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.
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 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. |
| • | 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 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:
Ignition Control Module (ICM)
Engine Coolant Temperature (ECT) Sensor
Intake Air Temperature (IAT) Sensor
Mass Air Flow (MAF) Sensor
Trans Range Inputs from Transaxle 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) 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, 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. |
How DTCs P1350/P1361 are Set
The IC output circuitry in the PCM issues 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 its own IC output, and expects to see no pulses on the IC circuit
when it has not yet applied 5 volts on the bypass control circuit.
When the second 3X reference pulse at the start of crank is seen by the PCM,
it applies 5 volts to the bypass control circuit and the IC pulses
should no longer be grounded by the ignition control module. The PCM constantly
monitors its IC output, and should detect the IC pulses only when commanding
the IC Mode.
If the IC circuit is open, the PCM will detect IC output pulses while
attempting to start the engine (in the Bypass Mode) due to the ignition control
module not being able to ground the IC pulses. Three things will occur:
| • | The PCM will not apply 5 volts to the bypass circuit. |
| • | The engine will start and run in Bypass Mode. |
If IC circuit is grounded, the PCM would not detect a problem until
the change to IC Mode is commanded by the PCM. When the PCM applies 5 volts
to the bypass control circuit, the ignition control module will switch to
IC Mode. With the IC circuit grounded, there would be no IC pulses for the
ignition control module to trigger the ignition coils, and the engine may
falter. The PCM will quickly revert back to Bypass Mode (turn Off the 5 volts
on the bypass circuit), DTC P1361 will set, and the ignition system will operate
in Bypass Mode until the fault is corrected and the engine is stopped and
restarted.
If bypass circuit is open or grounded, the ignition control module
cannot not switch to IC Mode. In this case, the IC pulses will stay grounded
in the ignition control module, and DTC P1361 will be set. The engine will
start and run in Bypass Mode.
Results of Incorrect Operation
An open or ground in the IC or bypass circuit will set DTC P1350 or
P1361. If a fault occurs in the IC output circuit when the engine is running,
the engine may falter or quit running but will restart and run in the Bypass
Mode once the ignition has been cycled. A fault in either circuit will force
the ignition system to operate on Bypass Mode timing (10 degrees BTDC),
which will result in reduced performance and fuel economy.
The PCM uses information from the engine coolant temperature sensor
in addition to RPM to calculate spark advance values as follows:
| • | High RPM = more advance |
| • | Cold engine = more advance |
| • | Hot engine = less advance |
Therefore, detonation could be caused by high resistance in the engine
coolant temperature sensor circuit. Poor performance could be caused by low
resistance in the engine coolant temperature sensor circuit.
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 P1200, 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.
