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Coil Resistor?

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    Posted: 29 Feb. 2008 at 1:59pm
If you use a 12 volt battery in a 6 volt system, do you need to wire-in some sort of resistor for the coil, or can you use it as is?  Does anything else have to be changed?
 
Maybe I should just get a 6 volt batteryLOL
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote westforkboyd Quote  Post ReplyReply Direct Link To This Post Posted: 29 Feb. 2008 at 2:16pm
Jack
 
Yes you need the resistor. Also all you need a 12V gen or alt. All the lights need to be 12V. you can use the starter as long as you don't over crank it. Same with the horn.
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote rocnroll Quote  Post ReplyReply Direct Link To This Post Posted: 29 Feb. 2008 at 2:35pm
Why not just get a 12volt coil?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote westforkboyd Quote  Post ReplyReply Direct Link To This Post Posted: 29 Feb. 2008 at 3:02pm
You're right Roc
 
Was trying to point out there would be more to change than just the ignition. If you do convert to 12V you should get a  12V coil.
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote rocnroll Quote  Post ReplyReply Direct Link To This Post Posted: 29 Feb. 2008 at 5:59pm
............with a built in resistor.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote samcj2a Quote  Post ReplyReply Direct Link To This Post Posted: 29 Feb. 2008 at 6:09pm
In this discussion, I have been trying to remember what I used to know about earlier 12 volt ignition systems that used a different starting voltage than a "running" voltage.   What is it that I'm thinking of that was done back in the day along those lines? 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Jus*Jack Quote  Post ReplyReply Direct Link To This Post Posted: 29 Feb. 2008 at 6:34pm
Another change that should be mentioned is the fuel gauge...that's why most of them don't work any more, I think.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bkreutz Quote  Post ReplyReply Direct Link To This Post Posted: 29 Feb. 2008 at 6:45pm
Most all coils work on 6v, even the 12v ones. When they changed over to 12v systems, the different manufacturers handled this differently, some used an exterior resistor, others built the resistance into the batt wire to the coil (Ford) and others used a coil that used an internal resistor. I think the cleanest way to do it now would be to use the one with the internal resistor, make sure you ask before buying. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote rocnroll Quote  Post ReplyReply Direct Link To This Post Posted: 29 Feb. 2008 at 7:33pm
Originally posted by samcj2a samcj2a wrote:

  What is it that I'm thinking of that was done back in the day along those lines? 
 
Sam, I believe what you're thinking about is the Ford (or universal type) starter solenoid. One terminal gives full voltage for starting and then drops back some in running position....Note: Electricity is NOT my thing!..Shocked
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 46CJ2A Quote  Post ReplyReply Direct Link To This Post Posted: 29 Feb. 2008 at 7:36pm
Actually, all I want to do is to try starting my newly rebuilt motor.  I have all new or rebuilt 6 volt electrical parts that I plan to use on the finished Jeep but I haven't bought a harness  yet.  I just wanted to know if I could use the 12 volt battery to start the motor without the generator, voltage regulaer, amp meter, fuel guage, sender, etc.
 
Guess I shoulda asked that question, huh?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote rocnroll Quote  Post ReplyReply Direct Link To This Post Posted: 29 Feb. 2008 at 8:41pm
Yes to both.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote samcj2a Quote  Post ReplyReply Direct Link To This Post Posted: 29 Feb. 2008 at 8:47pm
Isn't there a risk of burning out the coil if there is no resistor?  I was tempted to answer just as roc did, but I'm not sure about the question I just raised. 
 
Edit:  With the same impedence in the coil, there would be twice the current flowing through the windings.  I just don't know the consequence of that over a short duration.   I do know that a resistor doesn't cost too much and is available at places like Advance Auto and such.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote rocnroll Quote  Post ReplyReply Direct Link To This Post Posted: 29 Feb. 2008 at 8:57pm
The 12 volt coil or the 6 volt?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote samcj2a Quote  Post ReplyReply Direct Link To This Post Posted: 29 Feb. 2008 at 9:02pm
I was thinking that the context was using the 6 volt coil briefly with a 12 volt battery.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote westforkboyd Quote  Post ReplyReply Direct Link To This Post Posted: 29 Feb. 2008 at 9:16pm
You guys know I'm not a n electrical expert either. Shocked But I think power only flows through it when the points are closed. I'd think you could run it for a short period with the 12V. However trying to start a newly rebuilt engine usually takes more than just a short start-up. Fuel, timing, compression and such. Better spring for the new 6 or 8 volt battery before you try it.
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote samcj2a Quote  Post ReplyReply Direct Link To This Post Posted: 29 Feb. 2008 at 9:48pm
Well, I thought that I understood this until I read this page http://users.mrbean.net.au/~rover/ketterin.htm  and saw that even though something less than 10 amps flows through the primary at a voltage difference of 6 volts, the effect of the change in the magnetic field created by the current in the primary coil windings when the points open and the flow of current stops results in approximately a rise in voltage to 250 volts across the primary windings.  At this time, the ratio of the turns in the primary to secondary windings of the coil (usually of magnatude 100 to 1) induces a voltage of up to 25,000 volts in the secondary windings of the coil which is enough to jump the plug gap and fire the air/fuel mixture.   The higher primary voltage will result in twice the current flowing through the primary windings which are pretty small, so they get hotter, and I presume that a much larger voltage is induced in the secondary windings so they get much hotter too.
 
The bottom line to me is a significant risk of burning up either the primary or secondary windings in the coil.   So, I would do as WFB says, get the correct battery or get a resistor.
 
Since this is all theory, practice could be different, and, as they say, your mileage could vary!
 
Edit:  The after-thought is roc's earlier reference to the higher voltage for starting used in some 12 volt systems.  That practice would suggest that there is enogh margin in the coil design to sustain a brief period of higher voltage.  I think that scheme bypasses the resistor so the full 12 volts flows through the coil and I think that the reason I recall that scheme was that I heard of a car that would crank and start but stop running as soon as the driver stopped cranking the engine.  The cause was that the resisitor was burned out but it would start because the starting circuit by-passed the resistor.
 
Okay, we're all tired of this subject by now, but I just found another reference that explains why the 12 volt system uses a bypass of the resistor during starting.  It is because the use of the starter draws so much current that the voltage available to drive the coil is often not enough to create enough spark to start the engine and a higher starting voltage is needed at the coil.   The catch is that this means that during starting in those 12 volt cars, it is not 12 volts flowing through the coil but a reduced voltage due to the big draw of the starter.  So, follow WFB's advice!  I'm done on this.  Confused
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Post Options Post Options   Thanks (0) Thanks(0)   Quote rocnroll Quote  Post ReplyReply Direct Link To This Post Posted: 29 Feb. 2008 at 10:30pm
Bite the bullet....either buy a 6volt battery or a 12 volt coil w/built in resistor and get it runnin'!!.............or I guess you could just keep reading. LOL
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Post Options Post Options   Thanks (0) Thanks(0)   Quote p3ferris Quote  Post ReplyReply Direct Link To This Post Posted: 29 Feb. 2008 at 10:56pm
I used a 6 volt battery, jumped it with my 12 volt battery, started my jeep, disconnected the 12 volt right away.  I had no problem This was done about 8 times.  I now have a 12v system with aa 6v.starter  If you leave the 12v on it will blow the coil.
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