coil impedence

Rainmaker Ron

Rainmaker Ron

Senior Member
46 flathead running crab dizzy with tube coil on 12 volts with Chrysler resister. Develops carbon tracks in the cap after a short time. What is the correct impedence required for the coil circuit?

Rainmaker Ron
 
Impedance, or resistance, in the primary of points triggered ignitions is in the range of 1.5 ohms or so. If too low, problems with points burning will occur. Carbon tracking in the cap may be caused by plug gaps too high or bad wires. The coil will try to reach maximum voltage until the plug fires. If the gap is too large, the voltage will increase to the point where it will go to ground where ever it can; like to an adjacent terminal in the cap and out to another plug.
 
The primary circuit resitance of a 12 volt ignition system is typically 3-4 ohms. The ballast ( or loom) resistor is usually 1.5 ohms and the coils resistance is also approx 1.5 ohms.
Using these numbers the total resistance is approx 3 ohms and divided into 12 volts ( higher when running and charging) will result in a current flow thru the system 4 amps.
Current is 3-4 amps but the voltage is what kills the parts, using a 12 volt battery the volt drop of the first resistor ( the ballast) (vd=amps x resistance) is 6 volts and the coil would then drop 6 volts as well. This perfect paper world would leave on a few mili-volts to travel across the contacts. (doesnt ever really happen this way but looks good on paper)
Sounds good until we start the engine , then the battery voltage increases to , lets say 13-14 volts. Then the increase in heat would take some of the excess and the final resistor ( the finely stranded wire from the coil to the contacts) would eat up the rest of the excess. ( special lead wire)
Each system is calibrated to use all the voltage ( electrical pressure) and allow the contacts to have a long and usefull life.

Excessive voltage and current is what is eating your parts up, look over the system and see what is missing in the deal...... :eek:
 
And...sounds like your coil and resistor are not related. An easy way to be sure of a match between the resistor and what is wound into coil would be to buy a coil and a resistor for the same application...say, a 1965 Plymouth 318 coil if you have a single resistor Chrysler resistance. Coils do vary, and some run without a separate resistor because there resistance has been built in internally. The Standard catalog here shows different ones rated between 1.3 to 1.87, along with many not rated because they assume use in a particular car with a particular coil.
 
If you want to use a Chrysler Resister you need a .56 ohms ,
ICR 23Echlin,and use a 12 volt tube coil IC 17,that fits ALL Chrysler 1956 thur 1969. Thats a matcher pair.
I use a 56thur69 ford coil with a 1.35 ohms resister.
This info from Echlin Ca
 
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