Ignition System - DURA-Spark II: Operation: Distributor
A reluctor, containing the same number of teeth as engine has cylinders, turns with the distributor shaft. The pick-up coil contains a permanent magnet, causing a magnetic field around pick-up coil.
As the teeth of reluctor pass the pick-up coil, the magnetic field builds and collapses, causing a signal to be sent to the electronic control module. The control module turns the ignition coil primary on and off, causing a high voltage surge in the secondary.
Dura-Spark II systems have an adapter between the distributor housing and cap. See Fig 2. Distributor caps are larger than for conventional distributors and have male terminals. Distributors have both centrifugal and vacuum advance mechanisms.
On single diaphragm vacuum units, increased vacuum causes pick-up coil to pivot on lower plate assembly, advancing spark timing. On dual diaphragm units, outer (primary) diaphragm operates from carburetor vacuum to provide timing advance during normal, idle off, driving conditions. It is connected to the pick-up coil assembly.
The inner (secondary) diaphragm operates from intake manifold vacuum and acts to retard ignition timing. The inner diaphragm is connected to outer diaphragm by means of sliding linkage. Stronger intake manifold vacuum can override carburetor vacuum during closed throttle operation, retarding spark timing.