What Does a Distributor Cap Do?

Depending on what type of ignition system your vehicle has, it may require a distributor cap. Most vehicles use either a distributor or a set of coil packs. While coil packs have become more popular, there are still countless vehicles on the road that have a distributor-based ignition system. And these vehicles require a distributor cap. What is a distributor cap exactly, and what does it do? 

Overview of Distributor Caps

Distributor caps are small, round caps that fit over the distributor. They are typically made of molded plastic with metal inserts. The metal inserts are conductive, meaning electricity can flow through them. 

Assuming your vehicle has a distributor-based ignition system, it will likely feature a single distributor cap. The distributor cap will cover the distributor as well as the rotor.

The Purpose of a Distributor Cap

Distributor caps serve several purposes. They protect the distributor from environmental contamination, for instance.

The distributor is designed to route or "distribute" electricity from your vehicle's ignition coil to its spark plugs. Without a distributor cap, debris may accumulate inside of it. As dirt and debris build up inside of the distributor, it may fail to route electricity to your vehicle's spark plugs. The distributor cap seals the distributor so that it's protected from environmental contamination.

In addition to protecting against environmental contamination, distributor caps distribute electricity to the spark plugs. They aren't made entirely of molded plastic. Rather, distributor caps feature metal inserts inside of them. These metal inserts allow electricity to travel from the distributor to the spark plugs. The distributor won't work without a distributor cap.

Signs of a Bad Distributor Cap

If your vehicle has a bad distributor cap, you may have trouble starting the engine. The ignition system, of course, is essentially what stars the engine. If the distributor is bad -- or if the distributor cap is bad -- it may not start. Other problems like a dead battery can prevent the engine from starting as well, but the distributor cap could be the culprit.

A bad distributor cap may trigger the Check Engine Light (CEL). If you notice the CEL lit up on your vehicle's dashboard, you may be dealing with a bad distributor cap.

Another sign of a bad distributor cap is shaking. Maybe you're driving, or perhaps you're idling at a red light. Regardless, if you feel your vehicle shaking more aggressively than usual, you should consider getting the bad distributor cap tested by a professional mechanic.