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Old 04-04-2017, 02:54 PM   #1
wmws
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Default Coil Polarity

I have seen many wiring diagrams of the Model A ignition system but when they show the coil they don't show what wires go to the + and what goes to the -. The black wire is coming from the ammeter and the red to the ignition switch. I currently have the black to the + on the coil and the red to the - on the coil. Is this correct for the proper polarity. I have tried the lead pencil trick but my eyes are not as good as they once were and I'm not sure if I am seeing the flair. Thanks for any help.
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Old 04-04-2017, 03:18 PM   #2
zzlegend
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Default Re: Coil Polarity

I use one of these.

A-12116
Documents

Model A Polarity Tester

Simply place this unit between the distributor cap and the coil wire, and it indicates if you have the wires on your coil properly as well as if you have spark or not. Comes with complete usage instructions, genuine Nu-Rex. U.S.A.



  • Quantity A-12116 $24.95 / ea.
    1928-31
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Old 04-04-2017, 03:31 PM   #3
Bob C
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Default Re: Coil Polarity

If you have a stock positive ground system then the red goes to the +.

Bob
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Old 04-04-2017, 03:31 PM   #4
Art Newland
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Default Re: Coil Polarity

The ignition switch heads straight to the points, then ground, so on a positive ground car, the red wire connects to the +.
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Old 04-04-2017, 03:32 PM   #5
Art Newland
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Default Re: Coil Polarity

Yep... what Bob C said. 8^)
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Old 04-04-2017, 05:52 PM   #6
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Default Re: Coil Polarity

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If it runs BETTER with a certain arrangement, THEN YOU GOT IT RIGHT!
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Old 04-04-2017, 06:02 PM   #7
JDupuis
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Default Re: Coil Polarity

I have a 6volt coil for my run-in stand. It does not say + or - it has "bat" and "dist" written on it. I think I've got it hooked up right. . Jeff
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Old 04-04-2017, 06:42 PM   #8
Tom Wesenberg
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Default Re: Coil Polarity

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Originally Posted by JDupuis View Post
I have a 6volt coil for my run-in stand. It does not say + or - it has "bat" and "dist" written on it. I think I've got it hooked up right. . Jeff
BAT and DIST is a dumb way to mark a coil because who knows what it was original intended for?
Best to just connect it and then check for spark polarity.

If anyone doesn't know about checking for spark polarity, just do a Google search for "coil polarity" and read the one by chicagolandmgclub.
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Old 04-04-2017, 07:06 PM   #9
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Default Re: Coil Polarity

Your better off with a coil that is marked +. Or -- So you know exactly how to wire it.
As Tom said one marked dist / batt could be intended for a neg ground system. If thats the case then you would hook up the Batt to the dist and the dist to the Batt for a pos ground car.

I also have the tool ZZ posted
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Old 04-04-2017, 07:54 PM   #10
Patrick L.
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Default Re: Coil Polarity

Coil polarity should match battery polarity.

Neg ground battery, neg coil to ground/points
Pos ground battery, pos coil to ground/points
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Old 04-04-2017, 10:13 PM   #11
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Default Re: Coil Polarity

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Originally Posted by Patrick L. View Post
Coil polarity should match battery polarity.

Neg ground battery, neg coil to ground/points
Pos ground battery, pos coil to ground/points
Patrick L., Good way to remember.
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Old 04-05-2017, 11:05 AM   #12
wmws
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Default Re: Coil Polarity

Thanks for the help. I have it hooked up right now and with the pencil I think I see the flair toward the plugs.
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Old 04-05-2017, 11:30 AM   #13
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Smile Re: Coil Polarity

and tell me wmws..... have you noticed any difference of the running of the engine with the polarity switched either way? I certainly haven't noticed any difference on my A with it either way... but if it revved to 4000 or more I guess that could be a different matter
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Old 04-05-2017, 11:38 AM   #14
Tom Wesenberg
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Default Re: Coil Polarity

I drove my Model A for 2 years with the coil wired wrong, and it never made any difference. Only when you need max voltage to fire the plugs you might feel a misfire.

The coil output is the same either way, but when the hot center electrode is negative it fires easier.
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Old 04-05-2017, 01:06 PM   #15
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Default Re: Coil Polarity

I have one Model A coil that doesn't have any markings on it.
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Old 04-05-2017, 01:51 PM   #16
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Default Re: Coil Polarity

Yep. You only should notice any difference under extreme conditions. Wrong coil polarity can reduce output by 20-30%.
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Old 04-05-2017, 02:31 PM   #17
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Default Re: Coil Polarity

No I don't notice any difference in performance but if it is that easy to have it the right way why not have it right.
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Old 04-05-2017, 02:50 PM   #18
Patrick L.
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Default Re: Coil Polarity

Quote:
Originally Posted by wmws View Post
No I don't notice any difference in performance but if it is that easy to have it the right way why not have it right.


Oh, I agree. I like right.
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Old 04-05-2017, 03:32 PM   #19
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Default Re: Coil Polarity

Quote:
Originally Posted by Fullraceflathead View Post
I have one Model A coil that doesn't have any markings on it.
I ran into this and that's exactly why I bought the tool I posted. I don't trust markings on a coil or the little red and yellow dots on tires, but that's another story.
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Old 04-05-2017, 03:34 PM   #20
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Default Re: Coil Polarity

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Originally Posted by zzlegend View Post
I ran into this and that's exactly why I bought the tool I posted. I don't trust markings on a coil or the little red and yellow dots on tires, but that's another story.
Times 2,,, especially the tires.....
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