There are only a few reasons why you would need to replace your distributor cap. Your may be rebuilding an engine, and the original cap is corroded, cracked, or missing. The other reason may be because you've noticed no current reaching the end of a spark plug wire, so there is no ignition in one of your cylinders. The way one determines that the cap is the problem and the wires are not is by running a resistance test through each wire. If resistance is within normal range, then you can safely assume that there is no risk for shorts or arcs. When replacing ignition parts, usually a new rotor is purchased with the distributor cap. It's not uncommon for one to go bad along with the other. Since the parts are not terribly expensive and your engine won't run right without them, there's no downside to buying them both. And if they are not working together in perfect unison, you will notice misfires in one or more of your cylinders. There's no reason why you should let a broken distributor cap cause such a big problem.
Your engine's distributor is in charge of sending current to your individual spark plugs in a predetermined sequence, or firing order. This is accomplished when about a dozen different components are working together like clockwork. One of those parts is the distributor cap, and it looks like a crown in the center of your engine. It has a post for each wire as well as a central post that connects to the central coil. A turning rotor underneath it makes contact at the bottom of each of those posts, closing a circuit right in time to send a charge through the ignition wires to the spark plug.
Installing a distributor cap is super easy. You would essentially just disconnect all the wires leading to it from the coil and spark plugs, and unclip it from the distributor. The cap should simply come right off and a new one will install in its place. Most distributor caps have an index slot that prevents it from being installed incorrectly. This guarantees correct firing order. At this time, you can take a minute to examine the rotor under the cap to see if that also needs to be replaced. It is just as easy to do as the distributor cap, so it's recommended they be changed at the same time.
When it comes time to purchase new ignition components for your engine, don't settle for anything less than the best. For cars with older style mechanical distributors, buying parts that work well together is important. At Car Parts Discount, we carry top quality aftermarket and original equipment distributor caps and other components from manufacturers like Bosch, Bremi, SMP, YEC, and more. Get the right distributor cap for your job, and get it done quick and easy.