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Old 09-18-2010, 01:14 PM   #1
Barry Wolk
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Default Ignition resistor

My '41 has an ignition resistor that's been bypassed. It has a modern coil instead of the unit mounted on the distributor. Is this a correct installation?
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Old 09-18-2010, 02:20 PM   #2
trainguy
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Default Re: Ignition resistor

Will work if the coil has a internal resistor.Phil
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Old 09-18-2010, 02:34 PM   #3
rotorwrench
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Default Re: Ignition resistor

The ballast was designed to make the best of the original coil installation. Many later models use no ballast and were designed that way. A coils primary resistance has to be matched to the condenser that is being used and a ballast resistor is usually put there to better control the current to the system depending on the voltage applied to it.

As far as having an internal resistor, the primary winding in a coil is a resistor of sorts wound around another coil of secondary windings to form an inductor. Induction coils are rated by their primary resistance. THere are a lot of threads on this board about coils if you run a search and check them out.

Kerby
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Old 09-18-2010, 05:50 PM   #4
CWPASADENA
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Default Re: Ignition resistor

The original Ford coils were not 6 Volt but somewhere nearer to 4 Volts. When the engine is running, the resistor in the system drops the charging system voltage from apx 7 volts to the design voltage of the coil, apx 4 volts.

The use of a low voltage coil and a resistor is to help insure adequate spark during starting, especially if the engine is cold.

The ignition resistor is a high temperature wire that will heat up when currect is passed thru it. When cold, the resistor will have very low resistance but as it heats up, its resistance value will increase.

When cranking the engine, the battery voltage will drop but beings the resistor is also cold and will have low resistance, the coil will have adequate voltage to provide enough spark for starting.

As soon as the engine starts, the battery voltage will increase to a value closer to that of the the charging voltage, apx 7 volts. As the engine warms up, current thru the resistor will cause it to heat up and as a result, its resistance will also increase thus dropping the voltage to the coil back to its design voltage of apx 4 volts.

GM and may be other manufacturers used a simular system, a coil designed for lower voltage then that of the charging system, and a "Ballast" or dropping resistor. During cranking, the starter solenoid had an additional contact to provided direct "Battery Voltage" to the coil. Thus, there was a hot spark for starting while cranking the engine but the coil (in combination with the resistor) would operate at its design voltage while the engine was running.

In the "Old Days" when a guy was having trouble getting his Ford to start, he would often have someone yell at him to "Put a Nickel In It". This came from the situation when (especially if the car was hard to start) the engine was cranked for a while and the resistor had warmed and increased in value to to the point where the coil was no longer getting enough voltage to provide an adequate spark. Sometimes, you could short across the resistor and increase the voltage to the coil enough to get the engine to start. While he may not have a jumper wire with him, he could take a coin and reach under the dash and short across the terminals of the resistor.

Some actually installed a relay under the dash of their Old Ford that was energized by the starter solenoid. The relay was wired to automatically short across the ignition resistor while the engine was cranking thus providing full voltage to the coil.

You can use a generic "6 Volt" Coil that is designed to be used without a ballast resistor but you would loose any advantage to having the coil operate at its design voltage during the time the battery voltage is low while cranking the engine. However, "Modern" coils are more efficent and the "cold start" feature of the ballast resistor may become less important. Definately Do Not use a ballast resister a with an aftermarket coil (designed to be used without a ballast resistor) or you may get into as situation where your car will be hard to start.

I hope I was not too "Long Winded" in my explination of why Ford used a resistor in the ignition circuits of early V-8s.

Thanks for your patience.

Chris

Last edited by CWPASADENA; 09-18-2010 at 06:07 PM. Reason: TYPO
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Old 09-18-2010, 05:56 PM   #5
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Default Re: Ignition resistor

Great explanation. Thank you.
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Old 09-18-2010, 07:00 PM   #6
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Default Re: Ignition resistor

Fords main reason for the resistor was to lengthen the life of the points by reducing the arc during the opening and closing of the points.
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Old 09-18-2010, 09:26 PM   #7
CWPASADENA
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Default Re: Ignition resistor

Quote:
Originally Posted by gmc1941 View Post
Fords main reason for the resistor was to lengthen the life of the points by reducing the arc during the opening and closing of the points.
If you DO NOT use a resistor with a Early Ford V-8 "low voltage" Coil, you will burn the points as these coils will draw too many amps. These coils have lower primary resistance and were designed to run on reduced voltage.

You can run a 4-Cyl (Model A or B) Coil or any aftermarket "6-Volt" coil (that is designed to operate without a resistor) and you WILL NOT burn the points because these coils have higher primary resistance and were designed to run on full voltage. However, if you run a "full Voltage" coil, you will loose the hotter spark during cranking, "cold start" feature of the original Early Ford V-8 System. I.E., Starting on full battery voltage and running on reduced battery voltage.

The voltage that the coil "Sees" and the resistance of the primary windings is what determines the amount of the current thru the points. Increasing the voltage to a coil will raise the primary current. Increasing the resistance of the primary windings will reduce the current. Too much current thru the primary windings will cause the points to burn. This is why you use a resistor with a "reduced Voltage" Early V-8 Ford coil and you do not use a resistor with a "full Voltage" coil.

Chris
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