How to Test a Ballast Resistor with a Multimeter? – 3 Steps

Written by

Charles Bolte


Vernon Hoppe 

how to test a ballast resistor

Electrical circuits are a huge part of car systems. When turning your key, you create a circuit whose current begins at the battery and passes through the ignition coil.

When this current is unregulated, your ignition coil can overheat, giving it a shorter lifespan. This is where ballast resistors come in. They are intended to limit the current that gets into your ignition coil.

However, due to power surges, design flaws, or just normal wear and tear, ballast resistors can fail. In this article, we will show you a step-by-step guide on how to test a ballast resistor.

Step-by-step Ballast Resistor Test


Step 1: Locate the ballast resistor.


Modern cars do not use ballast resistors and instead use distributor-less ignition. To find out if your car has a ballast resistor, you need to set your multimeter to volts, connect your leads to the coil, and see if you have a 12V ignition coil or a 6V, 9V one.

If you see the two latter ratings, you have a ballast resistor.

To access this component, shut off the car’s engine and lift the hood. The ballast resistor is typically mounted on the car’s firewall or close to the ignition coil, depending on the make and model of your vehicle.

Step 2: Check the resistance


Disconnect the ballast resistor from the wire connector and place the multimeter probes on the resistor. Make sure the black lead is connected to the COM port and the red lead is plugged into the VΩmA port first. Then check ballast resistor ohms with a multimeter.

A specified resistance value of this component can be found in the service manual for your vehicle. You can also check this ballast resistor wiring diagram for the approximate values.

If the value in the multimeter does not match the specifications, the ballast resistor has an issue that needs to be addressed immediately.

Step 3: Check the voltage


Reconnect the wire connector to the ignition coil ballast resistor. Place the probes on the positive and negative terminals of the ignition coil.

To measure ballast resistor volts, turn on the ignition while testing ballast with multimeter set to volts.

Check your car’s manual for the specified voltage reading of your ballast resistor. If the multimeter’s value does not match, have your ballast resistor checked.

Symptoms of a Bad Ballast Resistor


  • Engine not starting

The cause of an engine failing to start could indicate a failing ballast resistor. Low voltage could cause no spark from the spark plugs due to the resistor not sending them enough energy. The failure of the engine to start or stall out is one of the most typical signs of a bad ballast resistor.

  • Rough Idling

Incomplete combustion caused by weak voltage to your ignition coil can cause the car to rumble and shake during idle time. A bad ballast resistor cause no spark or does not send enough voltage to the ignition, making the latter work harder.

  • Overheating

Overheating of the ignition coil can stem from a bad ballast resistor. The ignition coil’s voltage is controlled by the ballast resistor, which keeps it from overheating. The coil may receive too much voltage if the ballast resistor fails, overheating and possibly failing as a result.

Common Problems for Ballast Resistors


Aside from the problems mentioned above, a bad ballast resistor can cause your vehicle to burn more fuel and your dashboard engine light to malfunction.

If you’re constantly going through ballast resistors, there may be other faults in the electrical system, such as shorts or improper charging. Such issues can cause Dodge ballast resistor problems, as do bad coils.

Frequently Asked Questions


What does a ballast resistor do?

The ballast resistor purpose is to regulate the current from the battery to the ignition coil. It maintains the required number of volts needed to power the engine.

Without it, the unregulated current can cause the ignition to overheat and burn. However, most modern cars no longer have ballast resistors. Classic cars from the 70s typically have them.

Old Chrysler cars with ballast resistors typically used a dual-ballast design.

What makes a ballast resistor go bad?

One of the things that can make a ballast resistor go bad is a ballast resistor bypass. By jumping the resistor, you are sending voltage where it’s not meant to go, causing more electrical problems for the car.


Ballast resistors are rare components usually found in classic cars. They were designed to limit the voltage sent by your batteries to your ignition coil.

As useful as they are, these components can age and decline in quality. So, the next time your Churysler ballast resistor or Mopar ballast resistor has problems, remember how to test a ballast resistor following the steps here.

5/5 - (2 votes)

Magazine Latest Posts

Who Makes Quaker State Oil

Who Makes Quaker State Oil?

Have you heard about the Quaker State oil products? If yes, you might have thought ...


Is Marvel Mystery Oil Good?

  When was the last time you used a fuel or oil additive? If you ...

Predator 212 Oil Type & Capacity

Predator 212 Oil Type & Capacity

Whether used for go-karting or generating, choosing the right Predator 212 oil type & capacity ...

Quaker State vs Pennzoil

Quaker State vs Pennzoil: A Quick Comparsion

Quaker State Vs Pennzoil? Choosing the right engine oil from the same brand can be ...

How Tight Should an Oil Filter Be

How Tight Should an Oil Filter Be? How to Tighten Properly?

An oil filter is essential in maintaining the overall condition of your car engine, as ...

Is Havoline Good Oil

Is Havoline Good Oil? (Pros & Cons, Performance and More)

Havoline is a popular motor oil brand with great products that offer adequate engine protection. ...