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Summary
  • The ballast resistor lowers the voltage between the ignition switch and the ignition coil’s positive wire so that only about 8 volts are delivered to the ignition coil when the ignition switch is in the “run” position.
  • Ballast resistors are no longer present in modern vehicles because of electronic ignition systems.
  • When ballast resistors develop issues, your vehicle might show symptoms like stalling, misfiring, and premature ignition coil failure.

Ballast resistors play an important role in a vehicle’s ignition system. They’re responsible for lowering the voltage received by the ignition coils from the battery.

While ballast resistors are mostly no longer present in modern vehicles because of electronic ignition systems, there were some electronic ignition systems that were equipped with ballast resistors. One notable exception was Ford. Fords didn’t use a ballast resistor, but used a resistor wire that was built into the harness between the ignition switch and the coil positive feed.

ignition system with ballast resistor diagram
Diagram of an ignition system with ballast resistor | Image Source: Richard McCuistian

What Does a Ballast Resistor Do?

The ballast resistor drops voltage between the ignition switch and the ignition coil’s positive wire so that only about 8 volts is delivered to the ignition coil when the ignition switch is in the “run” position.

In the “start” position, a full 12 volts is delivered to the ignition coil, bypassing the resistor. This is why older solenoids have an “S” terminal for the solenoid and an “I” terminal for bypassing the ballast resistor during start. Older GM starter solenoids had the same two terminals for the same reason.

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The 12 volts of power supplied by vehicle batteries is typically too much power for the vehicle’s ignition coils. When vehicles had contact points, running the system with the ballast resistor bypassed (as some people would do when it burned out), would burn up the contact points in the distributor in a very short period of time.

Before the introduction of ballast resistors, ignition coils used to overheat and require frequent replacement in trucks.

ballast resistor image
A ballast resistor uses a resistive material to lower the voltage in a circuit. | Image Source: Richard McCuistian

The ballast resistor regulates the current flowing to the ignition coil by dropping it 3 to 4 volts, so that it operates at peak performance and has a longer lifespan. Without the ballast resistor, the ignition coils can overheat and wear out faster. 

Ballast resistors can be made of a variety of materials, including carbon, wound wire, or metal oxide, depending on the particular application and circuit requirements. They’re generally rated in Ohms and Wattage, which indicate their resistance and power-handling ability.

Bad Ballast Resistor Symptoms

Like any electronic component, ballast resistors degrade over time due to normal wear. If the ballast resistor fails, it can cause a number of symptoms that can affect your vehicle’s reliability and performance.

Start-and-Die

Since the ballast resistor is bypassed by voltage from the solenoid “I” terminal during engine start, a burned out ballast resistor can cause the engine to start initially. When the starter solenoid stops providing the bypassed voltage to the coil, the ignition system stops providing spark, resulting in a start-and-die issue.

See also  The Ignition System: Basic Parts and Systems

Rough Idle/Misfires

A defective ballast resistor can cause misfiring or harsh idling but only if it has developed more resistance and has reduced the available voltage to the ignition coil to the point that the coil can’t fire as it should. Ordinarily, the resistor doesn’t cause this problem. A weak spark is typically a problem with the ignition coil.

, Bad Ballast Resistor Symptoms

Pro Tips are nuggets of information direct from ASE-certified automobile technicians working with CarParts.com, which may include unique, personal insights based on their years of experience working in the automotive industry. These can help you make more informed decisions about your car.

Pro Tip: The resistor wire used instead of a ballast resistor on an older Ford can develop resistance as it ages and cause misfiring and idling issues.

Stalling

A faulty ballast resistor might not allow enough voltage to reach the spark plugs and cause a weak or inconsistent spark, which can lead to your engine stalling.

Premature Ignition Coil Failure

A defective ballast resistor can overheat the ignition coil, resulting in premature failure. Note, however, that this only happens if the resistor is internally shorted so that it has no resistance at all, which is very rare. Usually, an older resistor will, if it changes, develop more resistance, and not less.

Premature failure only happens if the ballast resistor is internally shorted so that it has no resistance at all, which is very rare. Usually, an older resistor will, if it changes, develop more resistance, and not less.

Richard McCuistian, ASE Certified Master Automobile Technician

If your vehicle’s ballast resistor causes the ignition coil to fail, then it can show similar symptoms like misfires and difficulty starting. A faulty ignition coil can also cause additional issues such as an illuminated check engine light, poor fuel economy, reduced power, and backfires.

See also  The Ignition System: Basic Parts and Systems

Diagnosing a faulty ballast resistor typically involves using a multimeter to check whether the resistor is functioning as it should.

However, this task can be difficult if you aren’t experienced in DIY repairs. If your vehicle is showing these symptoms, you should head to a trusted mechanic.

checking ballast resistors using  multimeters
If you have some DIY auto repair knowledge, you can try this method of using a multimeter to check if the ballast resistor is in good or bad condition. Set the meter on a 20-volt scale. With the engine spinning, measure the voltage drop across the resistor as shown in the illustration. | Image Source: Richard McCuistian

Getting Your Hands on a New Ballast Resistor

Driving with a bad ballast resistor can negatively affect your vehicle’s performance. Luckily, you can easily get a replacement that’s compatible with your vehicle thanks to CarParts.com.

The best part? You don’t even have to take one step outside your door to get your hands on a new ballast resistor. All you need to do is go online and visit our website. Simply input the exact year, make, and model to browse through compatible parts for your vehicle.

All products in our catalog are sourced from some of the most reliable names in the industry. And because we stand by their quality, we offer 60-day return and lifetime replacement guarantees.

Don’t let a malfunctioning ballast resistor ruin your next trip. Check out our selection of quality auto parts today!

About The Authors
Written By Automotive and Tech Writers

The CarParts.com Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by CarParts.com's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Reviewed By Technical Reviewer at CarParts.com

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

File Under : Ignition System , DIY Tagged With :
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