The distributor cap is a prime example of a component that eventually succumbs to heat and vibration. But even if its bakelite housing has not broken or cracked, carbon deposits and eroded metal terminals can cause distributor-cap failure. However it is a fairly easy and inexpensive part to replace.
The distributor cap has 3 to 9 posts on it. One post is for the coil voltage coming into the distributor. The other posts go to each spark plug respectively.
On the inside of the cap there is a terminal that corresponds to each post. The plug terminals are arranged around the circumference of the cap. Some distributors have the outside terminals in a straight line.
The rotor head (usually called "the rotor") is attached to the top of the distributor shaft which is driven by a gear on the engine's camshaft and thus synchronized to it.
The rotor is pressed against a carbon brush on the center terminal of the distributor cap. A spring is used to keep tension on the carbon point. On the inside of the cap, the coil terminal is in the center. The rotor is constructed such that the center tab is electrically connected to its edge, so the voltage coming in the coil post will travel through the carbon point to the center of the rotor, then to its edge.
Some rotors have an integrated resistor in the center tab for suppression of radio interference.
As the rotor rotates, its edge passes each of the plug terminals that are arranged around the inside of the cap, according to the firing order, sending the secondary voltage to the proper spark plug. The rotor edge however does not touch the plug terminals but the spark jumps when it is just opposite to it.
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