12 volt coil wiring

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LittleAl
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12 volt coil wiring

#1

Post by LittleAl » Thu May 07, 2009 3:37 am

my '54 FL is 12 volt. I'm just about finished putting her all back together. I bought it with a round 12 volt coil hung off one of the downtubes so I bought a replica 12 volt coil from NOS Parts but it has no markings on the terminal at all. Before I burn it out (that would be my luck, lol) I just want it confirmed by someone using their coil that when in the mounted position the primary terminal to the rear of the bike is for 12 volts and the terminal towards the front is for the timer wire.

I also have a question about testing for spark. If I have 12 volts to the correct low voltage terminal and the timer wire hooked up with the points closed and I manually open the points the spark plug should fire, right?

Thanks!



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Re: 12 volt coil wiring

#2

Post by steinauge » Thu May 07, 2009 11:42 am

Yes and yes.

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Re: 12 volt coil wiring

#3

Post by LittleAl » Thu May 07, 2009 1:34 pm

thanks!

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Re: 12 volt coil wiring

#4

Post by FlatHeadSix » Thu May 07, 2009 2:26 pm

actually Al, and Steinauge, conventional hook up for the OEM square coils is just the opposite of how you stated it. With the coil mounted the battery voltage coming from the switch should connect to the front (left) terminal, the wire coming from the circuit breaker connects to the rear (right) terminal. The coil will work either way and it will not harm it to connect it "backwards", it just changes the polarity. There has always been an argument about the affect of polarity on the intensity of the spark and life of the plug but I've never been convinced that it really makes any difference. Just like the the plug wires, some guys believe that the front cylinder should be connected to the rear terminal on the coil. Its a wasted spark system and it should not make ANY difference at all.

Your test method is correct, every time you open the points the plugs should fire. Try connecting the primary leads on the coil both ways and see if you get a fatter spark with the ignition wire connected to the front terminal, let us know what you find.

mike

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Re: 12 volt coil wiring

#5

Post by FlatHeadSix » Thu May 07, 2009 3:13 pm

See diagram, read the key for #26

Image

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Re: 12 volt coil wiring

#6

Post by LittleAl » Thu May 07, 2009 8:36 pm

Mike, that's pretty interesting. I can't wait to get home now and put to practice what you stated. I'll be especially curious about the spark with the primary connections in the two different configuration. I will certainly report back what I find. I have one of those thumb screw adjustable spark "testers" so I might actually be able to see how big a gap the spark can jump in either configuration, rather then just observing the intensity on a spark plug.

hummm, interesting little experimenting tonight....

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Re: 12 volt coil wiring

#7

Post by FlatHeadSix » Fri May 08, 2009 12:06 am

I've been waiting for one of you eagle-eyed sharp shooters to point out the error in the '49-'54 wiring diagram I posted. The drawing is from the Clymer's 48-65 Panhead Manual that's been in print forever, and it has always been wrong. Take a close look at the coil in the drawing, the mounting bracket is facing the viewer of the drawing which would actually reverse the positions of the terminals when mounted on the bike.

The text, however, is correct. In every factory shop manual, for all twins which use a square coil, the text instructions all tell you to connect the lead from the circuit breaker to the rear terminal on the coil.

mike

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Re: 12 volt coil wiring

#8

Post by FlatHeadSix » Fri May 08, 2009 1:52 am

just to keep this interesting, and to give Al another chance to indulge his science experiment appetite, I'll throw in this link we've posted before. It is probably the best and simplest explanation of plug polarity that I've seen, thanks to the folks at the MG car club.

http://www.chicagolandmgclub.com/techti ... l/574.html

there is lots of info out there regarding plug polarity and, depending on the source, it appears that hooking them up the wrong way will make them less efficient by requiring anywhere from 15% to 40% more energy to make the spark jump.

mike

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Re: 12 volt coil wiring

#9

Post by awander » Fri May 08, 2009 3:55 am

Mike:

I did notice that the picture was backwards-chalked it up to "artistic license".

Of course, I will now have to go check my bike to make sure it is hooked up per the text, and not the picture...

I have heard about the coil polarity making a difference before, specifically in regards to Farmall trractors, but I had never seen any kind of explanation before-thanks for posting that link!

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Re: 12 volt coil wiring

#10

Post by LittleAl » Fri May 08, 2009 6:02 pm

nice article Mike.

I didn't get to my science project last night. A close buddy with a problem needed a ride to the ER and it took hours. Lucky he's ok, but I didn't get home until like 3 am.

tomorrow I have the whole day for my weird science, lol

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Re: 12 volt coil wiring

#11

Post by LittleAl » Sat May 09, 2009 9:11 pm

For all of you interested in the results of my science experiment, this I what I was able to observe:

1) opening the points manually will fire the plugs with the coil primary connections either way, 12 volts to rear terminal/timer to front or the reverse, 12 volts to front terminal/timer to rear.

2) with my spark tester, the type that is basically a pointed screw to a pointed terminal with a ground clip the spark will jump the gap up to about a 1/2 inch. Beyond a 1/2 inch neither primary wiring configurations will jump it.

3) trying both wiring configurations with spark plugs well grounded the spark changes between the 2 wiring setups. With 12 volts to rear terminal/timer to front an adequate spark is observed. Reversed, with 12 volts to front terminal/timer to rear the spark is definitely what I would think is better, no yellow in the spark, a nice deep dark blue and has what I would describe as more "snap" to it.

Un-scientific, absolutely. Take what you will from it but it has convinced me that for my bike, my coil, my plugs I'm keeping the 12 volys on the front terminal and the timer wire to the rear.

well, that was fun! lol

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Re: 12 volt coil wiring

#12

Post by FlatHeadSix » Thu May 28, 2009 1:06 am

Thanks for the report Al, good observations.

I was going to mention that the reproduction 12 volt coils may not be wound in the same direction as the OEM units, but it looks like they are. At least yours is anyway. Your connections will be exactly like the shop manual instructs.

mike

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Re: 12 volt coil wiring

#13

Post by DCoul » Wed Jan 23, 2019 6:08 am

I think those old British cars were positive ground.

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Re: 12 volt coil wiring

#14

Post by RooDog » Wed Jan 23, 2019 2:57 pm

While wiring up my project 1950 Panhed I had the same question regarding the polarity of the dual post, dual fire coil. After much research, looking at Harley wiring schematics, pictures , and Harley ignition theory, my conclusion is that it makes no difference as to which terminal you feef the + power to, nor run to the points from. Much as it makes no diff as to which plug wire goes to which cylinder. I don't have reference leads for you at this time, but what I say is no doubt why the coils for a Harley dual fire system are not marked. Now, if you were to run single fire coils on your bike, or a normal points system in a car engine, then polarity does matter. As always, Harleys are a different animal from the rest of the herd.....
RooDog

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