The electronic ignition system controls fuel combustion
by providing a spark to ignite the compressed air/fuel mixture at the correct
time. To provide optimum engine performance, fuel economy, and control
of exhaust emissions, the PCM controls the spark advance of the
ignition system.
Electronic ignition has the following advantages over a mechanical
distributor system:
| • | Remote mounting capability |
| • | No mechanical load on the engine |
| • | More coil cooling time between firing events |
| • | Elimination of mechanical timing adjustments |
| • | Increased available ignition coil saturation time |
The electronic ignition system does not use the conventional
distributor and coil. The ignition system consists of 3 ignition coils,
an ignition control module (ICM), a camshaft position (CMP) sensor,
7X crankshaft position (CKP) sensor in the block, a 24X CKP sensor
behind the crankshaft balancer, the related connecting wires, and
the ignition control (IC) and fuel metering portion of the PCM.
Conventional ignition coils have one end of the secondary winding connected
to the engine ground. In this ignition system, neither end of the secondary
winding is grounded. Instead, each end of a coil's
secondary winding is attached to a spark plug.
Each cylinder is paired with the cylinder that
is opposite (1-4, 2-5, 3-6). These 2 plugs
are on companion cylinders, i.e., on top dead center,
at the same time.
When the coil discharges, both plugs fire at the same time to complete
the series circuit. The cylinder on compression is said to be the event cylinder
and the one on exhaust is the waste cylinder.
The cylinder on the exhaust stroke requires very
little of the available energy to fire the spark plug.
The remaining energy will be used as required by the
cylinder on the compression stroke. The same process is
repeated when the cylinders reverse roles. This
method of ignition is called a "waste spark"
ignition system.
Since the polarity of the ignition coil primary and secondary windings
is fixed, one spark plug always fires with normal polarity and the companion
plug fires with reverse polarity. This differs
from a conventional ignition system that fires
all the plugs with the same polarity. Because the ignition
coil requires approximately 30 percent more voltage
to fire a spark plug with reverse polarity, the
ignition coil design is improved, with the saturation
time and the primary current flow increased. This redesign
of the system allows a higher secondary voltage to
be available from the ignition coils -- more than
40,000 volts at any engine RPM. The voltage required
by each spark plug is determined by the polarity and
the cylinder pressure. The cylinder on compression
requires more voltage to fire the spark plug
than the cylinder on exhaust.
It is possible for one spark plug to fire even though a plug wire from
the same coil may be disconnected from the companion plug. The disconnected
plug wire acts as one plate of a capacitor,
with the engine being the other plate. These
2 capacitor plates are charged as a spark jumps
across the gap of the connected spark plug. The plates
are then discharged as the secondary energy is dissipated
in an oscillating current across the gap of the
spark plug that is still connected. Secondary voltage
requirements are very high with an open spark
plug or spark plug wire. The ignition coil has
enough reserve energy to fire the plug that is still connected
at idle, but the coil may not fire the spark plug under
high engine load. A more noticeable misfire
may be evident under load, as both spark plugs
may then be misfiring.
24X and 7X Crankshaft Position Sensors/Harmonic Balancer Interrupter
Ring
The 24X crankshaft position
(CKP) sensor (1), secured in a mounting bracket (3) and bolted
to the front side of the engine timing chain cover (2),
is partially behind the crankshaft balancer.
The 7X CKP sensor uses a 2-wire connector at the sensor and a
3-way connector at the ignition control module (ICM).
The 24X CKP sensor contains a Hall-effect switch. The magnet and Hall-effect
switch are separated by an air gap. A Hall-effect
switch reacts like a solid state switch, grounding
a low current signal voltage when a magnetic
field is present. When the magnetic field is
shielded from the switch by a piece of steel placed
in the air gap between the magnet and
the switch, the signal voltage is not grounded. If the
piece of steel, called an interrupter, is repeatedly
moved in and out of the air gap, the
signal voltage will appear to go on-off, on-off,
on-off. Compared to a conventional mechanical
distributor, this on-off signal is similar
to the signal that a set of breaker points in the
distributor would generate as the distributor
shaft turned and the points opened and
closed.
In the case of the electronic ignition system, the piece of steel is
a concentric interrupter ring mounted to the rear of the crankshaft balancer.
The interrupter ring has blades and
windows that either block the magnetic
field or allow the field to reach the Hall-effect
switch with the crankshaft rotation. The Hall-effect
switch is called a 24X crankshaft position
sensor, because the interrupter ring has 24 evenly
spaced blades and windows. The 24X
crankshaft position sensor produces 24 on-off pulses
per crankshaft revolution.
The interrupter ring is a special wheel cast on the crankshaft that
has 7 machined slots, 6 of which are equally spaced 60 degrees apart.
The seventh slot is spaced 10 degrees
from one of the other slots. As the
interrupter ring rotates with the crankshaft, the slots
change the magnetic field. This will
cause the 7X to ground the 3X signal voltage
that is supplied by the ICM. The ICM interprets the
7X on-off signals as an indication of crankshaft
position. The ICM must have the 7X signal to fire
the correct ignition coil.
The 7X interrupter ring and Hall-effect switch react similarly. The
24X signal is used for better resolution at a calibrated RPM.
Camshaft Position (CMP) Sensor
The CMP sensor is located
on the timing cover behind the water pump near the camshaft sprocket.
As the camshaft sprocket turns, a magnet in the sprocket activates the
Hall-effect switch in the CMP sensor. When the Hall-effect
switch is activated, the switch grounds the signal line to the
PCM, pulling the applied voltage of the CMP sensor signal circuit
low. This is interpreted as a CAM signal.
The CAM signal is created as piston #1 is on the intake stroke. If
the correct CAM signal is not received by the PCM, DTC P0341 will
be set.
Ignition Coils
Three twin-tower ignition coils are individually mounted to the ICM.
Each coil provides spark for 2 spark plugs
simultaneously. Each coil is serviced separately.
Two terminals connect each coil pack
to the module. Each coil is provided a fused ignition
feed. The other terminal at each coil
is individually connected to the module, which
will energize one coil at a time by completing and interrupting
the primary circuit ground path to
each coil at the proper time.
Ignition Control Module (ICM)
The ignition control
module (ICM) performs the following functions:
| • | Determines the correct ignition coil firing sequence based on
the 7X pulses. This coil sequencing occurs at start-up. After the engine is
running, the module determines the
sequence and continues triggering the ignition coils in the
proper sequence. |
| • | Sends the 3X crankshaft reference signal to the PCM. The PCM
determines the engine RPM from this signal. This signal is also
used by the PCM to determine the crankshaft
speed for the ignition control (IC) spark
advance calculations. |
The 3X reference signal sent to the PCM by the ICM is an on-off pulse
occurring 3 times per crankshaft revolution.
Circuits Affecting Ignition Control
In order to properly control the ignition timing, the PCM relies on
the following information:
| • | The engine load (manifold pressure or vacuum) |
| • | The barometric pressure |
| • | The intake air temperature |
| • | The crankshaft position |
The ignition control (IC) system consists of the following components:
| • | The ignition control module |
| • | The 7X crankshaft position sensor |
| • | The 24X crankshaft position sensor |
| • | The powertrain control module |
| • | All of the connecting wires |
The electronic ICM connector terminals are identified as shown in the
Electronic Ignition System graphic. These circuits
perform the following functions:
| • | The 3X reference high - the 7X crankshaft position sensor
sends a signal to the ICM which generates a reference
pulse that is sent to the PCM. The PCM uses this
signal to calculate the crankshaft position
and the engine speed, and to trigger the fuel injectors. |
| • | 3X reference low - this wire is grounded through the ICM
and assures the ground circuit has no voltage drop between the ICM and
the PCM. |
| • | The ignition control bypass - during initial cranking,
the PCM will look for synchronizing pulses from the CMP sensor and
the 7X CKP sensor. The pulses indicate
the position of the #1 piston and the
#1 intake valve. Five volts are applied to the bypass
circuit at precisely the same time these
signals are received by the PCM. This
generally occurs within one or two revolutions of the crankshaft.
An open or grounded bypass circuit
will set DTC P1351 and the engine
will run at base ignition timing. A small amount of
spark advance is built into the ICM in order
to enhance performance. |
| • | The ignition control (IC) - the PCM uses this circuit to
trigger the ICM. The PCM uses the crankshaft reference
signal to calculate the amount of spark advance
needed. |
| • | The 24X reference signal - the 24X CKP sensor increases
the idle quality and the low speed driveability by providing better resolution
at a calibrated RPM. |
Ignition System
There are important considerations to point out when servicing the
ignition system. The following Noteworthy Information will list some of these,
to help the technician in servicing
the ignition system.
| • | The ignition coils' secondary voltage output capabilities are
very high--more than 40,000 volts. Avoid body contact with ignition
high voltage secondary components when
the engine is running or personal injury
may result. |
| • | The 7X crankshaft position sensor is the most critical part of
the ignition system. If the sensor is damaged so that pulses are not generated,
the engine will not start. |
| • | The crankshaft position sensor clearance is very important! The
sensor must NOT contact the rotating interrupter ring at any time or sensor
damage will result. If the balancer
interrupter ring is bent, the interrupter
ring blades will destroy the sensor. |
| • | The ignition timing is not adjustable. There are no timing marks
on the crankshaft balancer or the timing chain cover. |
| • | If crankshaft position sensor replacement is necessary, the crankshaft
balancer must be removed first. The balancer is a press-fit onto the crankshaft.
Removing the serpentine accessory drive belt and balancer
attaching bolt will allow balancer removal
with the J 38197-A
balancer
remover. When the balancer is reinstalled, proper
torquing of the balancer attachment bolt is critical to
ensure the balancer stays attached
to the crankshaft. |
| • | If a crankshaft position sensor assembly is replaced, check the
crankshaft balancer interrupter ring for any bent blades. If this is
not checked closely and a bent blade exists,
the new crankshaft position sensor
can be destroyed by the bent blade with only one crankshaft
revolution. |
| • | Neither side of the ignition coil primary or secondary windings
is connected to engine ground. Although the ignition coil packs are secured
to the ICM, it is not an electrical
connection to ground. |
| • | Be careful not to damage the secondary ignition wires or the
boots when servicing the ignition system. Rotate each boot to dislodge the
boot from the plug or coil tower before pulling a boot from either a spark
plug or the ignition coil. Never pierce a secondary ignition wire or boot
for any testing purposes! Future problems are guaranteed
if pinpoints or test lights are pushed through
the insulation for testing. |
| • | The ICM is grounded to the engine block through 3 mounting studs
used to secure the module to the mounting bracket. If service
is required, ensure that good electrical contact
is made between the module and the
mounting bracket, including the proper hardware and torque. |
| • | A conventional tachometer used to check RPM on a primary ignition
tach lead will not work on this ignition system. In order to check the RPM,
use a scan tool. |
Powertrain Control Module (PCM)
The PCM is responsible
for maintaining proper spark and fuel injection timing for all driving conditions.
To provide optimum driveability and
emissions, the PCM monitors input signals
from the following components in calculating the ignition
control (IC) spark timing:
| • | The ignition control module (ICM) |
| • | The engine coolant temperature (ECT) sensor |
| • | The intake air temperature (IAT) sensor |
| • | The mass air flow (MAF) sensor |
| • | The transaxle range inputs from transaxle range switch |
| • | The throttle position (TP) sensor |
| • | The vehicle speed sensor (VSS) |
Modes of Operation
The ignition control (IC) spark timing is the PCM method of controlling
spark advance and ignition dwell when the ignition system is operating
in the IC Mode. There are 2 modes
of ignition system operation:
In Bypass mode, the ignition system operates independently of the PCM,
with the Bypass mode spark advance always at 10 (BTDC). The PCM has no control
of the ignition system when in this
mode. In fact, the PCM could be disconnected
from the vehicle and the ignition system would still
fire the spark plugs, as long as the
other ignition system components were functioning.
(This would provide spark but no fuel injector pulses.
The engine will not start in this situation.)
The PCM switches to the PCM controlled IC
mode of spark advance as soon as the engine begins
cranking. After the switch is made, IC mode will
stay in effect until one of the following
conditions occur:
| • | The engine is turned OFF. |
| • | The engine quits running. |
| • | A PCM/IC module fault (DTC P1351, P1352, P1361, or P1362)
is detected. |
If a PCM/IC module fault is detected while the engine is running, the
ignition system will switch to Bypass mode operation. The engine may quit
running, but will restart and stay
in Bypass mode with a noticeable loss of performance.
In the IC mode, the ignition spark timing and the ignition dwell time
are fully controlled by the PCM. IC spark advance and ignition dwell is calculated
by the PCM using the following inputs:
| • | The crankshaft position |
| • | The engine coolant temperature (ECT) |
| • | The throttle position (TP) |
| • | The Park/Neutral position input |
| • | The PCM and ignition system supply voltage |
The following describes the PCM-to-ICM circuits:
| • | The 3X reference PCM input. The ICM generates the 3X reference
signal from the 7X CKP sensor. The ICM sends the 3X reference signal
to the PCM. The PCM uses this signal to calculate the engine RPM
and the crankshaft position at speeds of more than 1600 RPM.
If the PCM receives no pulses on this circuit,
the PCM will use the 24X reference pulses to calculate the RPM and
the crankshaft position. The engine will continue to run
and start normally as long as 7X CKP sensor pulses are being received,
but DTC P1374 will be set. |
| • | The 24X reference PCM input. The 24X CKP sensor generates the
24X reference signal to calculate engine speed and crankshaft position at
engine speeds of less than 1600 RPM. The 24X reference signal
provides better resolution within the calibrated RPM range. This
increases the idle quality and low speed driveability. When the
engine speed exceeds 1600 RPM, the PCM begins using the
3X reference signal to control the spark timing. If the 24X reference
signal is not received by the PCM while the engine is running,
DTC P0336 will set, the 3X reference will be used to control
the spark advance under 1600 RPM, and Bypass mode will be
in effect under 400 RPM. The engine will continue to run
and start normally. |
| • | The reference low PCM input. The reference low circuit establishes
a common ground between the ICM and the PCM. The wire is connected
to engine ground only through the ICM. The circuit minimizes any
electrical ground differences between the PCM and the ICM. The PCM
uses the reference low circuit to clearly recognize the 3X reference
signals. If the circuit is open or connected to ground at the PCM,
poor engine performance and a MIL with no DTC set may result. |
| • | The knock sensor (KS) PCM input. The PCM contains integrated
KS diagnostic circuitry. The KS system is comprised of the knock
sensor, the PCM, and the related wiring. The PCM monitors the KS signal
to detect engine detonation. When spark knock occurs, the PCM retards
the spark timing in order to reduce detonation. A retarded spark
timing may also be the result of excessive engine mechanical or
transaxle noise. If a KS signal is found varying within the average
voltage, DTC P0327 may set. |
| • | The bypass signal PCM output. The ICM controls the spark timing
until the PCM detects a calibrated number of 3X reference pulses.
When the PCM receives these pulses, the PCM then provides 5.0 volts
to the ICM on the Bypass circuit. This signals the ICM to transfer
the spark timing control to the PCM. Proper sequencing of the
3 ignition coils, i.e. which coil to fire, is always the job of the
ICM. If the PCM detects a short to voltage on the Bypass circuit DTC P1362
will set. An open in the Bypass circuit will set DTC P1352. |
| • | The ignition control (IC) PCM output. The PCM sends out timing
pulses to the ICM on the IC circuit. When the ignition system is in
the Bypass mode and the PCM has not sent the 5.0 volt bypass
signal, the ICM grounds these pulses. When the system is in IC mode
and the PCM has supplied the bypass signal, these pulses are sent
to the ICM to control the ignition spark timing. If the IC circuit
is grounded when the engine is started, DTC P1361 will
set and the ignition system will remain in the Bypass mode. If the
IC circuit becomes open or grounded during IC mode operation, DTC P1351
or DTC P1361 may set. When this happens, the engine will
quit running but will restart. Upon restart following an ignition
cycle, DTC P1361 will set and the ignition system will operate
in Bypass mode. |
