When the ignition points are suddenly opened, the primary magnetic field collapses
rapidly, producing enough voltage to carry through the secondary system and jump
the gap at the plug. Two important factors must be kept in mind:
 

• The spark plug fires when the points open.
• As the primary winding builds up a magnetic field, it also produces an increased

Each of the parts in the system play an important role:
 
 

Ignition Switch - provides the driver's control over the system, allowing it to
be switched on and off.

Ballast Resistor (and bypass) - a ballast resistor and bypass are connected
between the ignition switch and the + terminal of the ignition coil. The ballast resistor
limits current flow in the primary winding to prevent it from overheating, and the
bypass allows the resistor to be excluded from the circuit during engine cranking, as
battery voltage can drop as low as 10 volts during starting, and may not be sufficient
enough to produce the high voltage required to jump the gap at the spark plug.

Primary Coil Winding - produces the moving magnetic field required to induce a
voltage in the secondary winding.

Ignition Points - provide an automatic switch controlled by a cam in the distributor
(which in turn is rotated by the engine camshaft) to open the circuit (a spring on the
points closes them).

Condenser - provides a temporary storage for electrons that would attempt to
jump the gap as the points open. If the points arc - they will burn, and the magnetic
field will not be able to collapse because current will still be flowing in the primary
coil winding (keep in mind that this primary voltage can be as high as 250 volts).
 

Coil Wire - carries the high voltage from the coil to the distributor cap.

Distributor Cap - provides a junction point for all the secondary wires and
arranges the spark plug wires in the correct firing order.

Rotor - provides a rotating switch under the cap to carry the high voltage from
the coil terminal in the cap to each of the spark plug terminals in turn. The rotor
is attached to, and rotates with, the same distributor shaft that operates the cam
to open the points.

Spark Plug Wires - these heavily insulated wires carry the high voltage from
the distributor cap to the spark plugs.

Spark Plugs - provide an air gap in the cylinder that the spark has to jump to
ignite the air/fuel mixture in the engine cylinder.

Centrifugal Advance

This consists of weights that swing outwards as engine speed increases. These
weights are connected to the cam that opens the ignition points - therefore as
speed increases, the points are opened sooner - and the spark occurs sooner.

Vacuum Advance

This consists of a diaphragm that is connected to the ignition points. As engine
speed increases the engine vacuum will also increase. The diaphraghm will then
pull on the ignition points, causing them to come in contact with the cam sooner -
resulting in them opening sooner, causing the spark to occur earlier in the
cylinder. Under heavy load or full throttle, engine vacuum drops significantly.
This causes the points to move back - thereby retarding the spark. This is
important, because if the spark occurs too soon in the cylinder under heavy load
it can cause preignition pinging that may result in piston damage.

To Summarize - the centrifugal advance responds to engine speed, the
vacuum advance to engine load. Remember this - as it will become significant
as you move on to the more advanced systems!

The point type ignition system has two major problems:
 

• As the points wear, the gap changes - causing the time at which the points

• As the points open and close, they tend to arc because of the high primary