Electronic Ignition (EI) System Operation
The electronic ignition (EI) system produces and controls the high energy secondary
spark. This spark ignites the compressed air/fuel mixture at precisely the correct
time, providing optimal performance, fuel economy, and control of exhaust emissions.
The powertrain control module (PCM) primarily collects information from the crankshaft
position (CKP) and camshaft position (CMP) sensors to control the sequence, dwell,
and timing of the spark.
Crankshaft Reluctor Wheel
The crankshaft reluctor wheel is mounted in the middle of the crankshaft. The
wheel is comprised of three 120-degree segments. Each segment represents a pair of
cylinders at top dead center (TDC), and is further divided into six 20-degree segments.
Within each 20-degree segment is a notch of 2 different sizes. Each 120-degree
segment has a unique pattern of notches. This is known as pulse width encoding. This
pulse width encoding pattern allows the powertrain control module (PCM) to quickly
recognize which pair of cylinders are at TDC. The reluctor wheel is also a dual track,
or mirror image, design. This means there is an additional wheel pressed against the
first wheel with a gap of equal size to each notch of the mating wheel. When one sensing
element of the crankshaft position (CKP) sensor is reading a notch, the other is reading
a set of teeth. The resulting signals are then converted into a digital square wave
output by the circuitry within the CKP sensor.
Crankshaft Position (CKP) Sensor
The crankshaft position (CKP) sensor is a 3-wire sensor based on the magneto
resistive principle. A magneto resistive sensor uses 2 magnetic pickups between
a permanent magnet. As an element such as a reluctor wheel passes the magnets, the
resulting change in the magnetic field is used by the sensor electronics to produce
a digital output pulse. The CKP sensor returns a digital ON/OFF pulse 24 times
per crankshaft revolution. The pulse width encoding pattern is used to synchronize
the coil firing sequence with the crankshaft position. The CKP sensor is used for
ignition timing, fuel injector timing, misfire diagnostics, and tachometer display.
The powertrain control module (PCM) supplies a 12-volt reference, a low reference,
and a medium resolution engine speed signal circuit to the CKP sensor.
Camshaft Position (CMP) Sensor
The camshaft position (CMP) sensor signal is a digital ON/OFF pulse output once
per revolution of the camshaft. The CMP sensor does not directly affect the operation
of the ignition system. The CMP sensor information is used by the powertrain control
module (PCM) to determine the position of the valve train relative to the crankshaft
position. By monitoring the CMP and crankshaft position (CKP) signals, the PCM can
accurately time the operation of the fuel injectors. The PCM supplies the sensor with
a 12-volt reference circuit, a low reference circuit, and a signal circuit.
Ignition Control Module (ICM)/Coils
There are 3 dual-tower ignition coils mounted to the ignition control module
(ICM). The ICM contains coil driver circuits that command the coils to operate. The
ICM has the following circuits:
• | An ignition voltage circuit |
• | An IC 1 control circuit for the 1-4 ignition coil |
• | An IC 2 control circuit for the 2-5 ignition coil |
• | An IC 3 control circuit for the 3-6 ignition coil |
• | A low reference circuit |
The powertrain control module (PCM) controls each dual-tower ignition coil by
transmitting timing pulses on the IC control circuit of the ICM for the proper coil
to enable a spark event.
The spark plugs are connected to each coil tower by spark plug wires. The spark
plug wires conduct the spark energy from the coil to the spark plug. The spark plug
electrode is tipped with platinum for long wear and higher efficiency.
Powertrain Control Module (PCM)
The powertrain control module (PCM) controls all ignition system functions,
and constantly corrects the basic spark timing. The PCM monitors information from
various sensor inputs that include the following:
• | The throttle position (TP) sensor |
• | The engine coolant temperature (ECT) sensor |
• | The mass airflow (MAF) sensor |
• | The intake air temperature (IAT) sensor |
• | The vehicle speed sensor (VSS) |
• | The transmission gear position or range information sensors |
• | The engine knock sensor (KS) |
Modes of Operation
There is one normal mode of operation with the spark under powertrain control
module (PCM) control. If the crankshaft position (CKP) pulses are lost, the engine
will not run. The loss of a camshaft position (CMP) signal may result in a longer
crank time since the PCM cannot determine which stroke the pistons are on. Diagnostic
trouble codes are available to accurately diagnose the ignition system with a scan
tool.