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Ballast resistors are found on older cars and trucks. You will not find an external ballast resistor on a new vehicle. If you own a classic car or truck, it pays to know a little bit about how your ignition system works in order to diagnose issues with the ballast resistor.



What are Ballast Resistors?

Engineered to limit voltage to the coils in older ignition systems, ballast resistors were made redundant by electronic ignition systems. If your vehicle is equipped with a points type ignition system, there’s a good chance you’ll find a ballast resistor somewhere in your ignition circuit wiring.

How do Ballast Resistors Work?

The ignition system on a classic car is often far from perfect. Ideally, your coils would like to see a certain voltage to operate at peak performance. Your vehicle’s charging system may produce more voltage than it needs to power the coils. When your coils are overpowered, they tend to wear out, break, and require frequent replacement.

This was a big problem on early cars and trucks, so engineers incorporated resistors in their ignition systems to limit voltage to the coils. The addition of a ballast resistor increased the life of ignition coils installed on vehicles with points-type ignition systems.

How are Ballast Resistors Made?

When a resistor is installed, it chokes voltage to a pre-determined level. Engineers design and build resistors out of a variety of materials and components, including metal, plastic, rubber, and ceramic.

Why do Ballast Resistors Fail?

  • Heat, vibration, and wear and tear brought about by old age cause all electrical components, including ballast resistors, to eventually wear out and fail.
  • Electrical malfunctions may cause your ballast resistor to fail.

What are the Symptoms of Ballast Resistor Failure?

  • Premature ignition coil failure.
  • Weak or no spark at the spark plugs.
  • The voltage coming out of the resistor matches the voltage coming into the resistor. Use a voltage meter or multimeter to diagnose this problem.

What are the Consequences of Ballast Resistor Failure?

  • Ignition coils are expensive and frequent replacement is not cost effective.
  • The vehicle may not start.

Ballast resistors are something of a throwback to the early days of automotive engineering. Newer vehicles use more complicated systems to accomplish the same task. If your classic car or truck has spark issues, replacing its ballast resistor may solve the problem.

If you have questions or concerns about ballast resistors or any of your vehicle’s ignition components, come into your local Pep Boys where we can answer any question, help you find any part, or perform any vehicle service you might need.