Ford DIS & EDIS ignitions are electronically controlled, distributorless systems. Instead of using a conventional rotating distributor, they both employ a primary/secondary electronic coil system that converts battery voltage into the much higher voltage needed to fire the spark plugs and ignite the compressed air/fuel mixture. The firing of the coils and plugs is controlled by the EEC computer ("PCM" in today's terms) which utilizes sensor signal feedback to help calculate and maintain proper timing and dwell.
Both DIS & EDIS are of a waste spark design. In other words, a single coil fires two seperate cylinders at the same time. The spark plugs in the two cylinders are in series electrically, with the circuit being completed through the cylinder block. When a coil fires, high voltage current flows INTO the top of first plug and OUT the top of the second plug, completing a big loop in each case. This means that the firing voltage of one spark plug is negative with respect to ground, while the other is positive with respect to ground. Half the spark plugs actually see a different polarity than the others do.
In Ford waste-spark systems, the spark plug on the compression stroke uses the majority of the coil’s stored energy, while the other spark plug, on its exhaust stroke, uses very little of that energy. This occurs because of the much higher cylinder pressure that is generated on the compression stroke. The higher the pressure, the greater the electrical resistance. Accordingly, more voltage is necessary to overcome the increased resistance of the highly-compressed air/fuel mixture. In a nutshell, the amount of pressure in a cylinder - and the associated electrical resistance that it creates - determines which spark plug (compression stroke) gets the majority of the coil's voltage, and conversely, which plug (exhaust stroke) gets the "waste" voltage.
Spark is generated as follows: an electronic switch-to-ground is employed in the coil primary circuit. When the switch is closed, battery positive voltage (+12v) is applied to the primary circuit and builds a magnetic field around the primary coil. When the switch opens, the power is interrupted and the primary field collapses, inducing high voltage pulses into the secondary coil windings. These high voltage pulses are carried by the plug wires to the spark plugs which create spark at the gap, igniting the compressed air/fuel mixture.
A kickback voltage spike occurs when the primary field collapses, creating an Ignition Diagnostic Monitor (IDM) signal which is used by the PCM to keep tabs on the overall health of the ignition system (for more on the IDM, see "System Monitoring" below). Coil switching - including timing and dwell - is ultimately determined by the PCM, although it is carried out by a separate Ignition Control Module in many applications, as discussed below.