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 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 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-A
. 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. |
| | Important: 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.
|
| • | 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. |
| • | 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
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,
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. |
