How a Distributor Cap Works

Most people don’t take much notice of a distributor cap. It is not something they give a great deal of thought to when thinking about their engine, but the distributor cap is actually a vital piece of the mechanism that involves starting the car.

Inside the Distributor Cap

Inside your distributor cap you will see an arm, usually black, which is known as a rotor. This sits atop the distributor shaft. The rotor ‘rotates’ to varying degrees. The shaft rotates in time with the crankshaft of your engine. The carbon brushes, which you can see inside the distributor cap, make contact with a cable. This cable passes high voltage from the coil to the brushes.

Where it Goes

There are output contacts that make contact with the rotor, which connects, via the distributor cap, to the high tension leads. You will recognize these as the leads which pass between the distributor cap and the spark plugs. There is a tiny pin-size gap in the rotor arm (in older cars these used to be referred to as points). They create an electrical spark via the plugs, which all co-ordinate in perfect timing to allow your vehicle to start correctly.


If one element is out of time or dysfunctional your engine will either fail to start completely or it will misfire. If you have ever heard the phrase ‘not firing on all cylinders’ you will know this means that one of the spark plugs is failing, but it is not enough to prevent the car from starting— just enough to prevent it from running smoothly. The shaft in the distributor has a cam that maneuvers a contact breaker, so that when the points are opened they produce a high voltage in the ignition coil.

How the Distributor Cap Works

Your vehicle will have a 12 volt battery in it. This battery is kept charged during use of the engine. The battery connects to the electrical system on your vehicle. When you start your vehicle the process seems almost instantaneous, but there is a well timed set of procedures that all take place within split seconds of you turning the key. The sequence begins with the key, which turns the engine and the cam that is inside the distributor cap. This enables the compression cycle to create a break in the points. When the ignition coil is sparked the points within the distributor cap break the circuit to set the rotor arm rotating via the cam shaft.


The current that flows via the battery goes around the whole circuit and back to the battery again, creating a magnetic field. If the distributor cap has a break, crack or gap, there will be no closed vacuum in which the sparks can operate. The cylinders inside the cap will misfire and ignition will fail. It is like skipping a heart beat. One little piece of the puzzle is missing and the without it the whole structure collapses. If the rotor arm doesn’t function, the points will fail. If the points fail, the rotor arm won’t work and so on. It is a precision movement and requires optimum capacity and precision timing.