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, 2 crankshaft
position sensors, 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.
The 24X crankshaft position sensor (1), secured in
an aluminum 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).
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 an aluminum 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 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, 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 7X crankshaft position sensor is the other Hall effect switch
closer to the crankshaft. 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 the Hall effect switch 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
To properly control ignition timing, the PCM relies on the following
information:
| • | Engine load (manifold pressure or vacuum). |
| • | Atmospheric (barometric) pressure. |
| • | Intake air temperature. |
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 (CKT 430)--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 P1350
and the engine will run at base ignition timing. A small amount of spark
advance is built into the ignition contro module to enhance performance. |
| • | Ignition Control (IC) (CKT 423)--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. |
Noteworthy Ignition Information
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 may 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:
Ignition Control (IC) Module.
Engine Coolant Temperature (ECT) sensor.
Intake Air Temperature (IAT) sensor.
Mass Air Flow (MAF) sensor (1).
Transaxle Range switch.
Throttle Position (TP) Sensor
Vehicle Speed Sensor (VSS).
Modes of Operation
The ignition system uses the same four ignition module to PCM circuits
as did previous Delco engine management systems using distributor type
ignition. Ignition Control (IC) spark timing is the PCMs 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) a 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, a KS module, 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. |
If the engine cranks but will not run or immediately stalls,
Engine Cranks But Will Not Run diagnostic table must be used to determine
if the failure is in the ignition system or the fuel system. If DTC P0300,
P0321, P0341, P0336, P0201-P0206, P1350 P1361 or P1374 is set, the
appropriate diagnostic trouble code chart must be used for diagnosis.
If a misfire is being experienced with no DTC set, refer to Symptoms
section for diagnosis.
