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Fig. 1 High Energy Ignition (HEI) distributor. Internal coil unit shown:

Fig. 2 High Energy Ignition (HEI) distributor internal components. Internal coil unit shown:

Fig. 3 Typical Electronic Spark Control (ESC) system:

The HEI system utilizes an all electronic module, pickup coil and timer core in place of the conventional breaker points, condenser and distributor cam. In addition, a specially designed ignition coil, distributor cap, rotor and high tension leads are used to provide and distribute high intensity secondary system voltages to the spark plugs. Typical HEI system components are illustrated in Figs. 1 and 2.
The magnetic pickup consists of a rotating timer core attached to the distributor shaft, a stationary pole piece, permanent magnet and pickup coil. When the distributor shaft rotates, the teeth of the timer core line up and pass the teeth of the pole piece inducing voltage in the pickup coil which signals the electronic module to open the ignition coil primary circuit. Since this is a full 12 volt system that does not use a resistance wire, high current saturation occurs in the coil primary windings. Maximum inductance occurs at the moment the timer core teeth are lined up with the teeth on the pole piece. At the instant the timer core teeth start to pass the pole teeth, the module opens the primary circuit, and the current decay causes a high voltage to be induced in the ignition coil secondary winding. The high secondary voltage is directed through a specially designed cap, rotor and high voltage leads to fire the spark plugs.
HEI systems use conventional vacuum and centrifugal advance mechanisms. The vacuum diaphragm is connected by linkage to the pole piece. When the diaphragm moves against spring pressure it rotates the pole piece allowing the poles to advance relative to the timer core. The timer core is rotated about the shaft by conventional advance weights, thus providing centrifugal advance.
Some models are equipped with and Electronic Spark Control (ESC) system that controls spark timing in order to provide maximum engine performance while preventing detonation. The ESC system consists of an engine mounted knock sensor and an electronic controller, Fig. 3. ESC is a closed loop system that monitors engine detonation through a sensor and constantly adjusts ignition timing to provide the maximum usable spark advance while preventing prolonged detonation.
The ESC knock sensor monitors the presence and intensity of engine detonation by sensing the resultant vibrations. The sensor produces a voltage signal which is proportional to the intensity of the detonation and this voltage signal is transmitted to the controller. The ESC controller is a hard wired signal processor/amplifier that operates in the 6-16 volt range, and has no memory storage provisions. The controller monitors knock sensor voltage output, processes these signals and controls the amount of spark advance through a special circuit in the HEI ignition module.
In addition, models with automatic transmissions include an ESC vacuum switch. The vacuum switch provides a signal to the ESC controller during throttle tip-in which causes the module to momentarily retard spark timing to prevent detonation on acceleration.