Although newer vehicles no longer have distributors (they rely on individual ignition coils, instead), there are still many older models on the road with distributor-style ignition systems. If you’re reading this article, your car is probably one of them.
Like spark plugs and spark plug wires, distributor caps are considered a maintenance item that wears out over time. You should replace your car’s distributor cap whenever you perform a tune-up. If you neglect to service the cap, it will eventually fail, resulting in one or more noticeable symptoms.
What is a Distributor Cap?
Distributor caps are only found on vehicles that have a distributor-style ignition system. It’s important to know how this type of ignition system works to understand the role of the distributor cap.
Like any ignition system, a distributor setup is designed to fire the spark plugs to ignite the air-fuel mixture inside the engine. As you might guess, the heart of a distributor-style system is the distributor—an engine-driven device that passes high voltage from the ignition coil to the spark plugs.
The distributor cap, which is made of plastic and has metal terminals, sits on top of the distributor. High voltage from the ignition coil is sent to the middle terminal of the distributor cap whenever the engine is running. From there, the voltage travels to the rotor inside of the distributor. The rotor turns with the distributor, delivering voltage to the cap’s terminals one at a time.
Each terminal connects to a spark plug wire that provides a path for voltage to reach one of the engine’s spark plugs and make the plug fire. The firing spark plugs produce a series of small explosions, creating the energy needed to propel your vehicle down the road.
Common Signs of a Bad Distributor Cap
Do you think you might be dealing with a bad distributor cap? If your vehicle is exhibiting one or more of the following symptoms, you might be right.
Note: Because other problems can present the same symptoms as a bad distributor cap, you’ll want to perform a thorough diagnosis of the vehicle before doing any repairs.
Misfiring and Rough Running
Issues with the cap—including worn terminals, corrosion, cracks, and carbon tracking—can prevent voltage from reaching the spark plugs as it should. As a result, the engine will misfire, run rough, and exhibit an overall lack of performance.
Illuminated Check Engine Light
On all vehicles built after 1996 (and some built prior), the engine computer, which is often referred to as the powertrain control module, monitors the engine for misfires. If the PCM detects a misfire for any reason, including a bad distributor cap, the module will turn on the check engine light and store a diagnostic trouble code in its memory.
The same distributor cap problems that can lead to misfiring and rough running can also result in extended cranking and difficulty starting.
Engine Cranks But Doesn’t Start
Problems with the distributor cap (e.g., cracks, a worn center terminal, or severe carbon tracking) can also lead to an engine that cranks but doesn’t start.
Distributor Cap FAQ
How to Test for a Bad Distributor Cap
One of the best ways to check a distributor cap is to remove it and perform a visual inspection. You’ll want to look for problems, such as cracks, tiny holes, corrosion on the terminals, and excessive terminal wear.
You can also test the cap for carbon tracking—a tiny line of carbon that creates an unwanted path for electricity—using a digital multimeter (DMM) set to ohms. The same method can reveal whether the internal path between the cap’s ignition coil terminal and center terminal is intact. Here’s how it’s done:
Testing the Path Between the Coil Terminal and Center Terminal
- Place one meter lead on the cap’s coil terminal and the other meter lead on the center terminal.
- There should be continuity between the two terminals. If not, the cap is defective and needs to be replaced.
Testing For Carbon Tracking Between Terminals
- Place one meter lead on one of the cap’s outer terminals.
- Place the other meter lead on each of the remaining outer terminals one at a time.
- There should NOT be continuity between any of the two outer terminals. If there is, the cap is defective and needs to be replaced.
- Place one meter lead on the cap’s center terminal.
- Place the other meter lead on each of the spark plug terminals one at a time.
- There should NOT be continuity between the center terminal and any of the spark plug terminals. If there is, the cap is defective and needs to be replaced.
Why Do Distributor Caps Go Bad?
Distributor caps operate under high voltage in extreme conditions. As such, they’re considered maintenance items that should be replaced periodically. It’s a good idea to replace the distributor cap and rotor whenever you do a tune-up.
How Much Does it Cost to Replace a Distributor Cap
If you choose to have a professional replace your car’s distributor cap, you can usually expect to pay somewhere between $75 and $200 to get the job done. Of course, the exact cost will depend on various factors, such as the year, make, and model of your vehicle.
You can save money by replacing the distributor cap yourself if you have the tools and the know-how. CarParts.com has a wide variety of replacement distributor caps available for various makes and models.
Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic.