Ignition System: Description and Operation

System Description:

To produce a spark that will ignite the air/fuel mixture within an engine's cylinders, ignition systems generate high voltage and direct it to spark plugs at each cylinder. Voltage must be high enough to jump the spark plug gap under compression and it must be supplied at the correct time under a wide range of operating conditions. Various systems of controlling spark generation and timing have been devised. These range from completely mechanical to ECU activated electronic to distributorless.

This vehicle utilizes an electronic ignition system with mechanical and vacuum advance mechanisms. Components include:

^ Ignition coil.
^ Camshaft driven distributor with vacuum control unit, governor weight system for mechanical advance, pick-up coil and igniter assembly, reluctor (signal rotor).
^ Distributor cap and high tension wiring for distributing high voltage to the spark plugs.
^ Necessary wiring and vacuum hoses.

When the ignition is switched "ON," current is supplied to the coil primary windings and grounded through the igniter within the distributor. As the distributor shaft turns, the pick-up coil magnetic field is altered by the reluctor (once for each cylinder), signalling the igniter to interrupt the coil ground connection. Each time this sequence occurs, the magnetic field created by current flowing through the coil primary windings collapses, inducing high secondary voltage (20,000 plus volts) required to fire spark plugs.

Timing advance is controlled in two ways. A mechanical governor weight system moves the rotor ahead in relation to engine speed and distributor shaft rotation speed. As engine and distributor shaft speed increase, spring-loaded weights swing outward due to centrifugal force, moving the upper distributor shaft and rotor ahead in relation to the lower (driven) part of the shaft. At low speeds, the springs hold the weights in, preventing the upper distributor shaft and rotor from moving ahead (advancing).

A two-chamber vacuum diaphragm is used to advance and retard timing according to engine load. An arm from the vacuum unit is attached to the breaker plate, moving the pick-up coil in relation to the reluctor as vacuum varies with load.

To calculate engine speed (rpm), the ECU receives a signal from the ignition coil negative terminal.