Distributor: Description and Operation

Distributor Assembly:

The distributor is mounted on the rear face of the cylinder head and is driven off the end of the camshaft. Ignition advance is controlled two ways:
Mechanically
^ According to engine speed by weights inside the distributor body.
^ According to engine load by vacuum control.
Electronically
^ According to altitude by the ignition module via the Barometric Pressure Sensor input to the Electronic Control Assembly (ECA).


When the distributor shaft rotates, the magnetic pulse is generated in the pickup coil. The pickup coil is part of the electronic ignition module, which controls the current through the ignition coil primary winding. As the coil windings collapse, the field induces a high voltage pulse.

IGNITION MODULE

The ignition module receives input from the ECA which receives input from the barometric pressure sensor at altitudes of 3,280 or higher, this signals the ignition module to advance ignition timing. High voltage is created each time the magnetic field is built up and collapsed.

CENTRIFUGAL ADVANCE

The mechanical advance mechanism consists of two weights pivoted so that they move outward from the distributor shaft as the engine speed increases. As the weights move outward, they turn the breaker plate in relation to the lower distributor shaft, thus advancing the ignition timing. Each weight is restrained by two springs of different tension, giving a progressive advance action. The amount the weights move outward is in direct proportion to the distributor shaft speed.

VACUUM ADVANCE

The dual-diaphragm vacuum advance mechanism acts on the stator plate which is free to rotate in the distributor housing. The larger diaphragm chamber is connected to the spark port on the throttle body, just above the throttle at the idle setting. The smaller diaphragm chamber is connected to the intake manifold, below the throttle. The resultant advance response is based on throttle position, engine speed, and engine load.