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HomeTips & GuidesWhat Is a Ballast Resistor? Symptoms of Failed Ballast Resistors, How To Test, etc.

What Is a Ballast Resistor? Symptoms of Failed Ballast Resistors, How To Test, etc.

What is a ballast resistor in a car?

Electrical ballasts are more synonymous with lighting devices like fluorescent lamps, LED lamps, neon lights, etc., where they limit the electrical current flow in the circuit. 

But they are also present in vehicles because they introduce a level of control, which is critical for the ignition system. 

So, what is a ballast resistor? And how does it work? Let’s find out.

What Is a Ballast Resistor?

A ballast resistor or ignition ballast resistor is a vital component in vehicle ignition systems because it regulates voltage and current flowing to the ignition coil to prevent overheating and ignition coil failure.

Table of Contents

Types of Ballast Resistors

  • Fixed Resistors
  • Self-Variable Resistors

Why Ballast Resistors are Necessary for Automotive Applications

Ignition ballast resistors sit between the primary voltage source (battery) and the ignition coil stud.

 When you turn on the ignition switch, this component lowers the coil voltage and current from 12V from the battery to around 6-9V.

This step down has two benefits.

Leaves More Power for the Spark Plugs

When starting a vehicle, the only power source is the battery. 

Traditional electronic ignition systems had a 12V ignition coil that matched the battery voltage.

This electrical circuit system is not problematic when the engine is running. 

And the primary source of electricity is sufficient for the coil.

But when starting, the high power drawn by the starter motor leaves no electricity for power spark plugs to initiate the fuel combustion cycle that runs the engine.

A mechanic holding a car starter motor

This issue is worse if the starter has worn parts or when it is cold outside because its motor draws more energy due to increased friction.

So manufacturers switched to 9V coils with ballast resistors to lower the current draw. 

These coils output the same power to the spark plugs as 12V coils when connected to the battery’s 12V feed but draw less power. 

This setup gives a better spark when starting while leaving the starter motor to draw the required current.

Four ignition coils for an internal combustion engine

Four ignition coils for an internal combustion engine

Keeps the Ignition Coil Running Cool

Ignition coils running at lower voltages and electric currents attain lower temperatures than those operating at 12V. 

Functioning at these low temperatures increases the ignition coil lifespan.

Symptoms of Damaged Ballast Resistors

A failing reactive ballast usually causes these symptoms.

Engine Not Starting

The ballast regulates power to the ignition coil, which amplifies the current going to the spark plugs to create sparks. 

So the electrical device controls the process.

A failing unit can have a significant voltage drop that underpowers the ignition coil, meaning little or no coil current to power the plugs. 

So the engine will have difficulty starting.

Therefore, check the ballast resistor if the starter cranks but the engine does not start.

A ballast resistor

A ballast resistor

Rough Idling

Gas or petrol engines are inefficient if they get weak sparks from the spark plugs.

 And if some plugs fail to fire, the incomplete combustion cycles can cause rough idling.

This issue can be due to the ballast resistor not sending enough secondary voltage to the ignition coil, which powers the spark plugs.

A set of four spark plugs for a four-cylinder engine

A set of four spark plugs for a four-cylinder engine

Ignition Coil Overheating

Ballast resistor failure can also cause excess voltage or current to flow into the 9V coil, resulting in overheating.

 If this occurs for extended periods, the ignition coil will fail, and the engine won’t start or run.

How To Test a Ballast Resistor

First, it is vital to note that newer vehicles with modern electronic circuitry don’t have ballast resistors. 

Instead, they feature distributor-less ignition systems. So this testing is irrelevant for such cars.

If unsure about your vehicle, use a multimeter to test the ignition coil. Adjust it to the 20V DC setting, then connect the probes to the ignition coil.

A ballast resistor

A ballast resistor

If it reads 12V, your car does not have a ballast resistor kit. But if you have a 6V or 9V coil, the vehicle has it.

To access this ballast coil resistor for testing, shut the engine, pop the hood, and look at the firewall area or close to the ignition coil.

 The electronic component should be around these areas. And you can test its voltage or resistance for diagnosis.

Voltage Test

Connect the multimeter probes to the ignition coil’s positive and negative terminals with the electric ballast resistor plugged into the wiring system.

The multimeter should be in the same voltage setting as above. Also, the black probe should be in the COM port, and the red lead in the VΩmA port.

A multimeter set to 20V DC measuring a car battery’s voltage

A multimeter set to 20V DC measuring a car battery’s voltage

Turn the ignition on, then check the voltage reading across the terminals. Check the car’s manual to see the specified ballast resistor voltage. 

This value should match the reading on the multimeter. If they don’t match, you might have a non-functional ballast coil.

Resistance Test

For this test, you’ll have to disconnect the resistive ballast from the electric circuit because the multimeter will inject some current into it and measure the resistance across the terminals.

Also, set the multimeter to resistance settings and connect the leads across the electrical ballast terminals. 

The resistance reading should match the ballast’s resistance value specified in the vehicle’s manual. 

Check the component’s wiring diagram to get the specifications.

If the two don’t match, get a ballast resistor replacement asap.

Factors To Consider When Buying Ballast Resistors

Consider these factors when shopping for a ballast resistor.

Power Capacity

A ballast resistor’s input power capacity indicates the amount of power it can handle before burning out. 

Given in watts, the higher the rating, the more current/voltage it can tolerate. 

But it is better to get a replacement that matches the power capacity of the damaged component you are replacing.


Resistance (given in ohms) determines the voltage and electrical current that can pass through to the ignition coil. 

The higher it is, the lower the voltage and flow of current to the other side. 

Ensure the resistor’s parameters match the required current and voltage by the ignition coil.


Electrical components can have variations to the actual output numbers. 

Ballast resistors with lower tolerances are more accurate than their high-tolerance counterparts. 

And they are more reliable, as well.

Although more expensive, we recommend low-tolerance units to reduce errors (over or underpowering) on the ignition coil.

A ballast resistor

A ballast resistor

Operating Temperature

Ballast resistors can handle different internal temperatures before breaking down, depending on the construction. 

If you drive your car in hot environments, replace this component with one that can handle higher operating temperatures.

 Also, check its installation area. Surrounding parts should not be dumping heat there.

Thermal Resistance

Thermal resistance goes hand-in-hand with operating temperature because it defines how well the resistor dissipates heat from the core element.

 High thermal resistance enables the component to run more efficiently because its operating temperature remains low.

Voltage Rating

Vehicles have different voltage ratings, which you can tell by checking the battery voltage in the hood. 

The ballast resistor’s input voltage should match the source voltage, and its power output should match the input power required by the ignition coil.

Mounting Style

Different ballast resistor mounting styles allow installation on boards, panels, wire rung ladders, or other chassis parts.

Board mounting gives easy access when doing repairs or troubleshooting and saves space.

 On the other hand, external installation is more straightforward and offers better protection from the elements. 

So consider the pros and cons of each style and pick the best one for your car.

An electric ballast

An electric ballast

Form Factor

While considering installation, the resistor’s size should fit into the required space. Tiny form factors are better for packed engine bays with minimal free space.


Reliability depends on factors like the ability to withstand shock, vibration, voltage surges, temperature fluctuations, etc. 

Determining the reliability of a product requires some research on the resistor’s model to ensure it serves you for a long time.


Cost also plays a significant role, but you should not compromise on quality. So weigh the factors above, then pick the ballast resistor that meets your budget.

Wrap Up

Ballast resistors might not be present in modern engines, but you might have to replace one if working on a hot rod or classic car engine. 

So consider your car restoration project, then pick the most appropriate electric ballast for your vehicle using the factors above. 

And if you’ve identified the specifications, we can help you find the product. Comment below to give us the details.

I am Lillian Yang, having been a sales manager for over 10 years.

I have received many positive reviews from customers. They have praised our excellent service, on-time delivery, and high-quality cable assemblies.

For your projects, please provide cable assembly files/images/smples, etc., so that I can send you a quotation within 24 hours.

Contact me now and let’s get started on building your wire harnesses!

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