When the current flowing through the coil is
broken (the primary circuit is opened), the magnetic
field collapses across the secondary windings. As the
magnetic field collapses, a high electrical voltage is
induced into the secondary circuit.
Since both the primary and secondary windings of
the coil are stationary, some means other than
movement of the windings must be found to change the
magnetic field surrounding the coils. In practice, a
switching device in the primary circuit creates this
effect. There are two common methods to break
current flow and fire the coilmechanical contact
points or an electronic switching device.
An ignition distributor can be a contact point or
pickup coil type, as shown in figure 2-45. A contact
point distributor is commonly found in older vehicles,
Figure 2-45.Comparison of a (A) contact point distributor
and a (B) pickup coil distributor.
whereas the pickup coil type distributor is used on
many modern vehicles. The ignition distributor has
several functions, which are as follows:
It actuates the ON/OFF cycles of current flow
through the primary windings of the coil.
It distributes the high voltage surges of the coil to
the spark plugs.
It causes the spark to occur at each spark plug
earlier in the compression stroke as speed
It changes spark timing with the changes in
engine load. As more load is placed on the
engine, the spark timing must occur later in the
compression stroke to prevent spark knock.
In some cases, the bottom of the distributor shaft
powers the engine oil pump.
In some electronic distributors, the distributors
house the ignition coil and the electronic
DISTRIBUTOR CAP.The distributor cap is an
insulating plastic component that covers the top of the
distributor housing. Its center terminal transfers
voltage from the coil wire to the rotor. The distributor
cap also has outer terminals that send electric arcs to
the spark plugs. Metal terminals are molded into the
plastic cap to provide electrical connections.
rotor transfers voltage from the coil wire to the spark
plug wires. The rotor is mounted on top of the
distributor shaft. It is an electrical switch that feeds
voltage to each spark plug wire in turn.
A metal terminal on the rotor touches the
distributor cap center terminal. The outer end of the
rotor ALMOST touches the outer cap terminals.
Voltage is high enough that it can jump the air space
between the rotor and cap. Approximately 4,000 volts
are required for the spark to jump this rotor-to-cap gap.
The spark plug consists of a porcelain insulator in
which there is an insulated electrode supported by a
metal shell with a grounded electrode. They have a
simple purpose of supplying a fixed gap in the cylinder
across which the high voltage surges from the coil must
jump after passing through the distributor.
The spark plugs use ignition coil high voltage to
ignite the fuel mixture. Somewhere between 4,000 and
10,000 volts are required to make current jump the gap