The typical multiple coil ignition system may have one of several different configurations. On Chrysler, Toyota and many other imports, the coils are mounted directly over the spark plugs. Many of these are the thin "pencil" style coils that extend down into recessed wells in the engine's valve covers. On other applications, such as GM's Quad 2.2L Four, the individual coils are mounted in a cassette or carrier that positions the coils over the spark plugs. On late model Corvette, Camaro and other V8s, a Coil-Near-Plug (CNP) setup is used because the spark plugs protrude from the side of the cylinder head and there isn't room to mount a coil on the end of each plug. Here, the individual coils are mounted on the valve cover and attached to the plugs by short plug wires.
In most of the older DIS ignition systems, an electronic module was part of the coil pack assembly and controlled the switching of the coils on and off. On most of the newer systems, the switching function is handled by the powertrain control module, though there may some additional electronics and diodes built into the top of each coil. The PCM receives a basic timing signal from the crankshaft position sensor and sometimes a camshaft position sensor to determine engine speed, firing order and timing. It then looks at inputs from the throttle position sensor, airflow sensor, coolant sensor, MAP sensor and even the transmission to determine how much timing advance to give each plug. Most of today's multi-coil ignition systems are capable of making timing adjustments between cylinder firings which makes these systems very responsive and quick to adapt to changing engine loads and driving conditions.
When a coil failure occurs on a distributor ignition system, it affects all the cylinders. The engine may not start or it may misfire badly when under load. But with multi-coil ignition systems, a single coil failure will only affect one cylinder (or paired cylinders in the case of waste spark DIS systems).
According to the original equipment supplies who make multi-coil ignition systems, having a separate coil for each cylinder also improves the engine's ability to handle more exhaust gas recirculation to reduce oxides of nitrogen emissions (important with today's low emission vehicle standards). A hotter spark also makes spark plugs more resistant to fouling and helps 100,000 mile plugs go the distance. A multi-coil ignition system also improves idle stability and idle emissions, too.