Electronic Ignition (EI) system. Description and operation.
The electronic ignition (EI) system consists of the following components
and wiring circuits:
| • | The ignition control (IC) module |
| • | The powertrain control (PCM) module |
| • | The crankshaft position (CKP) sensor |
| • | The camshaft position (CMP) sensor |
The EI wiring circuits listed here use service common names, they are
as follows: Between the IC module and the PCM.
| • | Medium resolution engine speed signal |
| • | Low resolution engine speed signal |
| • | Camshaft position signal |
Between the CKP sensor and the IC module:
Between the CMP and the IC module:
The CMP voltage and low reference circuits are shared with the CKP sensor.
The IC module is also supplied with the following:
Purpose
The EI system is responsible for producing and controlling a high energy
secondary spark. This spark is used to ignite the compressed air/fuel mixture
at precisely the correct time to provide optimal performance, fuel economy,
and control of exhaust emissions.
Operation
In this EI system there is one coil for each pair of cylinders. Each
pair is at top dead center (TDC) at the same time, and are known as companion
cylinders. The cylinder that is at TDC of the compression stroke is called
the event cylinder. The cylinder that is at TDC of the exhaust stroke is
called the waste cylinder. When the coil is triggered both plugs fire at
the same time, completing a series circuit. Because the lower pressure inside
the waste cylinder offers very little resistance the event cylinder uses
most of the available voltage producing a very high energy spark. This is
known as waste spark ignition.
EI component description
The following is a list of ignition system components and their functions.
Crankshaft position (CKP) sensor system
The 18x crankshaft position
(CKP) sensor contains two hall effect switches in one housing. A hall effect
switch is a solid state switching device, which produces an ON-OFF pulse
when a rotating element passes the sensor pick-up and interrupts the sensors
magnetic field.
The rotating element is called an interrupter ring or
blade. In this case there are two interrupter rings built into the crankshaft
balancer. The outer ring and outer switch provides the powertrain control
module (PCM) with the 18x signals, or 18 pulses per crankshaft revolution.
The inner ring and inner switch provide the PCM with a sync pulse three times
per crankshaft revolution. The ignition control (IC) module uses the 18x
and sync pulses to determine the engine position, by counting how many ON-OFF18x
pulses occur during a given sync pulse. Each of the 3 sync pulses represent
a pair of cylinders both at top dead center at the same time, which are called
companion cylinders. With this dual interrupter ring arrangement the IC module
can identify the correct pair of cylinders to fire within as little as 120 degrees
of crankshaft rotation.
Camshaft Position (CMP) Sensor
The camshaft position
(CMP) sensor is a hall effect sensor which is triggered by a magnet pressed
into the camshaft sprocket. The CMP sensor provides a signal pulse once every
camshaft revolution, known as the CMP sensor signal. The IC module uses this
signal to identify the position of the #1 cylinder at top dead center during
its power stroke.
Ignition coils and Ignition control (IC) module
Three dual tower ignition
coils are mounted to the IC module, and are serviced individually. The IC
module performs the following functions:
| • | The module supplies a power and low reference circuit to the CMP
and CKP sensors |
| • | The module recieves and processes the signals generated by the
CMP and CKP sensors |
| • | The module determines the correct coil triggering sequence, based
on how many 18x ON-OFF pulses occur during a sync pulse. This coil sequencing
occurs at start-up, and is remembered by the IC module. After the engine
is running, the module will continue to trigger the coils without the CKP
input. |
| • | The module determines the correct direction of the crankshaft
rotation, and cuts spark and fuel delivery to prevent damage from backfiring
if reverse rotation is detected. |
| • | The module sends 18x and 3x reference signals to the PCM |
The 3x reference signal is also known as the low resolution engine speed
signal. This signal is generated by the IC module using an internal divide-by-six
circuit. This circuit divides the 18x signal pulses by six. This divider
circuit will not begin operation without a sync pulse present at start-up,
and without 18x and 3x reference signals no fuel injection will occur.
Powertrain control (PCM) module
The PCM is responsible
for maintaining proper spark and fuel injection timing for all driving conditions.
Ignition control (IC) spark timing is the method the PCM uses to control
spark advance and ignition dwell. To provide optimum driveability and
emissions, the PCM monitors input signals from the following components
in calculating ignition control (IC) spark timing:
| • | The ignition control (IC) module |
| • | The engine coolant temperature (ECT) sensor |
| • | The intake air temperature (IAT) sensor |
| • | The mass air flow (MAF) sensor |
| • | The internal Mode or PNP inputs from Internal Mode switch or Park/Neutral
position switch |
| • | The throttle position (TP) sensor |
| • | The vehicle speed (VSS), or transmission output speed (TOSS) sensor |
The following describes the PCM to IC module circuits:
| • | Low resolution engine speed (3x reference)--PCM input --
From the ignition control module, the PCM uses this signal to calculate engine
RPM and crankshaft position above 1,200 RPM. The PCM also uses the
pulses on this circuit to initiate injector operation. |
| • | Medium resolution engine speed signal (18X reference)-- PCM
input -- The 18X reference signal is used to accurately control spark
timing at low RPM and allow IC operation during cranking. Below 1,200 RPM,
the PCM is monitoring the 18X reference signal and using the 18X reference
signal as the reference for ignition timing advance. When engine speed
exceeds 1,200 RPM, the PCM begins using the 3X reference signal
to control spark timing. |
| • | Camshaft position PCM input -- The PCM uses this signal to
determine the position of the cylinder #1 piston during its power stroke.
This signal is used by the PCM to calculate true Sequential Fuel Injection
(SFI) Mode of operation. The PCM compares the number of CAM pulses to
the number of 18X and 3X reference pulses. If the number of 18X and
3X reference pulses occurring between CAM pulses is incorrect, or if
no CAM pulses are received while the engine is running, the PCM will
set a DTC. If the cam signal is lost while the engine is running the
Fuel Injection System will shift to a calculated Sequential Fuel Injection
Mode based on the last CAM pulse, and the engine will continue to run.
The engine can be re-started and will run in the calculated Sequential
Mode as long as the condition is present with a 1 in 6 chance of
being correct. |
| • | Low reference -- PCM input -- this is a ground circuit
for the digital RPM counter inside the PCM, but the wire is connected to engine
ground only through the IC module. This circuit assures there is no ground
drop between the PCM and IC module. |
| • | IC timing signal --PCM output -- The IC module controls
spark timing while the engine is cranking, this is called bypass mode. Once
the PCM receives low resolution engine speed signals from the IC module,
the PCM applies 5 volts to the IC timing signal circuit allowing
the IC module to switch spark control to PCM control. |
| • | IC timing control-- PCM output -- The IC output circuitry
of the PCM sends out timing signals to the IC module on this circuit. When
in the Bypass Mode, the IC module grounds these signals. When in the IC
Mode, the signals are sent to the IC module to control coil dwell and spark
timing. Proper sequencing of the ignition coils is always controlled
by the IC module. |
Ignition System Modes of Operation
Anytime the PCM does not apply 5 volts to the IC timing signal
circuit, the IC module controls ignition by triggering each coil in
the proper sequence at a pre-determined dwell. This is called Bypass Mode
ignition used during cranking and running below a certain RPM, or during
a default mode due to a system failure.
When the PCM begins receiving 18x reference and 3X reference pulses,
the PCM applies 5 volts to the IC timing signal circuit. This
signals the IC module to allow the PCM to control the dwell and spark
timing. This is IC Mode ignition. During IC Mode, the PCM compensates
for all driving conditions. If the IC mode changes due to a system fault,
the IC will stay in default until the ignition is cycled OFF to ON, or the
fault is no longer present.
Ignition system service
Special care must be taken when diagnosing and handling EI systems.
The secondary voltage output can exceed 40,000 volts. Refer to the applicable
diagnosis or repair section for more information.
