See Figures 1 and 2
This system, used only on the 1984-86 Canadian trucks (Z24), is designed to change the ignition system from 2-plug ignition to 1-plug ignition during heavy load driving conditions in order to reduce engine noise. The system also functions to advance ignition timing by the proper amount during 1-plug ignition.
The main component of the system is an ignition control unit installed in the distributor. The unit has a switching function which allows it to change from 2-plug to 1-plug operation by means of a vacuum switch in the intake manifold.