How to Test a Ballast Resistor With a Multimeter? – 4 Steps


Written by

Thomas Carter



Marcos E. Williamson

how to test a ballast resistor with a multimeter

Ballast resistors are an essential component of the ignition system in many vehicles. These resistors limit the amount of current that flows through the ignition coil, preventing it from overheating and potentially causing damage to other components.

Over time, ballast resistors can become faulty, causing problems with starting or running the engine. If you suspect that your ballast resistor is not functioning correctly, testing it with a multimeter is an effective way to diagnose the issue.

In this blog post, we will guide you through how to test a ballast resistor with a multimeter and help you get to the root of the problem to keep your vehicle running smoothly.

Safety Precautions Before Starting


Ballast resistors can produce a significant amount of heat and carry high voltage, both of which pose a serious risk to anyone not properly prepared. So, it is essential to take safety precautions.

  • Wear Protective Gear: Before testing a ballast resistor, it is essential to wear appropriate protective gear, including rubber gloves.
  • Disconnect The Main Power: Make sure to disconnect the main power source to your vehicle before you do anything else.
  • Test In A Dry Environment: When testing a ballast resistor, it is important to do so in a dry environment to avoid electrical damage due to moisture.

Testing the Ballast Resistor With Multimeter for Resistance & Voltage

What You Will Need To Have:

  • A Multimeter
  • Wiring Diagram (You will not need to know the specifics of how to wire, but it helps to have a rough idea of the internal and external resistor coil wiring)
  • Your vehicle manual (to check the specified value range)

Step 1: Locating The Ballast Resistor In The Vehicle


The location of the ballast resistor in your vehicle will depend on the make and model of your vehicle and the type of ignition system it uses. Typically, ballast resistors are near the ignition coil or distributor.

Step 2: Check the manual

You should know that the standard battery voltage is 12V. With a ballast resistor, however, the voltage should be as low as 6 – 8 V. Check your vehicle’s owner’s manual first.

It should state that it is within this range (6 – 8V) to indicate that the ballast resistor operates correctly.

Step 3: Setting The Multimeter to Resistance Mode


To test the ballast resistor for resistance, you will need to set your multimeter to resistance mode. This option is denoted by Ω on most multimeters.

Step 4: Testing The Ballast Resistor With The Multimeter

  • Conduct the Ohms test (test the Ballast Resistor Ohms)


Connect the multimeter’s probes to the ballast resistor’s terminals. The multimeter should display a resistance value that is within the range specified for your particular ballast resistor (often 6 – 8V)

If the resistance falls outside of this range, the ballast resistor is not functioning correctly.

  • Conduct the Volts Test (test the Ballast Resistor Volts)


Now, set the multimeter to the voltage setting to check that it is letting enough voltage go through to the ignition coil.

If the reading is within the range, your ballast resistor is not faulty and you will not have to replace it.

Symptoms of a Bad Ballast Resistor

If you are experiencing engine problems like stalling, misfiring, or sluggish acceleration, and your ignition coil is overheating, your ballast resistor may be faulty.

The ballast resistor is responsible for controlling the amount of voltage sent to your ignition coils which helps ensure a steady and reliable spark in your engine. When the ballast resistor goes bad, it can cause issues with the electrical system that can lead to performance problems.

  • A common cause of this issue is simply wearing out over time due to high temperatures or prolonged use.
  • Additionally, moisture buildup or damage from exposure to corrosive chemicals, particularly in the ballast resistor wiring, can lead to early failure.

Troubleshooting & Diagnosing Issues


1. Common Issues with Ballast Resistors

Ballast resistors can fail due to a variety of issues. The most common reasons for failure include overheating, moisture damage, or physical damage from vibration or impact.

2. Differentiating Ballast Resistor Issues from Other Issues

It is important to differentiate ballast resistor issues from other issues that can cause similar symptoms. A faulty ignition coil, spark plug, or distributor can also cause problems with ignition and engine performance.

3. Steps to Take to Troubleshoot Ballast Resistor Issues

To troubleshoot an issue, start by checking ballast resistor with a multimeter as described earlier. If the readings indicate a problem, inspect the resistor for signs of damage or wear.

Check the surrounding components, such as the ignition coil and spark plugs, to ensure they are functioning properly.

Frequently Asked Questions (FAQs)


What does a ballast resistor do?

The ballast resistor limits the current through the car’s ignition coil, preventing it from overheating and causing damage to other electrical components. Doing so, it also increases the lifespan of the ignition system and improves the reliability of your vehicle.

Can a bad ballast resistor cause no spark?

If your ballast resistor is blown, a no-spark situation can occur. Otherwise, faulty ballast resistors tend to cause weak sparks.

Is running a coil without a ballast resistor a good idea?

No! When you bypass a ballast resistor, you prompt wear and tear, which cuts the coil’s life short.

Does a system with electronic ignition require a ballast resistor?

It does if it has breaker points. If it does not have breaker points, a ballast resistor is not necessary.

Can a faulty ballast resistor damage my engine?

Yes, a defective ballast resistor may result in the ignition coil to become overheated, which can cause engine damage if left unchecked. To avoid any damage, it is critical to test and replace the resistor as needed.

How often should I test my ballast resistor?

It is recommended to test your ballast resistor regularly as part of your vehicle’s routine maintenance schedule. However, suppose you suspect that the resistor is faulty and causing issues with your engine. In that case, you should test it immediately to avoid any further damage.


Now, you know how to test a ballast resistor with a multimeter. You can go ahead and apply what you have read to see if your ballast resistor is working smoothly to keep your 12V ignition coil or Ford ignition coil in tip-top condition.

Remember to always follow proper safety precautions when working and seek professional help if you are unsure about any aspect of the testing process.

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