The Ignition distributor consists of a housing, driving shaft with breaker cam, breaker plate with contact points, governor, condenser, rotor, advance mechanism, and cap. The shaft is driven by the engine camshaft directly or indirectly through the oil pump drive shaft. The driveshaft is rotated at half the engine speed in four-stroke engines.
When the shaft moves, it opens and closes the breaker points. The breaker contains the same number of lobes as the number of cylinders in the engine. A rotor is mounted on the breaker cam which is carried by the drive shaft. The rotor connects the center terminal of the cap with each outside terminal in turn so that high voltage surges from the coil are directed first to one spark plug and them to another, according to the firing order.
The distributor serves two functions in the ignition system:
- It opens and closed the circuit between the battery and the ignition coil. When the circuit closes, current flows in the ignition coil and builds up a magnetic field. When the circuit opens, the magnetic field in the coil collapses and a high voltage surge of current is produced by the coil.
- To distribute high tension current to the proper spark plug at the correct time.
There are two basic types of distributors – contact point distributor the one using the contact points to close and open the primary circuit t: and the other magnetic pick-up distributor using a magnetic pick-up and a transistor control unit to interrupt the current flow in the primary circuit.
The distributor with contact points has already been described above.
The figure shows a schematic diagram of the ignition system using a magnetic pick-up distributor. The control unit is connected between the ignition coil primary and the battery through the ignition switch. It permits battery current to flow to the coil primary winding and it interrupts this flow, on signals from the distributor. This action is similar to what happens when the points close and open in the contact point distributor.