The primary circuit of an electronic ignition
system is controlled electronically by one of the sensors just described and an
electronic control unit (module) that contains some type of switching device.
Primary Circuit Control
The system consists of a distributor with a
magnetic pulse pick-up unit and reluctor, an electronic control module, and a
ballast resistor. As described earlier, when the tooth of the reluctor passes
the pick-up, an electrical impulse is sent to the electronic module, which
contains the switching transistor. the pulse signals the transistor to open the
primary circuit, firing the plug. Once the plug stops firing, the transistor
cloeses the primary coil circuit. The length of time the transistor allows
current flow in the primary ignition circuit is determined by the electronic
circuitry in the control module. Some systems used a dual ballast resistor. The
ceramic ballast resistor assembly is mounted on the fire wall and has a ballast
resistor for primary current flow and an auxiliary resistor for the control
module. The ballast resistor has a 0.5-ohm resistance that maintains a constant
primary current. The auxiliary ballast resistor uses a 5-ohm resistance to limit
voltage to the electronic control unit.
There are some electronic ignition systems that
do not require a ballast resistor. For instance, some control units directly
regulate the current flow through the primary of the coil. Hall-effect systems
do not require ballast resistors either. The signal voltage is not changed by
the speed of the distributor as it is in an inductive magnetic signal generating
Some systems can be enhanced with additional
sensors that increase the capabilities of the control module. The module can be
equipped with either a barometric pressure switch or vacuum switch. The
barometric pressure switch enables the module to retard the ignition timing 3 to
6 degrees when the vehicle is operating at low elevations. The vacuum switch
does the same, when the engine is under hard acceleration or heavy load. Other
modules have the ability to retard ignition timing during start-up or when
engine knock is detected.
As stated, early electronic ignition systems
changed the timing mechanically just like breaker point systems.
CENTRIFUGAL ADVANCE At idle, the firing
of the spark plug usually occurs just before the piston reaches top dead center.
At higher engine rpm, however, the spark must be delivered to the cylinder much
earlier in the cycle to achieve maximum power from air/fuel mixture since the
engine is moving through the cycle more quickly. To change the timing of the
spark in relation to rpm, the centrifugal advance mechanism is used.
This mechanism consists of a set of weights and
springs connected to the distributor shaft and a distributor armature assembly.
During idle speeds, the springs keep the weights in place and the armature and
distributor shaft rotate as one assembly. When speed increases, centrifugal
force causes the weights to slowly move out against the tension of the springs.
This allows the armature assembly to move ahead in relation to the distributor
shaft rotation. The ignition's triggering device is mounted to the armature
assembly. Therefore, as the assembly moves ahead, ignition timing becomes more
VACUUM ADVANCE During part-throttle
engine operation, high vacuum is present in the intake manifold. To get the most
power and the best fuel economy from the engine, the plugs must fire even
earlier during the compression stroke than is provided by a centrifugal advance
The heart of the vacuum advance mechanism is
the spring-loaded diaphragm, which fits inside a metal housing and connects to a
movable plate on which the pick-up coil is mounted. Vacuum is applied to one
side of the diaphragm in the housing chamber while the other side of the
diaphragm is open to the atmosphere. Any increase in vacuum allows atmospheric
pressure to push the diaphragm. In turn, this causes the movable plate to
rotate. the more vacuum present on one side of the diaphragm, the more
atmospheric pressure is able to cause a change in timing. The rotation of the
movable plate moves the pick-up coil so the armature develops a signal earlier.
These units are also equipped with a spring that retards the timing as vacuum
The distributor cap and rotor receive the high
voltage from the secondary winding via a high-tension wire. The voltage enters
the distributor cap through the coil tower, or center terminal. The rotor then
sends the voltage from the coil tower to the spark plug electrodes inside the
distributor cap. The rotor mounts on the upper portion of the distributor shaft
and rotates with it.
The distributor cap is made from silicone
plastic or similar material that offers protection from chemical attack. It is
attached to the distributor housing with screws or spring-loaded clips. The coil
tower contains a carbon insert that carries the voltage from the high-tension
coil lead to the raised portion of the electrode on the rotor. Spaced evenly
around the coil tower are the spark plug electrodes and towers for each spark
An air gap of a few thousandths of an inch
exists between the tip of the rotor electrode and the spark plug electrode
inside the cap. This cap is necessary to prevent the two electrodes from making
contact. If they did make contact, both would wear out rapidly. This cap cannot
be measured when the distributor is assembled; therefore, the gap is usually
described in terms of the voltage needed to create an arc between the