Coil-on-plug assemblies are designed to convert a low voltage (primary side) to a high voltage (secondary side) to fire the spark plugs. They perform the functions of both the ignition coil, which creates the spark energy. Today’s coil-on-plug assemblies come in a variety of physical and wiring configurations. The typical configurations are two wires, three wires and four wires. Some ignition coils contain a solid-state driver module, which is part of the ignition coil and is controlled by the Powertrain Control Module (PCM). Others have the primary winding wired to and directly controlled by the Powertrain Control Module. One of the circuits that should be common to all ignition coils is the battery voltage power supply.
The typical wiring configuration for a two coil-on-plug assembly includes battery power supply and the direct control circuit (trigger) of the primary winding. This is very similar to the single ignition coil systems of the past. Most vehicles use a single strike to control the primary. The Ford example below uses multi-strike at idle, meaning that it fires multiple times in one firing event.The typical wiring for a three-wire ignition coil-on-plug assembly is battery voltage power supply, ground and control circuit (trigger) from the PCM to a transistor circuit in the coil on plug assembly.
Four-wire coil-on-plug assemblies can involve a lot of commonalities also such as, the battery voltage power supply, two ground circuits and the control circuit (trigger) from the PCM to the solid-state driver module. Some applications may use one the four wires of the coil-on-plug assembly as a feedback circuit to verify proper ignition coil system integrity.It is always recommended to refer to service information and wiring diagrams for the specific vehicle you are working on. Some ignition system can change wire colors depending on the bank and cylinder.