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Old 07-15-2020, 07:02 PM   #1
Bill Goddard
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Default Coil Resistance

I have a problem with coil resistance. I think I am doing everything right by using a 1.5-ohm coil with a 1.5-ohm ballast resistor and a 12volt coil.

When I run the car up to 45mph I get cutouts and some backfire. The coil gets too hot to handle. This cutting out could be a fuel problem but the coil should not get so hot should it?

I just read today the extensive discussion on cutting out at high speeds from some years ago but it didn't help me much. However; I have concluded that that the habit of mounting all coils on the model A terminals down is wrong. Oil-filled coils should be mounted terminals up and epoxy filled coils can be mounted terminals down.

This resistance and 6v/12v coil business are driving me nuts. Can anyone explain why I need a 1.5-ohm resistor with a 1.5 -ohm 12v coil or no resistor with a 12volt 3-ohm coil? I have a 12volt neg grnd car.

Thanks, Bill G
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Old 07-15-2020, 07:49 PM   #2
Jack Shaft
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Default Re: Coil Resistance

6v coil 1.5ohm
12v coil 3 ohm

1.5 ballast resistor and 1.5 ohm coil equals 3 ohm..but I wouldn't trust it,i would run a 3 ohm coil with a 12v system,ballast resistors are sketchy.
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Old 07-15-2020, 09:01 PM   #3
Bill G
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Default Re: Coil Resistance

From one Bill G to another, the math seems to work, but the hot coil is not good. What about trying another 12-V coil?
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Old 07-15-2020, 09:47 PM   #4
30 Closed Cab PU
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Default Re: Coil Resistance

The coil runs cooler with a 1.5 ohm ballast with a 1.5 ohm coil, in coparison to a 3 ohm coil. The total power dissipated is the same in each case, but the 1.5 ohm coil with a 1.5 ohm ballast resister share the power dissipation.
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Old 07-16-2020, 10:11 AM   #5
Purdy Swoft
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Default Re: Coil Resistance

I use a three OHM coil on twelve volts with NO ballast resister . If a one point five Ohm coil is used on twelve volts a resistor will be needed at the coil connection to prevent the full twelve volts from entering and burning out the six volt coil .
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Old 07-16-2020, 10:22 AM   #6
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Default Re: Coil Resistance

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Quote:
I have a problem with coil resistance. I think I am doing everything right by using a 1.5-ohm coil with a 1.5-ohm ballast resistor and a 12volt coil.
Are you putting the 1.5 ohm ballast resistor in series w/the 1.5 ohm coil, not in parallel?
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Old 07-16-2020, 12:49 PM   #7
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Default Re: Coil Resistance

An ignition system in good condition won't get all that hot while the engine is running. When the engine is not running, with ignition switch on and points closed, the coil becomes an electromagnet and will get hot. An ignition system with a bad condenser and a failing set of breaker points will not be dissipating the electromagnetic energy very well so it will get hot under those conditions but it will also be running like crap. If a coil has a damaged winding, it can have a higher than normal resistance and get very hot. The old coil testers from back in the day had a warm up cycle to get them up to operating temperature before the test would begin. It would let the technician know pretty quick if it was a good functioning coil or not.

While I can agree that a 1.5 ohm coil will dissipate heat better having an external ballast resistor just for the fact that is is external to the coil, I don't think it makes it any better than a 3 ohm coil that has epoxy insulation. They are made to take the heat that they normally dissipate since it is an inductor and not a resistor. Like I mentioned before, any coil in an ignition system that is working properly should not get all that hot. They should be touchable for several seconds without scorching your hand. The ignition system only build the magnetic field for a split second before it makes each induction cycle or spark. This is so rapid that it shouldn't get all that hot.

While some manufacturers may use oil for an insulator in coils of older design, most old originals had bitumen or what is commonly referred to as tar. Tar is basically a solid unless it gets very hot. It will liquify at higher than normal temperatures but this is telling a person that something is wrong with it. Many old coils were designed to be upside down in order for the high tension lead to fit properly. Model As all had coils with the terminal pointing down.
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Old 07-16-2020, 01:50 PM   #8
Purdy Swoft
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Default Re: Coil Resistance

A 1.5 ohm coil should work well with a six volt system and would need no resistor . I would use a 3 ohm coil with a twelve volt system and it would need no resistor . I prefer not to have to use a resistor . A resistor won't be needed if the correct coil is used . I use a 1.5 ohm coil with six volt applications and 3 ohm coils with twelve volts .
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Old 07-16-2020, 01:52 PM   #9
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Default Re: Coil Resistance

How hot does the ballast resistor get? I imagine you just have a failing coil. Is it an original?



Here's a little explanation on the 6/12v coil resistances. While a 1.5 ohm coil at 6v will draw as much current as a 3 ohm coil at 12v (4A) the 12v coil will be using 48W of power split between the coil and ballast while the 6V will only be using 24W. By contrast if you didn't use a ballast resistor on a 1.5 ohm coil you'd be drawing 8A and 96W of power! That coil would burn up quickly.
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Old 07-16-2020, 02:35 PM   #10
Purdy Swoft
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Default Re: Coil Resistance

What ohm coil do you use on six volts . model A's didn't come with a resistor . I've never needed a resistor with six volt applications .
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Old 07-16-2020, 05:44 PM   #11
Jack Shaft
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Default Re: Coil Resistance

Quote:
Originally Posted by Purdy Swoft View Post
What ohm coil do you use on six volts . model A's didn't come with a resistor . I've never needed a resistor with six volt applications .
or 3 ohm coils with a 12v application..I remember those ceramic spring type resistors from years ago..a friend had a 56 ford pickup with one,it wouldn't start in the morning,but once the sun heated the hood it would start..drove us crazy till we found the spring had broken,it would contract when cold,expand when hot..
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Old 07-17-2020, 08:20 AM   #12
Bill Goddard
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Default Re: Coil Resistance

Thanks for all the good replies.
I am currently using a 1.5 amp coil from Bratton's with a ceramic ballast wired in series in the red line passenger side -- negative ground.
Road test shows coil heats to 129 degrees F. I can hold my hand on it for several seconds -much hotter would hurt!
The coil is mounted upside down with terminals down. I believe it is an oil-filled coil. I didn't think to shake it before I put it in. I think I will be ok with this arrangement. Bill G
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Old 07-17-2020, 08:37 AM   #13
Patrick L.
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Default Re: Coil Resistance

Ballast resistors were common in the day. Always recommended customers keep one in the glove box and would show them how to quickly remove/attach the wires if needed.

Last edited by Patrick L.; 07-17-2020 at 03:40 PM.
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Old 07-17-2020, 02:34 PM   #14
rotorwrench
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Default Re: Coil Resistance

The V8 Fords up to 1949 had the bakelite case coils with different shapes for the different distributors that were used in those years from 1932 through 1948. The internal core and coils were set up differently than the can type coils used in the model A era and the 8BA era. A ballast was used on those early V8 cars to control the amperage even at 6-volts but the ballast is more like a 0.8 Ohm or something of that nature. Most of the early designs were Marion Mallory designs after 1934 or so. Henry Ford and Mr Mallory got along well and Ford Motors used Mallory as one of it's main suppliers of ignition parts till the 8BA era.

I looked at ballast resistors for some time to find an actual 1.5 Ohm unit but found that most are actually 1.3 Ohm. They work though if a person changes from 6 to 12-volt but uses the same can type coil.

Over the years of working in aviation, I've become accustomed to feeling for excessive temperature on components. If you touch it and it immediately burns you then it's likely too hot. If you can hold on to it for 3 seconds then it will be in the neighborhood of 160 to 180 degrees F. If you can hold on for 5 seconds then 130 to 160 degrees F is close. If you can comfortably hold it for a long time then it's running cool. I check transmissions and carrier bearings this way all the time but some manufacturers use Teletemp strips that stick to the component like tape. They have little squares that blacken as the temperature increases so a person can look at them and see if the component has been running hot. They make these in various temperature ranges.

A coil should be in the 5 second group. If it burns you then something is wrong with it.
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