ignition coil

Lawrie

Lawrie

Well-known member
Not for my 33 or 34. but for a model A
What. is the difference in the resistance mean for the spark and points,
I see running a points 6v pos earth distributor on my test machine, a .7 Ohm coil puts out a way better stronger looking spack than a 2.5 or 3.0 ohm one.
I have copper leads .
The coil. that seemed to work the best was a modern square one with .7 ohms.
any help.
what will maybe bugger up by using the .7 ohm coil
Thanks Lawrie
 
You generally like to see 4 amps maximum going through the points. More will tend to "burn" them, while less will produce a weaker spark. Using Ohm's Law, the .7 ohm coil will send abou 8.5 amps through the points, while a 3 ohm coil will send 2 amps. This is assuming a 6 volt system and no ballast resistor. The .7 amp coil should run better, but at the expense of accelerated point and coil damage. The 3 ohm coil (generally used with 12 volt systems) will produce a weak spark. 6 volt coils are usually 1.5 ohms and are also used on 12 volt systems with a 1.5 ohm ballast resistor. 6/1.5=4 and 12/3=4.

.7 ohms is a little low, is that "modern square one with .7 ohms" perchance for some sort of electronic ignition?
 
Thank you very much for that info and lesson on the Ohms law.
You know I can make a new distributor for the A engines we build ,with mech advance and vac as well, and they work great, But they most are Hall effect and 12v neg earth, this is the first one with 6v pos earth, hence the points.
I have spent the day with the distributor on my test bench trying a lot of different coils,
Funny thing,
at the end of the day and after frying a set of points ,(now I know why I fried the points, accidentally put 12v into the .7 ohm coil) I find the nicest spark and none at the points is a 1.2 ohm coil,(6V),
but tomorrow will try a 3 ohm coil/
will it mater if it is marked 12v? but I will use 6v?
But with the info you gave me on the Law, I can now understand it.
Lawrie
 
Lawrie,

If you have a multi-meter, you can measure the actual resistance of the coil. The "12 volt" / "6 volt" labels on the coil can be misleading, because of the presence or absence of a ballast resistor. Early in the 12 volt era, a lot pf 12 volt vehicles actually used 6 volt coils with a ballast resistor. Check it yourself and be sure (the resistance across the primary terminals). If marked "12 volt", a coil should be 3 ohms, but if you check, you will be sure.

One thing I hadn't taken into consideration is that I don't know if Model "A"'s used the resistance coil in the ignition system like the early V8's. If they do, it throws another monkey wrench in the works.
 
No, the A I'm distributor Im working on is just 6v pos earth,
But Im testing it on my machine that has a 12v and 6 v socket, both pos earth,BUT the 6v has the V8 reciter in the circuit's the coil has only been seeing 3-1/5 or so volts, but I have a bypass button on the 6v feed that bypasses the resister so I can get the 6v at the coil. yes I have a multi meter,(Fluke) . but never new the relationship between the ohms and volts and amps.
So tomorrow I will work on it again, but way more confident ,
so what is the resistance (ohms ) of the coil mean, more ohms = less or more spark.
But I assume that using the law,6v and lower ohms equals more amps, so more spark?
Lawrie
 
I had a think, the spark intensity must be related to the type of windings in the coil, number of turns and wire size ?
I see in my ford service books they list the number of turns etc in the coil primary and secondary circuits, there are lots of turns .
I will measure an old ford one and a repaired one to see what the ohms are,
I must have them both (33 and 34) as they do heaps of miles and no problems
Worth measuring them just to see.
Lawrie
 
Both , an original ford one and a skip repaired one for my stock 33 distributors are .6 ohm
Lawrie
 
Lawrie said:
Both , an original ford one and a skip repaired one for my stock 33 distributors are .6 ohm
Lawrie
Click to expand...


The 32-48 Fords have a ballast resistor on the firewall behind the dash, add the resistance of that to the coil to find your total resistance, then apply Ohm's law.
 
38 coupe said:
The 32-48 Fords have a ballast resistor on the firewall behind the dash, add the resistance of that to the coil to find your total resistance, then apply Ohm's law.
Click to expand...

Remember, he is working on a Model "A" here. I don't think they have that resistor, but I'm not sure.
 
On a Model A you need to be around 1.5 ohms. Anyting more the spark will decrese, anything too much less and your points will burn prematurely. A 1.5 Ohm 6V coil is about the most basic 6V coil Availavle. A V-8 Uses a .6-.8 ohm coil originally stacked with a .8 ohm resistor equating to the same 1.5 ohms. Model A's do not use the resistor although you could with the .7 ohm coil
 
Thank you very much for that info,
Funny,
What about using the modern plug wires.
I know on my 33 only to use copper ones .
thank you
Lawrie
 
Modern as in Solid core wires, will make no difference. Modern as in Resistor core wires, yes they will affect it but I won't run them so I don't know to what degree it would affect it.

Unless you have a radio; resistor core wires are not needed and will not last near as long as solid core.
 
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