Polarity tester

Brian SATX

Brian SATX

Well-known member
OK, I have one of those coil polarity testers sold by the vendors. The kind you use for a quick check to see if your wires are connected to the coil properly. It has been accurate every time I use it. Tried it on a friends car yesterday and it showed the wires were wrong on the coil(red light) We checked the coil visually and the wires were correct. Had to think why. His car has been changed to 12V Negative ground. Would that affect the reading? Is that tool set so that polarity of the car makes a difference?

As a side note, we were checking to see why he had an intermittent miss, usually in 3rd gear. We tried taking a nut off the + side of the coil and the post turned, not the nut. He got a new coil and problem solved.
 
Brian, Misery loves company.

I just went thru that drill yesterday with an all stock 6 volt Model A electrical system. I'm wondering about the polarity tester itself.

- Tested a stock '28-29 slant pole coil and the tester reported it's polarity was reversed. Funny I have one other slant with the same results ??

- Tested a Model A style replacement 'look alike' with a clearly marked positive post (distributor). Connected that way, the tester indicated it was reversed.

It was time to stop for the day, bourbon required.....jb
 
If the tester shows red, the wires are reversed, regardless of battery polarity.

Positive ground - black (-) wire to battery negative, red (+) to ignition switch/points.
Negative ground - armored (-) wire to points (ground), red wire (+) to switch/battery positive.
 
The polarity tester is correct. The wires need to be switched on the coil. Even if the wires are hooked up correctly by color or by the lettering on the coil, they are reversed. Switching battery voltage will make the difference.

If the coil is reverse wired it will cause a miss. The polarity of the coil does not change its function but it will change how the plugs work. The center electrode is hotter and the electrons have an easier time jumping to the grounded electrode than visa versa.

More information: Once the initial spark occurs there are several others as the coil and condensor resonate. The spark jumps back and forth in opposite directions. But because the air is ionized from the initial spark the electrons can flow back and forth with ease.
 
Am I confused? If you change the polarity of the battery ground as in 12 volt negative ground, you have to reverse the ammeter leads or the gauge will read incorrectly, as in backwards. Why would we not expect to reverse the coil knowing that it is polarity sensitive?

What is it about the windings of the ignition coil that determines the internal polarity?
 
The secondary windings, the ones that sends the spark to the plugs, is really what determines the polarity of the coil. As the points open, the magnetic field in the coil collapses which induces a voltage in the secondary windings. One side is grounded and the other side goes to the distributor and thus to the spark plugs. That side should be negative so that the negative charged electrons show up at the center electrode of the plug and can easily jump to the grounded electrode. The polarity of the primary windings determines which way the magnetic field is oriented and thus the polarity in the secondary windings when the magnetic field collapses.

Additional information: The antique model airplane engines had spark ignition and miniature spark plugs. Both electrodes had sharp points so that the polarity of the coils did not matter. The electrons could jump from the grounded electrode just as easily as from the center electrode.

More additional information: The first internal combustion engines had points that opened in the combustion chamber inside the engine. As the points opened a spark was created. The points were hooked to a battery and the wire going to the insulted point was wrapped around a steel bolt. The current when the points were closed caused a magnetic field to build up in the bolt that collapsed when the points opened and this is what caused the spark. Many years ago I fixed a hit-and-miss engine for a rancher friend of mine. He used it on his well to pump water to a holding tank. I had to wind the wire to the ignition points on a bolt to get the engine to run. The engine would fill up the holding tank and then run out of gasoline and stop. Next time the tank needed to be filled, the engine was filled with gasoline and the process repeated.
 
Last edited:
I don't trust a lot of modern stuff to be correct. If you find an internal diagram to a can type ignition coil, look at the way the primary winding is connected. Each end of the primary coil is connected to a different primary terminal but one terminal is also connected to one end of the fine wire secondary coil. That one connection is what determines which way the secondary high tension event moves as the breaker points open.

New coils are marked + pos and - neg but are they really properly connected? You can't
see in there so the only way to judge polarity is either with the pencil method or the little plug in tester. If it shows to be off by either method then switch the primary terminal leads on the coil and test it again. Many of the old coils were marked Bat and Dis but that was for positive ground systems. That changed after the 12-volt negative ground systems came in in 1956.

If a car is converted from positive ground to negative ground then all polarity sensitive items will need to be swapped terminal for terminal. Luckily, the coil is about the only polarity sensitive item but more modern radio sets are as well. The ammeter really only detects flow either to or from the battery but a change in system polarity will affect the way the current flows. If it flows backwards then discharge will become charge and visa versa. When it reads backwards just swap the leads. Most ammeters have no + or - markings on the back terminals.
 
Guys, here are my thoughts on Model A coil polarity.

Each time a given cylinder's piston approaches TDC on the power stroke, that cylinder's spark plug fires a series of sparks across the gap between its center electrode and its ground side electrode. When viewed on an oscilloscope, this is actually a series of sparks / flows of electrons / currents rather than a single one. Each successive spark in the series becomes less intense than the previous until the series dies out due to various resistances. From start to finish, each of the individual current flows / sparks will have reversed polarity from the previous, as in AC / residential electric services. This phenomenon is a result of the repeated build up of very high voltage, i.e. hundreds of volts, in the condensor and is akin to the ringing / resonance of a church bell or say the liberty bell.

It has been found for efficiency and strength of these individual series of sparks, that the initial polarity at the spark plug center electrode should be negative. This means that the current / electron flow / spark is from the plug's center electrode to the ground electrode. This first, initial spark of the series is much stronger, i.e. higher voltage, than its successors as it has to ionize the fuel and air mixture that occupies the space between the center electrode and the ground one. Once ionized the fuel air mixture offers less resistance to the alternating polarity, less intense series of sparks that follow.

The polarity of the initial, ionizing spark for a given cylinder's power stroke is determined by the direction of the initial current flow through the primary coil windings. The initial flow of current / electrons comes from the battery or the generator / alternator. This initial current flow determines polarity of the magnetic field that builds to a saturation point each time the points close.

Depending on which end of the primary coil winding is connected to the current source, i.e. one of the terminal posts in the terminal box, the polarity of the initial magnetic field will be reversed, as in N/S or S/N. (I'm thinking this is Lenz's law, the right hand thumb rule) On the Model A the initial flow of electrons into the primary winding is always flowing in the same direction. Reverse the flow of electrons through the primary winding and the initial magnetic field reverses. The initial N/S or S/N polarity of the initial magnetic field ultimately determines whether or not the initial strong spark at the plug flows from the center electrode to the ground side or vice versa.

The stock Model A uses a + positive ground electrical system. Automotive lead-acid batteries have an anode and a cathode. The aNode is the Negative post and the cathode is the positive post. The positive ground system is, IMHO, easiest to understand and diagnose if modern electron flow theory, which was proven around 1900, is used. This means that electrons flow from the - negative post of the battery through the cable and wire connections to the terminal box and on to one or the other sides of the coil. But what determines which end of the primary winding will always give us the correct N/S or S/N initial magnetic field polarity? I believe as Kirby does in his post above that it is the way the two windings are connected at one end, but I'm not sure. It may be something deeper in Lenz's or Faraday's laws.

As I interpret it, the coil doesn't care which end of the primary winding you connect the current source to. It's the spark plugs that need it to be connected correctly.
 
Last edited:
Rob, You just restated what I said in Post #7, but perhaps in a more elegant way and in a way that is easier to understand.

Perhaps you can explain to me how an electronic ignition circuit works. There is no condensor so there is no LC circuit to ring. It seems to me that there is only one spark, the initial one.

I have a miniature coil that I use for my antique model airplane engines. It has an electronic circuit glued to it that causes multiple sparks, like a Model T coil. I have no idea how that works, but I do know that it is easier to start the engine with this setup.
 
Last edited:
Neil, thanks for the response. I meant to credit your description, Tom's and Kirby's too. but it took so long to write my comments, I forgot to do it. I was exhausted.

Reading your's and Tom's description, triggered a light bulb for me.It was mainly your last sentence in the first paragraph, though. After rereading it multiple times coming back to it, a specific memory of a few pictures depicting magnetic fields and coils from a book I was loaned back in 1968 popped into my mind. I'm pretty sure it was depicting Lenz's right hand thumb rule. The book was a basic course in HAM radio! I've spent a good deal of time trying to drive the full understanding of the Model A ignition system deep into my aging head over the last couple of years. I've read several articles.

Also, the article in the link put up by ModelA29 https://mossmotoring.com/coil-polarity/ was helpful, but didn't quite do it for me, but the pictures of the coil windings in the article home right in on what Kirby said about the shared connection at one of the primary windings terminals.

I have no knowledge of how the electronic ignition works, but I can easily imagine that it puts out a single powerful spark across the plug. I have a Lincoln Welder that weighs 40 pounds. It uses 115 volt wall plug and will weld 1/8" steel slick as a whistle. It's modern inverter technology.
 
Re: electronic ignition- As I recall, there was a product advertised in car magazines a long time ago that claimed to continuously fire the plug after the initial timed spark. I have a vague idea that it had a clever name (multi-spark?) but it’s all a bit hazy. Now I read that the regular old points-condenser-coil system does it too, though the subsequent sparks aren’t as high voltage.
Lifetime learning, courtesy of the Model A community.
 
Katy, thanks for the link. From the article it seems that the electronic ignition produces one spark, although hotter than the traditional points, coil, condensor system. I know drag racers use a very hot spark and a large gap for the plugs. The plugs are replaced every race because the electrodes burn away.

Full Race Flat head, You can just use the pencil trick. If you put a graphite pencil in the path of the spark the sparks coming off the pencil indicate the direction of electron flow. Best done in low light.
 
The electronic ignition just uses a different trigger to affect the collapse of the magnetic field in the coil core. The capacitor was there to keep the breaker points from arcing and to prolong the high tension event for a split second longer. The Hall effect trigger doesn't need any protection from arcing but there is likely still a capacitor to give the high tension event a good kick and to protect the coil from back voltage. All this stuff in potted on the modern coil per cylinder set ups so you can't see the make up of the coil trigger circuit. The old GM HEI had the trigger circuit in the module. All the modern stuff is relatively reliable but it's hell to find a weak coil on a Ford modular V8 when it won't throw a code. The electronic oscilloscope is the only way to monitor what's going on. The modern engine control unit scan tools are expensive.
 
You must log in or register to reply here.
Back
Top