prevents radio interference. Also known as radio
interference wires, they have approximately
10,000 ohms per foot. This prevents high-
voltage-induced popping or cracking of the radio
On the outer ends of the spark plug wires, boots
protect the metal connectors from corrosion, oil, and
moisture that would permit high voltage to leak across
the terminal to the shell of the spark plug.
CONTACT POINT IGNITION
Before studying today's electronic ignition
systems, you should have a basic understanding of the
contact point ignition system. The two systems use
many of the same components. These include the
battery, the ignition coil, the ignition distributor, the
spark plugs, and wires and cables that connect them.
Contact Point Ignitions System
The internal components of the distributor for a
contact point ignition consist of the following:
DISTRIBUTOR CAMThe distributor cam is
part of, or is attached to, the distributor shaft and
has one lobe for each cylinder. As the cam
rotates with the shaft at one half of engine speed,
the lobes cause the contact points to open and
close the primary circuit.
CONTACT POINTSThe contact points, also
called breaker points, act like spring-loaded
electrical switches in the distributor. Its function
is to cause intermittent current flow in the
primary circuit, thus causing the magnetic field
in the coil to build up and collapse when it
reaches maximum strength. Wires from the
condenser and ignition coil primary circuit
connect to the points.
CONDENSERThe condenser, also known as
a capacitor, is wired in parallel with the contact
points and grounded through the distributor
housing. The condenser prevents arcing or
burning at the distributor contact points when the
points are first open. The condenser provides a
place where current can flow until the contact
points are fully open.
Contact Point Ignition System Operation
With the engine running, the distributor shaft and
distributor cam rotate. This action causes the
distributor cam to open and close the contact points.
With the contact points wired to the primary
windings of the ignition coil, the contact points make
and break the ignition coil primary circuit. With the
contact points closed, the magnetic field builds up in
the coil. As the points open, the magnetic field
collapses and voltage is sent to the spark plugs.
With the distributor operating at one half of engine
speed and with only one cam for each engine cylinder,
each spark plug only fires once during a complete
revolution of the distributor cam.
To ensure that the contact points are closed for a set
time, point dwell, also known as cam angle, is set by
using a dwell meter. Point dwell is the amount of time
given in degrees of distributor rotation that the points
remain closed between each opening.
A dwell period is required to assure that the coil
has enough time to build up a strong magnetic field. If
the point dwell is too small, the current will have
insufficient time to pass through the primary windings
of the ignition coil, resulting in a weak spark.
However, if the point dwell is too great, the contact
points will not open far enough, resulting in arcing or
burning of the points.
ELECTRONIC IGNITION SYSTEM
The basic difference between the contact point and
the electronic ignition system is in the primary circuit.
The primary circuit in a contact point ignition system is
open and closed by contact points. In the electronic
system, the primary circuit is open and closed by the
electronic control unit (ECU).
The secondary circuits are practically the same for
the two systems. The difference is that the distributor,
ignition coil, and wiring are altered to handle the high
voltage produced by the electronic ignition system.
One advantage of this higher voltage (up to 60,000
volts) is that spark plugs with wider gaps can be used.
This results in a longer spark, which can ignite leaner
air-fuel mixtures. As a result engines can run on leaner
mixtures for better fuel economy and lower emissions.