See Figure 1
The High Energy Ignition (HEI) system controls the fuel combustion by providing a spark to ignite the compressed air/fuel mixture at the correct time. To provide improved engine performance, fuel economy, and control of exhaust emissions, the engine control module (ECM) controls distributor spark advance (timing) with an ignition control system.
The distributor may have an internal, or external ignition coil. To be certain of the type coil used for your vehicle, visually inspect the ignition system. If the ignition coil is inside the distributor cap, it connects through a resistance brush to the rotor. If your vehicle is equipped with an external ignition coil, it connects to the rotor through a high tension wire.
The distributor contains the ignition control module, and the magnetic triggering device. The magnetic pickup assembly contains a permanent magnet, a pole piece with internal "teeth", and a pickup coil (not to be confused with the ignition coil).
All spark timing changes are done electronically by the engine control module (ECM) which monitors information from various engine sensors. The ECM computes the desired spark timing and then signals the distributor ignition module to change the timing accordingly. No vacuum or mechanical advance systems are used.
In the HEI system, as in other electronic ignition systems, the breaker points have been replaced with an electronic switch - a transistor - which is located within the ignition module. This switching transistor performs the same function the points did in a conventional ignition system; it simply turns the coil's primary current on and off at the correct time. Essentially, electronic and conventional ignition systems operate on the same principle.
The module which houses the switching transistor is controlled (turned on and off) by a magnetically generated impulse induced in the pickup coil. When the teeth of the rotating timer align with the teeth of the pole piece, the induced voltage in the pickup coil signals the electronic module to open the coil primary circuit. The primary current then decreases, and a high voltage is induced in the ignition coil secondary windings, which is then directed through the rotor and high voltage leads (spark plug wires) to fire the spark plugs.
In essence, the pickup coil module system simply replaces the conventional breaker points and condenser. The condenser found within the distributor is for radio suppression purposes only and has nothing to do with the ignition process. The ignition module automatically controls the dwell period, increasing it with increasing engine speed. Since dwell is controlled in this manner, it cannot be adjusted. The module itself is non-adjustable/non-repairable and must be replaced if found defective.