Ignition System: Operation

The pick-up coil assembly consists of a permanent magnet, a pole piece and a pick-up coil. The pick-up coil assembly is stationary, unless it is advanced or retarded by the vacuum diaphragm.

The timer core mounted on the distributor shaft, rotates with the shaft inside the pole piece portion of the pick-up coil assembly. When the external teeth of the trigger wheel line up with the internal teeth of the pole piece, a voltage is induced in the pick-up coil. This signals the electronic module inside the distributor, which opens the ignition coil primary circuit. See Fig 1.

Current then decreases in the primary circuit and high voltage is induced in the ignition coil's secondary circuit. This travels through the rotor, distributor cap contact and secondary wires to fire the spark plugs.

A vacuum advance unit adjusts position of pick-up coil and pole piece, providing vacuum spark advance. Conventional centrifugal advance weights shift the trigger wheel on the distributor shaft, providing centrifugal spark advance.

The electronic module automatically controls dwell period, extending it with increased engine speed. Dwell is not adjustable and periodic checks are not necessary. The HEI system features a longer spark duration, which is desirable for firing lean air/fuel mixtures.