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 follows:

TRIGGER WHEEL - The 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 cylinders.

PICKUP COIL - The 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 AMPLIFIER - The 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.

Hall-Effect Sensor

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 ping.