To better understand the operation of current ignition systems, it is helpful to first review how older, fully mechanical distributor systems worked.

Breaker Point Ignition

Breaker point ignition systems were used on vehicles for more than 60 years but were abandoned many years ago as engineers looked for ways to decrease emissions and increase fuel efficiency. The breaker point system had three major functions.

SWITCHING DEVICE The distributor assembly acted as a mechanical switch to turn the primary circuit on and off. The distributor's shaft, cam, breaker points, and condenser performed this function.

The breaker point assembly, which was mounted on the breaker plate inside the distributor, consisted of a fixed contact, movable contact, movable arm, rubbing block, pivot, and spring. the fixed contact was grounded through the distributor housing, and the movable contact was connected to the negative terminal of the coil's primary winding. As the cam was turned by the camshaft, the movable arm opened and closed, which opened and closed the primary circuit in the coil. When the points were closed, primary current flow attempted to saturate the coil. When the points opened, primary current stopped and the magnetic field collapsed causing high voltage to be induced in the secondary. The firing of the plug was the result of opening the points.

Because voltage was still present at the movable arm when the breaker arms opened, current could continue to arc across the open point gap, which could damage the points. To prevent this, a condenser was attached to the movable arm. In this way, the voltage at the movable arm was retained by the condenser instead of arcing across the gap.

A primary or ballast resistor was located in series between the battery and the primary coil winding and was responsible for keeping the primary voltage at the desired level (about 9 or 10 volts). This prevented the contact points from burning due to high voltage. The ballast resistor could be either a separate unit or a specially made wire. During starting, the ballast resistor was bypassed to provide maximum current flow to the primary circuit.

TIMING ADJUSTER The distributor also mechanically adjusted the time the spark arrived at the cylinder through the use of two mechanisms: the centrifugal advance and the vacuum advance units. This improved engine performance, fuel efficiency, and emission levels.

SPARK DISTRIBUTION As its name implies, the distributor mechanically distributed the spark so it arrived at the right time during the compression stroke of each cylinder. The distributor's shaft, rotor, and cap performed this function.

Electronic or Solid State Ignition

From the fully mechanical breaker point system, ignition technology progressed to basic electronic or solid state ignitions. Breaker points were replaced with electronic triggering and switching devices. The electronic switching components are normally inside a separate housing known as a control module or control unit. The original (solid state) electronic ignitions still relied on mechanical and vacuum advance mechanisms in the distributor.

As technology advanced, many manufacturers expanded the ability of the ignition control modules. For example, by tying a manifold vacuum sensor into the ignition module circuitry, the module could now detect when the engine was under heavy load and retard the timing automatically. Similar add-on sensors and circuits were designed to control spark knock, start-up emissions, and altitude compensation.

Computer-Controlled Electronic Ignition

Computer-controlled ignition systems offer continuous spark timing control through a network of engine sensors and a central microprocessor. Based on the inputs it receives, the central mircoprocessor or computer makes decisions regarding spark timing and sends signals to the ignition module to fire the spark plugs according to those inputs and the programs in its memory.

Computer-controlled ignition systems may or may not use a distributor secondary voltage to the spark plugs. As mention earlier, distributorless systems use multiple coils and modules to provide and distribute high secondary voltages directly from the coil to the plug.