See Figure 1
The General Motors HEI system is a pulse-triggered, transistor controlled, inductive discharge electronic ignition system. The entire ignition system is contained within the distributor cap.
The distributor, in addition to housing the mechanical and vacuum advance mechanisms (1975 through 1980), contains the ignition coil, the electronic control module, and the magnetic triggering device. The magnetic pick-up assembly contains a permanent magnet, a pole piece with internal teeth, and a pick-up coil (not to be confused with the ignition coil).
All spark timing changes in the 1981 and later distributors are done electronically by the Electronic Control Module (ECM), which monitors information from various engine sensors, computes the desired spark timing and then signals the distributor to change the timing accordingly. No vacuum or mechanical advance units 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 control module. This switching transistor performs the same function the points did in older conventional ignition systems; it simply turns coil primary current on and off at the correct time. So, electronic and conventional points-type ignition systems operate on the same basic principle.
The module which houses the switching transistor is controlled (turned on and off) by a magnetically generated impulse, induced in the pick-up coil. When the teeth of the rotating "timer" align with the teeth of the pole piece, the induced voltage in the pick-up coil signals the electronic module to allow current to flow to the coil primary circuit. As the rotating "timer" moves away from the pole piece, the electronic module is signaled to stop the current flow to the primary circuit, and high voltage is induced in the ignition coil secondary windings which is then directed through the rotor and spark plug wires to fire the spark plugs.
In essence, the pick-up 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 module automatically controls the dwell period, increasing it with increasing engine speed. Since dwell is automatically controlled, it cannot be adjusted. The module itself is non-adjustable and non-repairable and must be replaced if found defective.