A spark plug wire diagram shows both the location of an engine's spark plugs and the order in which they fire. There is no universal spark plug wire diagram, as every vehicle has its own distinct spark plug firing order and wire diagram.
Using a spark plug wire diagram is necessary when changing the spark plugs and spark plug wires in an engine. Spark plug wires carry the electrical current produced by the engine's distributor to the specific spark plug to which the electrical contacts inside the distributor correspond. Replacing spark plugs in the incorrect firing order causes the engine to run roughly or not at all.