Distributors are electro-mechanical devices used to deliver spark to the spark plug at the correct time during the ignition cycle. The distributor can be a contact point type or an electronic type with a pick up coil. The contact point style consists of points, condenser, and a distributor cam on the lobed part of the distributor shaft. The electronic type consists of an electronic module and a pickup coil or hall effect switch..
Point style distributors have lobes on the distributor shaft that open and close the contact points, making and breaking the ignition coil primary circuit.
When the points are closed a magnetic field builds in the coil. As the points open the magnetic field collapses, sending voltage out of the coil tower through the rotor contacts and down to the spark plugs. Dwell is the amount of time the points remain closed between each point opening. The dwell period is needed to build up a strong magnetic field in the ignition coil to deliver the proper voltage to the plugs. Too much dwell time results in point burning or arcing, too little results in a weak spark.. Timing is vacuum and mechanically advanced.
Electronic distributors consist of an electronic control circuit and a magnetic pick up coil or hall effect switch. A trigger wheel located on the end of the distributor shaft replaces the cam lobes used in a contact type distributor. Electronic pulses are produced by the pickup coil or hall effect switch as the distributor rotates and the reluctor wheel passes the pickup assembly. The electronic "switch" located in the ignition module, turns the primary coil circuit on and off. Dwell time is regulated in the ignition module under the control of the Electronic Control Module. Timing is electronically and mechanically advanced (on some applications).
The main advantage of electronic distributors over contact points style is there are no mechanical points to wear out or burn, thus affecting the dwell, engine performance and emissions. The electronic distributor is also able to produce a higher spark thus resulting in better performance and reduced emissions levels.
Failure of distributors can be attributed to bushing/shaft wear, worn, frayed, or broken wires, breaker plate wear, lack of lubrication, and/or improper installation of the unit.