Ignition Coil ballast Resistor
 

The question of what is the function of an ignition coil's ballast resistor often comes up, so this is an explanation of what it does.
Simply put, the ballast resistor reduces surging / high current flow into the coil's primary winding to protect the coil from damage and service life reduction.

When starting the engine, the starter motor causes a high surge current in the electrical system because it draws 175+ Amps from the battery to crank the engine. Once the engine starts, the generator / alternator surges current into the electrical system to charge the battery. The surge currents would also go through the ignition coil, but the ballast resistor drops the voltage and so reduces the surge current going into the coil's primary windings.

The ignition coil actually runs with less voltage than the battery / generator / alternator produces because of the ballast resistor.

I hope this simplified explanation makes sense to you all.

Re: Ignition Coil ballast Resistor
 

I’m not familiar with this. Is this something that is built into the coil? Or, something aftermarket that is added to a coil?


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Re: Ignition Coil ballast Resistor
 

This is done only on 12v systems. When the starter is engaged, the coil receives full voltage for easier starting.

Re: Ignition Coil ballast Resistor
 

In the early days, cars were 6 volts. Along came 12 volt cars and to use the existing stock of 6 volt coils, ballast resistors were used to knock down the voltage at the coil to 6 volts. It was then discovered that the ballast resistor could be bypassed when the car was started so that the coil would put out a hotter spark, especially when the starter drew down the battery voltage to about 8 volts.

When 12 volt cars were around for a while, people started building the ballast resisters into the coils. So now you can buy coils with the ballast resistors built in.

Along came modern cars that have totally different ignition systems, mostly with individual coils for each spark plug. These are designed to use 12 volts without any built in ballast resistors. The difference is in the primary windings.

Re: Ignition Coil ballast Resistor
 

The 6-volt systems worked well with the 1.5 Ohm can type coil and had no need for a ballast resistor. In the V8 era things changed. Mallory and Ford developed the coil/distributor combo that was used up through 1941. The Mallory type Ford coils needed more current control due to the coil design so they used a ballast resistor with an approximate value of 0.8-Ohms to get the current in the right place for most efficient and reliable operation of the system. This continued till Ford went back to the can type coil in the 8BA era. In 1956 at the 12-volt change over, they added a ballast to continue use of the 1.5-Ohm coils till the mid to late 1960s and beyond on some applications.

Some insist that it was to drop voltage and others go along with the current control theory but is really does both per Ohms law so it's not worth much of an argument.

Re: Ignition Coil ballast Resistor
 

Ditto

Re: Ignition Coil ballast Resistor
 

I had a 35 Ford a few years ago,and when going over it to get it started I found a ballast resistor up under the dash.I'd owned a few of the 30's vintage V8's before,and never knew they had one in them.It was fried,and I bought one from a V8 guy in Uxbridge Mass,and everything was fine.Never knew a 6 volt system to use one before I saw that.

Re: Ignition Coil ballast Resistor
 

Re: Ignition Coil ballast Resistor
 

Bob, I don't know if what you have written makes sense to others or not, but I do know that it is mostly incorrect, especially on a stock Model A with the 6 volt system (which does not even use a ballast resistor).

Even on a Model A that has been converted to 12 volts, or a later model vehicle that was always 12 volts and uses a ballast resistor, what you have written is mostly incorrect.

This entire post should be deleted, so as to not confuse other new car owners and novice mechanic readers at a later time. Introducing amp draw to the starter and other stuff is irrelevant and confusing.

Re: Ignition Coil ballast Resistor
 

Ditto on post 9.

Re: Ignition Coil ballast Resistor
 

With point type distributors, the maximum amperage the points can handle reliably for long life without excessive pitting or burning is about 5 amps. With a 6 volt system, when limiting the current to 5 amps you need a coil with a resistance of no less than 1.2 ohm. This comes from using the formula, "Voltage=Current x Resistance" (V=IxR) which is one of the basic electrical formulas.
Model A Coils are about 1.2 Ohm, so with 6 volts, the resultant current is 5 amps.
With a 12 volt system, if you want to limit the current thru the points to 5 amps, then the coil must have a resistance of about 2.4 Ohm. (V=IxR)
Now, general speaking, 12 volt coils come 2 ways, with a built-in ballast resistor and without a built-in ballast resistor. A 12 volt coil without a built in ballast resistor will usually be marked that it is to be used with an external ballast resistor.

A 12 volt coil with a built-in ballast resistor will have a total resistance of about 2.4 ohm. A 12 volt coil that does not have a built-in ballast resistor will usually have a resistance of apx. 1.2 Ohm (same as a 6 volt coil). A ballast resistor is also usually about 1.2 Ohm so the total resistance of the coil and ballast resistor is 2.4 ohm. Therefore, if you are converting a 6 volt system to 12 volts, you do not have to change the coil, you only have to add a ballast resistor.

Now, starting in 1932 and continuing thru 1948, With the new V-8 engine, Ford used a 3 volt coil and a ballast resistor. These resistors were made with ni-chrome wire, the same as you would find in a toaster. When ni-chrome wire is cold, it has no resistance but as you pass current thru it and it becomes hot and its resistance will increase. With this system, when first starting the car and the resistor is cold, the coil will be receiving full battery voltage and the spark to the plugs will be hot and strong to assist in starting, especially in cold weather. Keep in mind, the starter on the V.-8 engine when cranking will take a lot of current and pull the battery voltage down to may be 4-5 volts so during starting cold, the voltage to the coil is also in the range of 4-5 volts. After the engine starts and the ballast resistor heats up, the current thru the points will be reduced to a manageable amount and the points will not burn.

This system worked well except on a hot day when the engine was hot. If you stalled the car, it may crank but not start due to not enough voltage to the coil to produce a hot enough spark when cranking the engine. In the "old days" when this happened, you could take a coin and reach under the dash and short across the ballast resistor and get enough voltage to the coil to produce a hot enough spark to start the engine. If you would wait a few minutes, the resistor would cool enough so its resistance would be reduced enough to allow enough voltage to the coil to get a good hot spark.

After going to 12 volts, most manufacturers used a system that used a coil and a separate ballast resistor. During cranking, the ballast resistor was by-passed and the coil would receive full battery voltage, so you had a good hot spark for starting.

Now, please understand, this is a generalization of how basic point type ignition systems work. Resistance values may be slightly different for different applications. Higher performance coils may have a little less resistance allowing a little more current and a hotter spark but usually with these applications, you are changing the points a little more often.

Anyway, sorry for the long-winded explanation but this is the basics of the point type ignition system.

Chris W.

Re: Ignition Coil ballast Resistor
 

A lot boiled down the what materials were used to make ignition coils over the years. If the resistance of the primary was too high then the unit would build up excessive heat during normal use. They used tar as an insulator in a lot of coils. The tar melts and can leak if the case is compromised. The big problem with the old coils would come if a person had an engine stop running for other than ignition off selection. If a person forgets to turn the ignition off when this happens with the added problem of the ignition breaker points being closed then the coil turns into an electro magnet. They can heat up enough to melt stuff inside the coil assembly.

I never refer to a coil as a 6-volt or a 12-volt. Ballast resistors were used to protect the coil. The condenser is in there to extend the high tension pulse but it also protects the breaker points which tend to arc with back flow from the primary. The Mallory design ignition systems of the 30s & early to late 40s used a bit higher capacity condenser than the model A or 8BA designs. Those Mallory coils were a lot different than the basic can type coil.

In the modern era, a particular formula of epoxy is used as an insulator so they can take a primary resistance of 3.0-Ohms with no problems. Now days one coil fires one cylinder electronically so they last fairly well. The modern Motorcraft coils for the modular engines seem to last about 100K miles before they get weak and start to misfire. They're tiny compared to an old can coil.

Re: Ignition Coil ballast Resistor
 

I’m looking for my ballast resistor. What do they look like?

Re: Ignition Coil ballast Resistor
 

What make,model,and year is it in? Is it bone stock? all are different.

Re: Ignition Coil ballast Resistor
 

Chris W - Thank you for your explanation. I vaguely remembered the term “shunt” in reference to bypassing the resistor when cranking.

Re: Ignition Coil ballast Resistor
 

Model A vehicles had no ballast but the 32 through 48 cars had one like this link illustrates. They were mounted on an insulated little panel with the headlight breaker up under the dash.
https://thirdgenauto.com/product/new...tion-resistor/

Re: Ignition Coil ballast Resistor
 

You are correct.

I had an old hot rod with a modified engine that was running on 6 volts. It had a Harman-Collins dual coil distributor that used 2 46-48 Ford coils and resistors.
The resistors were mounted on the coils which were under the hood.

If I stalled the thing when the engine was hot, I had to get out and jump across one of the resistors to get it to re-start. I finally installed 2 relays that automatically shunted the resistors during cranking that solved that problem.

Chris W.

Re: Ignition Coil ballast Resistor
 

I remember that style being mounted on the engine side of the firewall, 1940s Ford vehicles. I still have one in my stock of "spare parts".

Re: Ignition Coil ballast Resistor
 

In 1942 with the intro of the crab type distributor, The coil was redesigned and mounted with two long screws on a bracket that attached to the front left side of the intake manifold. This setup was continued after the war on the rabbit ear distributors till the 8BA era.

Beginning in 1941, the 11A-12250 circuit breaker and ignition ballast resistor were mounted to a rectangular piece of phenolic insulator board mounted under the dash. The open circuit breaker was replaced with two closed short stop circuit breakers in 1946 or 47 but it was still mounted in the same way.

The 11A-12250 link.
http://www.vanpeltsales.com/FH_web/F...r-resistor.jpg

Re: Ignition Coil ballast Resistor
 

I thought the main reason ( for a ballast resistor)
was to knock down the voltage after the vehicle starts when
first started thus, saving the points from high voltage / damage.