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.
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 percent 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.
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 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
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 (CKT 453)--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 (CKT 424)--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 control 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. |
| • | 24 X 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. |
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
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 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,
at a fixed spark timing. 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. |
The IC output circuitry in the PCM generates IC output pulses anytime
crankshaft reference signal input pulses are being received. When the ignition
system is operating in the Bypass Mode (no voltage on the bypass control
circuit), the ignition control module grounds the IC pulses coming from
the PCM. The ignition control module will remove the ground from the
IC circuit only after switching to the IC Mode. The PCM commands switching
to IC Mode by applying 5 volts on the bypass circuit to the ignition
control module. The PCM monitors the IC and Bypass circuits for electrical
malfunctions affecting proper ignition system operation. If a malfunction
occurs, diagnosis is included in DTC P1351, P1352, P1361 and P1362 diagnostic
tables. If diagnostic trouble codes are encountered, go to the DTC tables
for diagnosis.
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 ignition control module maintains
a fixed spark timing while the engine cranking (Bypass mode). Once the PCM
receives 3X reference pulses, 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). |
| • | 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 sent to the ignition control
module to control coil dwell and spark timing. Proper sequencing of the
3 ignition coils, i.e., which coil to fire, is always the job of the ignition
control module. |
| • | Knock Sensor (KS) PCM input--The PCM contains integrated
knock sensor (KS) diagnostic circuitry. The KS system is comprised of A knock
sensor, PCM, and related wiring. The PCM monitors the knock sensor signal
to detect engine detonation (spark knock). When the spark knock occurs,
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. |
