Coil packs are a component typically used in automotive distributorless ignition systems (DIS). They perform roughly the same function as traditional ignition coils, though there will usually be a coil for each cylinder instead of one for the entire engine. Each coil pack may be separated out, or they may be combined into a single unit. When coil packs are split into individual units, they are typically located near the cylinders they fire instead of in a central location. Coil packs are typically activated by an electronic control unit (ECU) or ignition module, so that each one can fire its cylinder at the necessary time.
In a traditional ignition system, a coil or magneto typically generates a large voltage that can then be sent to the correct cylinder using a distributor and rotor system. Distributorless ignitions eliminate the need for a number of the components associated with those systems, primarily the distributor and rotor. Instead of using mechanical motion from the camshaft to time the spark via a rotor, these systems can use the computer to fire individual coils at the appropriate time. This is often achieved through the use of a cam or crankshaft sensor.
Each coil pack typically consists of at least a primary and a secondary winding, though there may also be other components. The coil packs are all supplied with battery voltage, which can be used by the primary winding to create a magnetic field. When signaled by the ECU or through other means, the primary winding voltage can be cut off. This can then generate a much higher voltage in the secondary winding of the coil pack, which may be used to fire a spark plug. In most distributorless ignition systems, each coil pack can be connected to its own spark plug via a plug wire.
Certain distributorless ignition systems use a slightly different configuration. Some larger engines can have two or more coil pack units, each of which may have a number of internal windings and are responsible for a several cylinders. Each set of windings in these coil packs will typically still correspond to a single cylinder and be connected to a spark plug wire. Other configurations can have as many coil packs as cylinders, each located near the spark plug it's responsible for firing. Another variation is coil on plug (COP), which can integrate each coil pack with a spark plug boot.