It is the Ford spark plug wires that transfer the electrical pulse from the distributor, coil pack, or ignition control module to the spark plug so the fuel in the combustion chamber can be ignited. These are an electrical component and must be periodically changed because over time or their efficiency in transferring the spark will diminish. There is usually a spark plug wire for each cylinder on your Ford engine. They are connected to the spark plug at one end and the distributor on the other for older models. The newer models they are connected to the coil and the ignition controller. Care must be taken when separating the spark plug wire from the plug. If the polymer boot on the plug is fused in place, a slight twisting motion will help break it free. If this is not done and the Ford spark plug wire is just pulled, the end of the wire could become disconnected and replacement is your only option. The testing of a spark plug wire on Ford (or other) vehicles is done using an ohmmeter. If there is little to no resistance in this wire, then it is still functioning. As the resistance increased, the efficiency in which the wire can perform the transfer of the electrical impulse is diminished. When just one spark plug wire is found to be needing replacement on your Ford vehicle, all of them should be done at the same time. This helps to ensure an even distribution of the electrical impulse to all of the cylinders is nearly equal for each one. This will make it possible for your Ford engine to run smoother. If you have an older style engine with a distributor, the shortest wire should be connected to the distributor and the coil connection. Remember only replace one wire at a time so the firing order is not changed.