The ignition distributor is at the heart of the engines ignition system. Its 3 basic functions are to trigger the firing of the ignition coil, to control timing advance, and to distribute the coils high voltage output to the individual spark plug.
On older applications with breaker point ignition systems, the contact points switch the ignition coil's primary voltage on and off. The points are mounted on a movable plate in the distributor housing. Cam lobes on the distributor shaft rotate against a plastic block on the points to open and close the points. On newer applications with electronic ignition, the distributor can contain an inductive pickup, an optical sensor or even a hall affect switch instead of points to generate a signal for the ignition module, which in turn switches the coil on and off. On many applications, the ignition module itself can also be located inside the distributor or mounted on the distributor housing.
Wear in the distributor drive gear can introduce play that can affect ignition timing, while worn distributor shaft bearings or bushings (often the result of infrequent oil changes and varnish build-up on the distributor shaft) can also cause erratic ignition timing. Both conditions can affect engine performance, fuel economy and emissions. If the distributor is worn, repair or replacement is the only option.
The spark from the coil is generated at precise moments during engine piston travel. For optimum performance and fuel economy, ignition should occur just before the piston reaches the top of its stroke, so that the air-fuel mixture in the combustion chamber is completely burned at the top of the piston's compression stroke. The heat and pressure of the expanding gases in the combustion chamber force the piston downward. At higher engine speeds, the piston travels faster meaning ignition needs to occur earlier, so that combustion is complete when the piston reaches the top of its compression stroke. Coolant temperature, air temperature, air pressure and altitude are all variables that can affect the ignition timing.
With respect to ignition timing, the distributor on older, contact points type applications, have both a centrifugal advance mechanism and vacuum advance diaphragm. Flyweights and springs in the centrifugal advance mechanism add timing advance as engine speed increases. The vacuum advance diaphragm rotates the breaker plate to advance timing when engine intake vacuum is high to improve fuel economy, and retards timing when the engine is under heavy load to prevent detonation or pinging.
Distributor rotor & cap
The distributor rotor directs the ignition coil's high voltage output to the various spark plug terminals in the distributor cap. The condition of the cap and rotor are very important, as is the "air gap" or distance between the tip of the rotor and cap terminals. If the cap or rotor are cracked, worn, eroded or have carbon tracks, either or both should be replaced.
RAE has Australia’s largest range of ignition distributors with over 500 part numbers available in both New and remanufactured.