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 cool down 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 three ignition
coils, an ignition control module, a camshaft position sensor,
a dual Hall-effect crankshaft position sensor, an engine crankshaft
balancer with interrupter rings attached to the rear, 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 it (1-4, 2-5,
3-6). These two 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 its 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% more
voltage to fire a spark plug with reverse polarity, the ignition
coil design is improved, with saturation time and primary
current flow increased. This redesign of the system allows higher
secondary voltage to be available from the ignition coils--greater
than 40 kilovolts (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 one on exhaust.
It is possible for one spark plug to fire even though a plug wire from
the same coil may be disconnected from its companion plug. The disconnected
plug wire acts as one plate of a capacitor, with the engine
being the other plate. These two 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, both spark plugs may
then be misfiring.
24X and 7X Crankshaft Position Sensors/Harmonic Balancer Interrupter
Ring
The 24X crankshaft position
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 (refer to the 24X
Crankshaft Position Sensor graphic).
The 7X crankshaft position sensor uses a two wire connector at the
sensor and a three-way connector at the ignition control module.
The 24X crankshaft position sensor contains a Hall-effect switch.
The magnet and Hall-effect switch are separated by an air gap. A Hall-ffect
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,
with crankshaft rotation, either block the magnetic
field or allow it to reach the Hall-effect switch.
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 seven machined slots, six 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 ignition control module. The
ignition control module interprets the 7X on-off
signals as an indication of crankshaft position. The
ignition control module must have the 7X signal to fire
the correct ignition coil.
The 24X 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 camshaft position
sensor is located on the timing cover behind the water pump near the camshaft
sprocket. As the camshaft sprocket turns, a magnet
in it activates the Hall-effect switch in the camshaft
position sensor. When the Hall-effect switch is
activated, it grounds the signal line to the PCM, pulling
the camshaft position sensor signal circuit's applied voltage
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 ignition control module. Each coil
provides spark for two plugs simultaneously (waste
spark distribution). 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 performs the following functions:
| • | It determines the correct ignition coil firing sequence, based
on 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 proper
sequence. |
| • | It sends the 3X crankshaft reference (fuel control) signal to
the PCM. The PCM determines engine RPM from this signal. this signal is also
used by the PCM to determine crankshaft speed for
Ignition Control (IC) spark advance calculations. |
The 3X reference signal sent to the PCM by the ignition control module
is an on, off pulse occurring 3 times per crankshaft revolution.
Circuits Affecting Ignition Control
In order to properly control ignition timing, the PCM relies on the
following information:
| • | Engine load (manifold pressure or vacuum) |
| • | Atmospheric (barometric) pressure |
The Ignition Control (IC) system consists of the following components:
| • | Ignition control module |
| • | 7X crankshaft position sensor |
| • | 24X crankshaft position sensor |
| • | Powertrain control module |
The electronic Ignition Control Module (ICM) connector terminals are
identified as shown in the Electronic Ignition System graphic. These circuits
perform the following functions:
| • | 3X reference high--The 7X crankshaft position sensor sends
a signal to the electronic ignition control module which generates a reference
pulse that is sent to the PCM. The PCM uses this signal
to calculate crankshaft position and engine speed
(also used 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. |
| • | Ignition control bypass--During initial cranking, the PCM
will look for synchronizing pulses from the camshaft position sensor and
the 7X crankshaft position sensor. The pulses indicate
the position of the #1 piston and the #1 intake
valve. Five volts is 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 a DTC P1351 and the engine will
run at base ignition timing. A small amount of spark advance is
built into the ignition control module to enhance
performance. |
| • | Ignition Control (IC)--The PCM uses this circuit to trigger
the electronic ignition control module. The PCM uses the crankshaft reference
signal to calculate the amount of spark advance
needed. |
| • | 24X reference signal--The 24X crankshaft position sensor
increases idle quality and 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. |
| • | 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. |
| • | Ignition timing is not adjustable. There are no timing marks
on the crankshaft balancer or 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 its removal with
special tool J 38197. When 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 blades being bent. 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 ignition control module, it is not an electrical
connection to ground. |
| • | Be careful not to damage the secondary ignition wires or boots
when servicing the ignition system. Rotate each boot to dislodge it from the
plug or coil tower before pulling it 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 ignition control module is grounded to the engine block through
3 mounting studs used to secure the module to its mounting bracket. If servicing
is required, ensure that good electrical contact
is made between the module and its mounting bracket,
including 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 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 Ignition Control (IC) spark timing:
| • | Ignitions Control module (ICM). |
| • | Engine Coolant Temperature (ECT) sensor. |
| • | Intake Air Temperature (IAT) sensor. |
| • | Mass Air Flow (MAF) sensor. |
| • | Trans Range inputs from Transaxle Range switch. |
| • | Throttle Position (TP) sensor. |
| • | Vehicle Speed Sensor (VSS). |
Modes of Operation
Ignition Control (IC) spark timing is the PCM's method of controlling
spark advance and ignition dwell when the ignition system is operating
in the IC Mode. There are two modes of ignition
system operation:
In Bypass Mode, the ignition system operates independently of the PCM,
with 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 IC Mode (PCM controlled spark advance)
as soon as the engine begins cranking. After the switch
is made to IC Mode, it will stay in effect until one
of the following conditions occur:
| • | The engine is turned off. |
| • | The engine quits running. |
| • | A PCM/IC fault (DTC P1350 or DTC P1361) is detected. |
If a PCM/IC 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 ignition dwell time is
fully controlled by the PCM. IC spark advance and ignition dwell is calculated
by the PCM using the following inputs:
| • | Engine speed (24X reference or 3X reference). |
| • | Crankshaft position (24X reference or 3X reference and Camshaft
position PCM input signal). |
| • | Engine Coolant Temperature (ECT sensor). |
| • | Throttle Position (TP sensor). |
| • | Knock Signal (Knock sensor). |
| • | Park/Neutral Position (PRNDL input). |
| • | Vehicle Speed (Vehicle Speed Sensor). |
| • | PCM and ignition system supply voltage. |
The following describes the PCM to ignition control module circuits:
| • | 3X reference PCM input--From the ignition control module,
the PCM uses this signal to calculate engine RPM and crankshaft position.
The PCM compares pulses on this circuit to any that
are on the Reference Low circuit, ignoring any pulses
that appear on both. The PCM also uses the pulses
on this circuit to initiate injector pulses. If the PCM receives
no pulses on this circuit, the PCM will use the 24X
reference pulses to calculate RPM and crankshaft
position. The engine will continue to run and start
normally, but DTC P1374 will be set. |
| • | 24X reference PCM input--The 24X reference signal is used
to accurately control spark timing at low RPM and allow IC operation during
crank. Below 1200 RPM, the PCM is monitoring the
24X reference signal and using it as the reference
for ignition timing advance. When engine speed exceeds
1200 RPM, the PCM begins using the, 3X reference signal
to control spark timing. If the 24X reference signal is not
received by the PCM while the engine is running,
a DTC P0336 will be set and 3X reference will
be used to control spark advance under 1200 RPM, and
Bypass Mode will be in effect at under 400 RPM. The engine will
continue to run and start normally. |
| • | Reference low 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 ignition control module. Although
this circuit is electrically connected to the PCM,
it is not connected to ground at the PCM. The PCM
compares voltage pulses on the 3X or 24X reference input
to those on this circuit, ignoring pulses that appear on both.
If the circuit is open, or connected to ground at
the PCM, it may cause poor engine performance and
possibly a MIL (Service Engine Soon) with no DTC. |
| • | Bypass signal PCM output--The PCM either allows the ignition
control module to keep the spark advance at Bypass Mode 10 degrees
BTDC, or the PCM commands the ignition module to
allow the PCM to control the spark advance (IC Mode).
The ignition control module determines correct operating
mode based on the voltage level that the PCM sends to
the ignition control module on the bypass circuit. The PCM provides
5 volts on the bypass circuit if the PCM
is going to control spark timing (IC Mode). If the
PCM does not apply 5 volts to the bypass circuit, or
if the ignition control module doesn't sense the 5 volts,
the ignition control module will control spark timing
(Bypass Mode). An open or grounded bypass circuit
will set DTC P1361 and the ignition system
will stay at Bypass Mode advance. |
| • | Ignition Control (IC) PCM output--The IC output circuitry
of the PCM sends out timing pulses to the ignition control module on this
circuit. When in the Bypass Mode, the ignition control
module grounds these pulses. When in the IC Mode,
these pulses are the ignition timing pulses used
by the ignition control module to energize one of the ignition
coils. Proper sequencing of the 3 ignition coils, i.e.
which coil to fire, is always the job of the ignition
control module. If the IC circuit is grounded when
the engine is started, DTC P1361 will set and the ignition
system will stay in the Bypass Mode. If the IC circuit
becomes open or grounded during IC Mode operation,
DTC P1350 or P1361 may set. When this happens,
the engine will quit running but will restart. Upon restart
following an ignition cycle, DTC P1361 will be
set, and the ignition system will operate in Bypass Mode. |
| • | Knock Sensor (KS) PCM input--The KS system is comprised
of A knock sensor, and the PCM. The PCM monitors the knock sensor signal to
determine when engine detonation occurs. When the
knock sensor detects detonation, the PCM retards
the spark timing (IC) to reduce detonation. Retarded
timing can also be a result of excessive valve lifter, pushrod
or other mechanical engine or transaxle noise. |
| • | Camshaft Position PCM input (CAM signal)--The PCM uses this
signal to determine the position of the cylinder #1 piston during its
intake 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 24X and 3X reference pulses. 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 fault is present with a 1 in 6 chance of being correct. |
