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 two 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 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 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 CKP sensor is a three wire sensor based on the magneto resistive principle.
A magneto resistive sensor uses two 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 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 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 PCM to determine the position of
the valve train relative to the crankshaft position. By monitoring the CMP and 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 that are part of 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 PCM controls each dual-tower ignition coil by transmitting timing pulses
on the IC control circuit to 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 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 PCM control. If
the CKP pulses are lost, the engine will not run. The loss of a 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.