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