Electronic Ignition System Components
The components of an electronic ignition system
regardless of the manufacturer all perform the same
functions. Each manufacturer has it own preferred
terminology and location of the components. The basic
components of an electronic ignition system are as
TRIGGER WHEELThe trigger wheel, also
known as a reluctor, pole piece, or armature, is
connected to the upper end of the distributor
shaft. The trigger wheel replaces the distributor
cam. Like the distributor cam lobes, the teeth on
the trigger wheel equal the number of engine
PICKUP COILThe pickup coil, also known as
a sensor assembly, sensor coil, or magnetic
pickup assembly, produces tiny voltage surges
for the ignition systems electronic control unit.
The pickup coil is a small set of windings
forming a coil.
ELECRTONIC CONTROL UNIT AM-
PLIFIERThe ignition system electronic
control unit amplifier or control module is an
"electronic switch" that turns the ignition coil
primary current ON and OFF. The ECU
performs the same function as the contact points.
The ignition ECU is a network of transistors,
capacitors, resistors, and other electronic
components sealed in a metal or plastic housing.
The ECU can be located (1) in the engine
compartment, (2) on the side of the distributor,
(3) inside the distributor, or (4) under the vehicle
dash. ECU dwell time (number of degrees the
circuit conducts current to the ignition coil) is
designed into the electronic circuit of the ECU
and is NOT adjustable.
Electronic Ignition System Operation
With the engine running, the trigger wheel rotates
inside the distributor. As a tooth of the trigger wheel
passes the pickup coil, the magnetic field strengthens
around the pickup coil. This action changes the output
voltage or current flow through the coil. As a result, an
electrical surge is sent to the electronic control unit, as
the trigger wheel teeth pass the pickup coil.
The electronic control unit increases the electrical
surges into ON/OFF cycles for the ignition coil. When
the ECU is ON, current passes through the primary
windings of the ignition coil, thereby developing a
magnetic field. Then, when the trigger wheel and
pickup coil turn OFF the ECU, the magnetic field
inside the ignition coil collapses and fires a sparkplug.
Some electronic distributors have a magnetic
sensor using the Hall effect. When a steel shutter
moves between the two poles of a magnet, it cuts off
the magnetism between the two poles. The Hall-effect
distributor has a rotor with curved plates, called
shutters. These shutters are curved so they can pass
through the air gap between the two poles of the
magnetic sensor, as the rotor turns. Like the trigger
wheel, there are the same number of shutters as there
are engine cylinders.
Each time a shutter moves through the air gap
between the two poles of the magnetic sensor, it cuts
off the magnetic field between the poles. This action
provides a signal to the ECU. When a shutter is not in
the way, the magnetic sensor is producing voltage.
This voltage is signaling the ECU to allow current to
flow through the ignition coils primary winding.
However, when the shutter moves to cut off the
magnetic field, the signal voltage drops to zero. The
ECU then cuts off the current to the ignition coils
primary winding. The magnetic field collapses,
causing the coil secondary winding to produce a high
voltage surge. This high voltage surge is sent by the
rotor to the proper spark plug.
IGNITION TIMING DEVICES
Ignition timing refers to how early or late the spark
plugs fire in relation to the position of the engine
pistons. Ignition timing must vary with engine speed,
load, and temperature.
Timing advance happens when the spark plugs fire
sooner than the compression strokes of the engine. The
timing is set several degrees before top dead center
(TDC). More time advance is required at higher speeds
to give combustion enough time to develop pressure on
the power stroke.
Timing retard happens when the spark plugs fire
later on the compression strokes. This is the opposite of
timing advance. Spark retard is required at lower
speeds and under high load conditions. Timing retard
prevents the fuel from burning too much on the
compression stroke, which would cause spark knock or