The electronic ignition system used on S14 and M20 engines is a combination of a distributor with the conventional breaker point ignition system found on earlier vehicles and a fully electronic system. This system uses a distributor cap and ignition rotor to distribute the ignition spark from a single ignition coil to each cylinder's spark plug in the correct order and requires the cap and rotor to be inspected, cleaned, and/or replaced during periodic maintenance.
Aside from the distributor cap and rotor, the main difference between this system and the breaker points type of ignition system is that it does not have any moving parts that require periodic adjustments, unlike breaker points, which require periodic rubbing block lubrication, adjustment and replacement. To advance the ignition timing relative to engine speed, the breaker point ignition systems used a mechanical, centrifugal advance unit and, if equipped, a vacuum diaphragm to fine tune the ignition timing based on throttle position. During full throttle operation, the ignition timing would advance based solely on the engine's speed. Those systems worked well for their day, however to meet today's more stringent emission standards, the engine's operating efficiency must be optimized. To meet these standards, and optimize an engine's efficiency requires precise control of the ignition timing and fuel management system.
To achieve this level of efficiency, the ignition timing advance is controlled electronically by the Engine Control Module (ECM), commonly referred to as the Digital Motor Electronics (DME) system. The DME system allows the ignition timing to be controlled electronically based on input from a collection of electronic sensors. This system allows the ignition timing advance to be adjusted and optimized instantly, for changes in engine speed, throttle position, intake manifold airflow rate and the engine coolant temperature.
These sensors also assist the DME control unit to determine the proper fuel delivery. The ignition timing is fine tuned by the DME control unit for throttle position, intake manifold airflow rate and the engine coolant temperature. The two sensors that are used to plot the basic ignition advance and supply a voltage pulse signal to the ignition coil are the speed and reference sensors. Just as their names imply, the speed sensor monitors engine speed, and the reference sensor informs the DME control unit of the relative position of the crankshaft.
These sensors are triggered electronically by either the teeth of the ring gear on the flywheel or a reference wheel mounted behind the front crankshaft pulley. As the teeth of the ring gear or reference wheel pass very closely to the sensor, which is simply a small electric coil, the electric field of the sensor is energized. As the teeth move past the sensor, the electric field of the sensor is collapsed causing an electric pulse that is sent from the sensor to the DME control module. The control module uses these electric pulses to calculate the engine speed and the relative position of the crankshaft. The faster the engine spins, the faster the pulses are generated, allowing the DME control unit to calculate the engine's crankshaft speed.
It's also worth noting that the DME fuel pump relay is actuated when it receives a pulse signal from these sensors. Hence should a sensor fail or if the engine stops rotating, the signal to the DME fuel pump relay is lost and the relay stops supplying voltage to the fuel pump.