The distributor cap and rotor are responsible for transferring high voltage from the ignition coil to the appropriate cylinders. Here’s a closer look at how these components work within the ignition system.
The ignition coil provides the voltage for your spark plugs – which we know ignites fuel. In order for the fuel to be properly ignited, the spark plug needs to spark just as the piston rises toward the plug (see figure). This means the spark plugs must spark at varied, but uniform, times.
That’s where the cap and rotor come in.
Looking at the top side of the distributor cap, we can see a prong (or tower) for each cylinder the motor has. Depending on the motor, there may be an additional tower for an external ignition coil (for applications where the ignition coil is not mounted to or under the cap). The towers are where your spark plug wires attach.
Flipping the cap over, you can see that there are coordinating metal contacts under the towers. That is where the rotor interacts with the cap. The rotor snaps into place on the distributor and spins in time with the other moving components of the motor.
Let’s now look at how it all works together.
The spark originates at the ignition coil. That spark is transferred to the rotor (either directly or through a spark plug/coil wire). As the rotor spin, the spark jumps from the rotor to the different cap contacts. The electric power travels down that spark plug wire to the spark plug in the cylinder – thus allowing the spark plug to ignite the fuel in the cylinder.
Their importance isn’t an obvious one, unless you understand how they all work together. Now that we know how the cap and rotor function within the ignition system, it’s very clear how important it is for us to replace our caps and rotors regularly: to ensure that spark gets to where it needs to be, when it needs to be there.