points open and close when starting the car. If the car did not start, but the points were opening and closing it told you that the primary circuit was intact. A good diagnostic tool.

The attached article explains two other useful tools.

The old pencil tool has been around for years,


https://www.santaanitaas.org/wp-content/uploads/2022/01/Two-Helpful-Tools.pdf

Tom Endy
 
The link that LEM provided is correct. It is not the coil but the plug that cares which way the electrons are going. The spark plug's center electrode, the insulated one, runs hotter so the electrons jump from this electrode to the grounded one easier than the other way around. The electrons also jump easier from a sharp point. The easiest and more accurate way to test the coil polarity is with the tool that wrndln showed. You can use the graphite pencil as Tom talked about but it is easier with the tool.

The ammeter wiring was changed to include the ignition loss mid way during production of the Model A. If your car has not been changed it is a good idea to do so. Hand turn the engine over until the ignition points are closed. Turn the key on and observe whether the ammeter indicates a discharge. If it does not, change the wire from the coil to the junction box to the other side of the junction box and try again. If neither side of the junction box causes the ammeter to show a discharge then the points are not conducting or something else is wrong.
 
Last edited:
Electrons flow more easily from the hot surface which in this case is the centre electrode to the cooler outer electrode . In which case it doesnt matter which way the coil is wired when the engine is cold !
 
If you look closely at the OPs coil internal diagram, you can see how the one end of the secondary coil junctions into the primary coil circuit. The junction can only be on one side of the primary coil. This is what affects the polarity of the magnetic field that builds in the core when energized. It's basically an electromagnet until the breaker points open.

I don't qualify as an electronic engineer but aviation school had a pretty good course on basic electricity. I didn't know how to use trigonometry to measure wave lengths till I completed that course. We also had to know how to use Ohms Law to pass that course. The AC part was pretty basic but we had to know enough to have an understanding of any DC or AC components that may need repair or overhaul. The ignition systems basically function on pulsating DC current which allows a half wave AC to make the coil induce its high tension voltage.
 
Last edited:
Hi Bob,

The coil needs to create a discharge voltage high enough to jump the air gap at the plug. Several factors influence whether the coil is able to do this:

- When the throttle is open wider, more air enters the cylinder and is compressed more – more air molecules between the electrodes means higher resistance, and if the voltage is insufficient then the arc may fail to form.

- When engine rpm is high, there's less time for the arc to form, so if the voltage is too low then the arc may fail to form in time.

- Lastly, regardless of which ground polarity a car is configured with, it will always be the case that the resistance of the plug gap will be lower when the central electrode (the one connected to the terminal at the top of the plug) is negatively charged (meaning that electrons are trying to jump away from it) and the side electrode is positively charged. If the secondary winding discharges in such a direction that it charges the central electrode positively (meaning electrons are jumping from the side electrode to the central), resistance will be higher and the arc may fail to form.

How does this relate to hooking up the wires? Well, from the outside of any given coil, it can be difficult or impossible to tell whether connecting black on the left and red on the right, or vice versa, will result in the secondary winding pushing electrons toward the high-tension terminal or away from it. If the configuration is such that the ignition sequence, where the points open and the magnetic field of the iron core collapses, pulls electrons away from the high-tension terminal, the minimum voltage necessary to arc the air gap at the plug will always be higher for a given set of cylinder conditions than if the coil is set up to push electrons toward it.

Consequently, a coil that's hooked up "backwards" may cause misfires under conditions where the same coil hooked up "correctly" would not. The coil generates the same voltage either way, but if the polarity of the high-tension discharge is "wrong," that voltage might not be enough to overcome the resistance at the air gap.

Hope that makes sense.
 
Once the initial spark occurs the air molecules there are ionized which makes it easier for current to flow. Lightening works the same way. The coil/condensor work as a tuned LC circuit and there are multiple sparks that happen in a very short time. The LC circuit rings like a spring so the current flows in opposite directions for each spark. If you look at the wave form on an oscilloscope you can see this. Each spark has diminished voltage and eventually (in a very short time) the cycle stops. It is not just one jump. The hot center electrode makes it easier for the initial jump.

Different coils are marked differently and, as we know, the Model A can be negative or positive ground. So the best way to tell if the spark is going the right direction is with the tool shown in post #3.

Model airplane engines made in from the 1920 to the early 1940's ran on gasoline and had a spark plug. The spark plugs are tiny and have sharp points on both the center and ground electrodes. The instructions for hooking up the coil for the model airplanes say to hook up the battery either way. My experience running antique model airplane engines verifies that the polarity on the coil does not matter. The sharp points on both electrodes makes it easy for the electrons to do the initial jump either way.
 
Last edited:
Bob Bidonde said:
I am looking for a technical lesson from an electrical engineer on the Barn. It has been discussed that if the wire connections to the ignition coil are reversed, the coil's output is reduced, so engine performance suffers. Regarding the picture of the coil below, please explain what goes on in the coil to affect its output.
According to the factory wiring diagram, the hookup has a Black Wire from the Terminal Box (Battery power in) going to the Left Terminal A, and a Red Wire (power out) going from the Right Terminal A to the Points via the Ignition Switch. In this case, Coil A-A induces a hi-voltage in Coil B-B.
What happens when the Black Wire (Battery power in) is connected to the Right Terminal A? I am not sure I understand why the Coil's output is degraded.
Click to expand...

Ignition coils are wound with the primary on the bottom or next to the core because the field around low voltage coil is lower so the closer to the iron the better it works.
 
Thanks all for your replies. I have a better understanding of what happens when the coil connections are reversed.
 
See the attached link for a good explanation for the resulting difference in discharge characteristics at the spark plug. Reverse polarity at the coil affects the direction of the current or electron flow across the spark plug gap. The author gives a good visual example of the spark discharge characteristics when the polarity is reversed.
https://mgaguru.com/mgtech/ignition/ig104.htm
 
You must log in or register to reply here.
Back
Top