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Old 05-29-2018, 06:55 PM   #1
Brkile
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Default Are Model A coils polarity sensitive ??

I've always just slapped them on and they seem to work ok. On coils that are marked + and -, which wire goes where ?? thanks
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Old 05-29-2018, 07:22 PM   #2
harryc
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Default Re: Are Model A coils polarity sensitive ??

some cars are very sensitive and others aren't. Like you I have just slapped the wires on for many years and they have worked fine on nearly all of my cars. Recently I was having a problem locating the reason for repeated spark plug fouling. I finally broke down and checked the polarity properly and discovered it was reversed. I corrected it and the fouling ended and I was one happy guy.
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Old 05-29-2018, 07:44 PM   #3
MAG
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Default Re: Are Model A coils polarity sensitive ??

The polarity on the coil is related to the polarity of the battery. On a Model A, if wired as original, the positive battery post is grounded to the chassis.
Constant voltage is applied to the (-) post of the coil.
The ignition coil should be wired so that the positive side (+ post) of the coil is going into the direction of the chassis, i.e. positive ground.
The wire going to the ignition switch from the coil (+) will ultimately find its way to the distributor. In the distributor, when the points close, ground is provided to the coil turning it ON.
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Old 05-30-2018, 12:00 AM   #4
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Default Re: Are Model A coils polarity sensitive ??

The coil will provide a spark no matter how the coil is wired BUT the spark will be hotter if the polarity is correct.


It has to do with the spark jumping from the center electrode of the spark plug to ground or from ground to the center electrode.


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Old 05-30-2018, 05:31 AM   #5
Patrick L.
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Default Re: Are Model A coils polarity sensitive ??

Yes they are to a certain extent. They'll if hooked up wrong but at about a 30% or so reduction. Spark likes to jump in the right direction.

The coil should be hooked up according to the battery. Positive ground, positive wire to coil to points.

I'm basically just repeating what the other fellas said.
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Old 05-30-2018, 08:12 AM   #6
cpf240
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Default Re: Are Model A coils polarity sensitive ??

How do I determine which post is which on an unmarked coil?
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Old 05-30-2018, 09:28 AM   #7
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Default Re: Are Model A coils polarity sensitive ??

To test whether or not the polarity of the coil is properly connected, with the motor running and a loose spark plug wire arcing to ground and creating a spark. Take a lead pencil and put the lead of the pencil into the jumping arc with an air gap on both sides of the pencil's lead. The lead acts as a capacitor and slows the arc to the point that you can see which way the arc is jumping. The spark will be jumping to the ground, or it will jumping from the ground.... You want it to be jumping to the ground. To reverse the spark's jumping direction, reverse the primary leads on the ignition coil.
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Old 05-30-2018, 09:38 AM   #8
steve s
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Default Re: Are Model A coils polarity sensitive ??

The science of what's going on is pretty simple. The executive summary is that it's easier for electrons to jump from the hotter spark plug tip than from the cooler spark plug rim.

More details:

The coil is basically an electrical transformer. It responds to a changing amount of current flowing thru its low voltage winding (due to points opening and closing) by creating a MUCH higher voltage, or energy, difference between the two ends of its secondary winding. In order for Conservation of Energy to be obeyed, the higher, secondary voltage can generate only a much smaller amount of current (albeit with a powerful "poke"!). If there's no viable, conductive path connecting the two ends of the secondary winding to each other, the electrons will be jamming up at the negative end and wanting in the worst way to get to the positive end--possibly even to the point of jumping through air to do it.

A sparkplug's spark consists of electrons jumping from one side of the gap to the other as they endeavor to get from the negative to the positive end of the coil's secondary winding. Depending on how the battery is connected to the coil, one or the other end of the coil's secondary winding will be negative and thus the potential source for jumping electrons.

The hotter any metal is, the easier it is for electrons to jump off of it, AND since the tip of the spark plug is out there surrounded by either combustion or that white insulator stuff, whereas the edge of the spark plug has a better thermal connection to the "cooled" cylinder head, the tip will be hotter and provide a stronger spark if it's at the negative end of the coil's secondary winding.

So, you want to set things up so that electrons are coming down the wire from the coil to the distributor, then out of the sparkplug tip, across the gap, and back thru various ground connections to the other end of the coil's secondary winding (which is unseen and always grounded).

As others have indicated, once that coil pumps electrons' energy up to a few thousand volts, they will jump the gap in either direction, but they'll do it better in one direction than the other.
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Old 06-02-2018, 12:03 AM   #9
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Default Re: Are Model A coils polarity sensitive ??

Quote:
Originally Posted by SSsssteamer View Post
To test whether or not the polarity of the coil is properly connected, with the motor running and a loose spark plug wire arcing to ground and creating a spark. Take a lead pencil and put the lead of the pencil into the jumping arc with an air gap on both sides of the pencil's lead. The lead acts as a capacitor and slows the arc to the point that you can see which way the arc is jumping. The spark will be jumping to the ground, or it will jumping from the ground.... You want it to be jumping to the ground. To reverse the spark's jumping direction, reverse the primary leads on the ignition coil.
Pictures of what Ssssteamer is talking about can be found here.

http://modelabasics.com/Ignition.htm
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Old 06-02-2018, 08:21 AM   #10
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Default Re: Are Model A coils polarity sensitive ??

It does matter for engine performance. Simple to check. Check one plug. Remove the connector from the top of any plug. The flare should be towards the plug. If not reverse your coil wiring.
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Old 06-02-2018, 10:36 AM   #11
Purdy Swoft
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Default Re: Are Model A coils polarity sensitive ??

I like the ignition spark and coil tester sold by Brattons .This handy device shows when coil polarity is correct. This tool is on page 80 of the Brattons catalog Part number 17140 . I've never regretted buying the one that ive got . it will work on any old car six or twelve volt . I don't like getting shocked trying to use the pencil trick .
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Old 06-06-2018, 02:57 PM   #12
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Default Re: Are Model A coils polarity sensitive ??

Like Purdy said ...

If you are having slow day... grab the pencil by the metal cap for the eraser.

Kinda like Russian Roulette ... sometimes it will shock you ... sometimes NOT!
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Old 06-13-2018, 02:52 PM   #13
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Default Re: Are Model A coils polarity sensitive ??

Quote:
Originally Posted by Brkile View Post
I've always just slapped them on and they seem to work ok. On coils that are marked + and -, which wire goes where ?? thanks
The following URL will get you what you need and also provide the way to do it!

http://www.underhoodservice.com/tech...odel-vehicles/
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Old 06-13-2018, 08:20 PM   #14
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Default Re: Are Model A coils polarity sensitive ??

Quote:
Originally Posted by DHZIEMAN View Post
The following URL will get you what you need and also provide the way to do it!

http://www.underhoodservice.com/tech...odel-vehicles/
On this link, Randy Rundle has you set the VOM to the highest voltage range, but he does not say to use a DC range, not an AC range. You MUST use a DC range for this.

The actual difference between a coil for negative ground systems and positive ground systems rests upon which terminal the secondary winding returns to. When the points open and the coils magnetic field collapses, the secondary reaches a very high induced voltage. But,so does the primary winding. In the case of the correct coil for the polarity of the car, the primary's induced voltage adds to that of the secondary. If miss matched, the primary's induced voltage fights the secondary induced voltage.

It is easy to meter a coil and see which it s, but requires a very sensitive meter, as the ratio of primary to secondary winding is very large. I suppose no one wanted to know any of that, just spend the $10 or less on a new one and be sure.
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