Ignition System - DURA-Spark II: Operation: Distributor
A reluctor, containing the same number of teeth as the engine has 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 pass the pick-up coil the magnetic field builds and collapses, causing a signal to be sent to the electronic control module. In turn, 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 Fig 1. 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 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. 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 the outer diaphragm by means of sliding linkage. Stronger intake manifold vacuum can override carburetor vacuum during closed throttle operation, retarding spark timing.