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Ignition Systems
Lesson 1
Electronic Ignition Systems
Successful completion of this unit’s learning objectives will allow you to meet the
Integrated Curriculum Standards (ICS) and Mathematical Content Expectations
for the National Council of Teachers of Mathematics (NCTM) and National
Automotive Technicians Education Foundation (NATEF).
Successful completion of this lesson’s learning
objectives (technical competencies) will allow you
to meet the Integrated Curriculum Standards (ICS)
listed in the right margin.
163 Engine Performance
Learning Objectives
Upon completion and review of this lesson, you will be able to:
¾ Explain how distributor and distributorless ignition systems generate
and time an ignition spark through the primary and secondary circuits.
¾ Explain the primary and secondary ignition circuits and waveforms
they generate.
¾ Explain the operation of triggering devices (sensors), such as, PickUp Coils, Hall effect switches, Permanent Magnet (PM) generators,
Optical Pick-ups, and Magneto Resistive (MR).
Key Terms
• Camshaft Position (CMP) Sensor
• Crankshaft Position (CKP)
• Distributor Ignition
• Distributorless Ignition (DI)
• Dwell
• Electronic Ignition (EI)
• Flux Density
• Hall effect Switch
• Ignition Coil
• Magnetic Flux
Mutual Inductance
Primary circuit
Secondary circuit
Spark Duration
Spark Line
Spark Timing
Switching Device
Triggering Device
Waste Spark
Ignition Systems
Ignition System
The ignition system in a spark-ignited gasoline fueled engine provides an electrical
spark of enough intensity to ignite the air-fuel mixture within the combustion
chamber. This boils down to three main functions:
Spark generation: Generate an electrical spark that has enough heat to
ignite the air/ fuel mixture in the combustion chamber.
Spark duration: Maintain the spark long enough to allow for the combustion
of all the air and fuel in the cylinders.
Spark timing: Deliver that spark to each cylinder at the right time during the
compression stroke of each cylinder. Spark timing is the point at which the
spark plug is actually fired. Timing is measured in degrees of crankshaft
rotation relative to Top Dead Center (TDC), or the point at which the piston
reaches its full upward travel position.
All ignition systems are divided into three interconnected electrical sections: a
primary circuit (low voltage), which includes a triggering circuit, and a
secondary (high voltage) circuit (Figure 5-1).
Figure 5-1 Basic Ignition System Schematic
Ignition Systems
Primary Circuit
The following items shown in Figure 5-2 comprise the primary circuit:
Battery: The battery is the source of electrical voltage. The positive side is
connected to the ignition switch through a wiring harness with the ground
connected to engine block and frame.
Ignition Switch The ignition switch is the driver’s on/off control that
switches on voltage to the primary circuit.
Ignition Coil Primary Winding: This is a type of step-up transformer that
takes battery voltage and steps it up from 75 to 200 volts.
Switching device: This component provides the on/off signal for the ignition
coil primary windings. Prior to 1975, OEMs used switches called ignition
points to open and close the primary circuit. Currently, this switching
function is done electronically through various means, such as a NPN
transistor inside an electronic ignition module.
Triggering Devices: this signal device provides an input to the Ignition
Module (IM), Powertrain Control Module (PCM), or Electronic Control
Module (ECM) to control the switching on/off of the coil primary winding.
There are five categories of triggering devices: Permanent Magnet (PM),
Generators (AKA Variable Reluctance Devices), Hall effect switches, Optical
Pick-ups, and Magneto Resistive (MR). This lesson will examine their use in
the various systems. These triggering devices also provide crankshaft
position and RPM information to the ECM/PCM.
Switching Circuit/Control Module: All electronically controlled ignition
(distributor and distributorless) use a transistor type switching circuit to turn
the primary circuit (coil ground) on and off, which replaced the breaker
points. Most OEMs use the ECM/PCM to provide precise control of the
ignition spark.
Ignition Systems
Figure 5-2 Ignition System-Primary Circuit
When the driver turns on the ignition switch and cranks the engine, a primary
current of .000005 amps (5 micro-amps) is directed to the primary winding of the
ignition coil. This primary circuit is completed to ground through a switching device
(ignition module transistor or breaker points before 1975) that is controlled by a
triggering device. Triggering devices can consist of the breaker points and
distributor cam (prior to 1975), or their electronic counterparts, the electronic pickup coil (PM generator, Hall effect switch, etc.) Switching devices are transistors in
an electronic ignition module. Ignition breaker points prior to 1975 are both a
triggering device and a switching device because no electronics were involved in
the process.
When the points are closed or the transistor is on, current flows through the primary
coil. The total time of this current flow is also called the dwell time or period. This
time is needed to recharge the coil every time it is discharged. This current creates
a strong magnetic field around the primary winding. When the points open or the
switching device interrupts the current flow, the magnetic field in the primary
winding collapses. This causes the magnetic field to move toward the core. As the
magnetic field (flux) moves toward the core, it passes through the secondary coil
windings, which in turn induces a high voltage. The high voltage in the secondary
coil seeks a path of opposite polarity with a greater potential difference.
Ignition Systems
The primary coil (Figure 5-3) is within a secondary coil that contains 200 times the
number of turns of wire. This collapse of the primary winding induces the lines of
electromagnetic force to move across the secondary coil windings, generating a
higher voltage of about 40,000 volts to jump the gap in the spark plug.
Figure 5-3 Primary Coil
When current flows through a conductor, it immediately reaches a maximum value
set by circuit resistance. If this wire is wound into a coil, the maximum current is not
immediately present. A magnetic field forms as the current begins to flow. The
magnetic flux (lines of force) of one section of the winding passes over another
section of the winding and tends to cause an opposition to the current flow that
produced the magnetic flux (Figure 5-3). This opposition is called inductive
reactance or counter Electro-Motive Force (EMF). Reactance causes a temporary
resistance to current flow and keeps the flow of current from reaching it maximum
value for a period of time.
Ignition Systems
Coil Saturation
When maximum current flow is present in a winding, a maximum magnetic field is
present and the coil winding is considered saturated. Saturation (Figure 5-4) of the
primary windings only occurs if the ignition primary switching device provides a
ground path long enough to allow maximum current flow (5.5 amps maximum).
Figure 5-4 Coil Saturation
The duration of primary current flow is called dwell. Dwell is measured in degrees
of distributor rotation. One complete turn of the distributor shaft equals 360° and
represents one firing of all of the cylinders of the engine. If the engine is a
4-cylinder, every 90° of distributor rotation represents the amount of time available
for saturation of the primary coil and the firing of one cylinder. During this 90°
period, the module provides a ground for enough time to saturate the coil and
opens long enough for the coil to fully collapse. The time needed for coil saturation
is approximately .010 seconds or 10 milliseconds. Dwell needs to be long enough
for the current to reach approximately 3.0 amps. By keeping the coil primary
winding resistance around 0.5-1.0 ohm, saturation is reached more quickly. As the
engine RPM increases, the time available for dwell decreases, causing the
secondary output to decrease. Reading dwell on an ignition scope can show a
variance of up to 2° between cylinders.
Ignition Systems
Checking Available Primary Voltage
The available battery voltage across the primary circuit is approximately 10 to 11
volts at 70° F. In electronic ignition, the dwell increases as the available voltage
To check the available primary voltage, observe the following procedure:
1. Use a Digital Multimeter (DMM) to check the available battery voltage across
the main battery terminals while cranking.
2. Measure the voltage at the ignition module input terminal (HEI Distributor
Battery terminal) from the terminal to ground at the distributor or module
housing. You should not measure a difference greater than 1.0 volt. This
voltage difference is the voltage drop across the circuit.
3. If the drop is greater than 1.0 volt, attach the negative lead to the ignition
module input terminal.
4. Attach the DMM positive lead to the B+ battery terminal, crank the engine,
and note the voltage drop.
5. If the drop is now less, there is resistance in the switched ignition feed from
the ignition switch to the ignition module or distributor.
The pattern in Figure 5-5 is taken from the negative side of the primary ignition coil.
Any upward movement indicates positive polarity on the primary pattern select.
Read the pattern from left to right.
Figure 5-5 Primary Ignition Pattern
Ignition Systems
The vertical axis represents system voltage at approximately 14.2 volts. Position 2
represents 0 volts. The vertical line traced indicates extremely rapid switching. Note
the primary ignition coil, which just went from 14.2 volts to 0 volts at position 2. The
sudden drop in voltage is the result of the triggering device (pole piece) generating
250mV, and turning the ignition module transistor on.
With the transistor on, the tachometer terminal (TACH) goes to ground, beginning
the dwell period. Dwell begins from position 2. The TACH remains grounded until
the current limiter begins to add some resistance to the primary coil circuit. As a
result, the voltage starts to rise at position 3. The rise and fall between positions 3
and 4 is the actual voltage drop across the current limiter to maintain 3.0-5.5 amps.
This event may also be identified as controlled coil saturation. At position 5, the
primary current is once again stabilized, and the module transistor remains
turned on, approaching the end of the dwell period as the trigger timer core teeth
are directly aligned with the pole piece teeth.
Position 6 is the result of pole piece reverse polarity. The transistor has turned off
and the ground path for the primary ignition coil has opened. The straight vertical
rise from position 6 is now displayed over to the extreme left side of the scope,
where the electron gun starts to repeat the trace. The trace begins here only
because the scope triggers off the rapid rise of the primary voltage. The voltage
rise is so rapid that the trace appears as a straight vertical line. If the scope was
switched to a 5 millisecond time-base, then the trace would appear slightly angled.
Ignition Systems
Secondary Circuit
Secondary Circuit Components
Refer to Figure 5-6 for the visual representation of the following components:
Ignition Coil Secondary Winding: A step-up transformer to boost voltage
to the higher values necessary to jump the spark plug gap. High voltage
spark from the coil secondary winding goes via the distributor cap and rotor
to the spark plug cables and on to the spark plug or directly to the spark
plug in the case of direct ignition types.
Spark Plugs: This device one for each cylinder is screwed into the
combustion chamber. It contains a gap between the electrodes that an arc
occurs as the high secondary voltage tries to bridge it.
Distributor Cap and Rotor: Spark from the coil secondary winding is
distributed to the multiple cylinders via the rotor and onto the spark plug
cables via the distributor cap.
Spark Plug Cables: These high voltage carbon filled wires usually carry
the high voltage current to the spark plugs.
Figure 5-6 Secondary Circuit Components
Ignition Systems
Secondary Circuit Magnetism Definitions
Magnetic Flux: The lines of magnetic force crossing or cutting across a
conductor. The number of these lines passing perpendicularly through a
square centimeter is known as flux density.
Mutual Inductance: A voltage induced in one coil because of a changing
current in another coil. When the current is increasing in one loop the
expanding magnetic field will cut across some or all of the neighboring coil
loops and induce a voltage in these loops.
Self-Inductance: The inducing of a voltage in a current carrying wire when
the current in the wire itself is changing.
Ignition Coil
The ignition coil is one of the major components of the secondary circuit. It is
responsible for providing the required voltage to overcome the total secondary
resistance. The cap and rotor in Distributor Ignition (DI) systems transfer the
required voltage to the correct cylinder in the firing order. The spark plug wires and
the spark plugs complete the secondary circuit to the engine block. When the
primary coil has current flowing through the windings, the magnetic flux passes
through the secondary windings. This occurs when the module provides a ground
to the primary ignition coil. When the module transistor opens the ground path
(open collector-emitter circuit), the magnetic flux collapses toward the soft iron
core. As the magnetic flux (lines of force) passes through the secondary windings,
a voltage is induced. This is referred to as mutual induction. Mutual induction
cannot exist without a magnetic flux that changes in intensity. This flux is created
by either an increase or a decrease in current flow, or a magnetic field increase or
Ignition Systems
In Figure 5-7, the arrows indicate current flow (conventional theory) through the
primary windings of the ignition coil from positive to negative. When the ignition
module-switching transistor is on, the current increases from zero up to a limiting
3.0-5.5 amps. The coil induces a voltage as the current changes. The polarity of
the induced voltage opposes the change in current that produced it. Since the coil
itself becomes a source of voltage, the polarity of the induced voltage is positive at
point X and negative at point Y. With this polarity, the coil, as a source of voltage,
tends to send current out of point X, which is in direct opposition to the increasing
current supplied by the battery.
Figure 5-7 Secondary Circuit Components
The principle of self-induction is defined as the inducing of a voltage in a
current-carrying wire, when the current in the wire itself is changing. Since the
current creates a magnetic field consisting of concentric circles (flux) around the
wire which expand and contract as the current increases and decreases, the
magnetic flux (lines of force) cuts across the conductor and thereby induces a
voltage in the conductor. This principle of self-induction grounds the primary side
and prevents the current from immediately reaching 5.5 amps when the ignition
module is turned on.
Ignition Systems
In Figure 5-8, the transistor (switch) has opened (turned off). The coil current now
decreases from 5.5 amps to zero. This changing current induces a voltage in the
coil that attempts to keep the current flowing at 5.5 amps. The polarity of the
induced voltage reverses as the current decreases. The amount of voltage induced
is dependent on the number of coil windings, size of the windings, material of the
steel core, and maximum obtained current.
Figure 5-8 Self-Induction Schematic
Ignition Systems
Mutual Induction
During mutual induction (Figure 5-9), the primary and secondary coil windings
share a common steel core. The primary coil winding is wound closest to the steel
core. When the module is turned on, current increases in the primary, and the flux
lines cut across the secondary windings, as they are molded within 1/8 inch of the
primary. This causes an induced voltage in the secondary. Note that as the primary
current is increasing, the secondary has opposite polarity.
Figure 5-9 Mutual Induction Schematic
The total resistance (AT) of the secondary is too high to overcome when the
primary current is building. The induced voltage is relatively low at this time. When
the module turns off, the sudden decrease in primary current induces a voltage in
the secondary. Again, the primary coil current begins to decrease, inducing a
primary voltage from 75 to 200V AC. The magnitude of the voltage induced in the
secondary is determined by the number of turns in the coil. If the primary coil has
200V through self-induction, and the number of turns in the secondary is 200 times
more than the number in the primary, then: 200 • 200 = 40,000V (induced in the
Ignition Systems
If primary system voltage is 12V, which in turn is applied to an inductor, (primary
winding) the self-induction produces approximately 200 volts. The turn ratio of
200:1 would multiply the voltage, or increase by mutual induction in the secondary,
to 40,000V.
In Figure 5-10, note the polarity in relationship to system ground (the engine block).
The secondary lead at the coil (on the “X” end) is attached to ground. Current flows
from the coil to ground. The lead at the other end of the secondary coil is
connected to the distributor cap. The rotor connects a current path to an individual
circuit for each number of cylinders, in this case, cylinder number one. If, for
instance, the voltage required to overcome the individual circuit resistance of
cylinder number one were 9,500V or 9.5kV, then the circuit would have to build up
to 9,500V before spark would occur. Because of the negative polarity of the coil
end, the block is positive in relationship to the polarity of the coil lead.
Figure 5-10 Secondary Firing
Ignition Systems
Scope Patterns
When viewing the secondary circuit, scope patterns are divided into three sections:
the firing section, the intermediate section, and the dwell section (Figure 5-11).
Figure 5-11 Secondary Firing Pattern
The firing section (A) displays the build-up of voltage in each cylinder
circuit and the flow of current as the spark plugs arc to the side electrode,
ground. When viewing a trace of the secondary, the voltage rise of the
firing line is negative voltage, not a positive trace as it is on the primary
pattern. The scope internally flips the image so that when switching from
primary to secondary, they both have very similar patterns; however, they
are the opposite in polarity. The primary is positive on the vertical voltage
scale and the secondary is negative on the vertical scale.
The intermediate section (B) begins when the spark plug stops firing and
the remaining voltage from the secondary coil is dissipated.
The dwell section (C) is the beginning and ending of the module providing
a ground to the primary ignition coil. Whatever happens in the primary coil
is multiplied by the turns ratio of the secondary coil; the difference is the
resistance to the secondary voltage. When the magnetic flux in the primary
coil windings collapses, it induces a high voltage in the secondary coil
windings. This high negative polarity voltage seeks to complete a path for
current to flow through the secondary circuit.
Ignition Systems
Once the initial arc jumps across the spark plug gap (Figure 5-12), which is the
greatest resistance of the secondary, the voltage requirement drops by 70%. As a
result, there is a quick drop to the voltage required to maintain the spark. The
sudden drop from A to B is the result of the initial arcing across the spark plug
electrodes. Because there is less resistance once the air gap is ionized, the voltage
requirement is less. The voltage’s required maintaining spark is referred to as the
spark line. The duration of the spark line is based on total secondary circuit
resistance and coil voltage available.
Figure 5-12 Firing and Spark Lines
Ignition Systems
A good ignition coil will sustain spark duration for .85ms to 2ms. Using the 5ms
time base will help determine whether or not the spark duration is within
specifications. The spark line (Figure 5-13) is the most important area to examine
for locating a secondary ignition problem. Area B to C forms the spark line. It is
within this short .85 to 2.2ms time period that the hydrocarbons (HC) and oxygen
(O2) molecules are burning. Problems with the burning, or fuel propagation, will
show up within the spark line. There are times however when a problem exists but
cannot be seen examining the spark line. The time a coil takes to collapse is fairly
constant, but one thing must always occur: the air-fuel mixture must ignite and
sustain before the voltage is reduced to zero.
Figure 5-13 Spark Line Pattern
In Figure 5-13, using the 0-5 milliseconds scale along the bottom of the horizontal
axis, the spark line measures as 1.1ms. The written specification is .85-2.2ms, so
the spark line sits within desired specification. Because current flows in the
secondary circuit only during the spark duration, it is important to understand what
can alter the spark line trace.
Any change in resistance during spark duration will cause a change in the spark
line trace. If the resistance increases halfway through spark duration, then the trace
will show an increase in the needed voltage needed to maintain the spark, thus
Ignition Systems
increasing the height of the trace. The angle of the rise depends on the amount of
resistance encountered.
All conditions and components that affect the firing line may also affect the spark
line. If the coil does not have enough voltage left to jump across the spark plug
gap, then the spark will terminate.
Any increased resistance in the secondary decreases spark duration, thus, the
higher the firing line, the shorter the spark line (Figure 5-14). If the spark line is too
short, the fuel will not completely burn, causing loss of power, poor fuel
consumption, and high emissions levels. The two causes for a short spark line are
either a high secondary resistance or an excessively lean air-fuel ratio.
Figure 5-14 Spark Line Pattern
Ignition Systems
Figure 5-15 magnifies the spark line to show the relationship of an air fuel ratio to
spark ignition. Notice that a large number HC lowers the voltage requirement; HC is
conduces electricity. O2, however, is insulated and has high resistance.
Figure 5-15 Magnified Spark Line Pattern
The A-to-B pattern in Figure 5-15 shows these effects on the spark line. Many O2
molecules cause the resistance and the voltage requirement to rise, and many HC
molecules cause higher conductivity and lower the voltage requirement. To obtain
total combustion, the spark must be sustained as long as combustible mixtures are
present in the combustion chamber, and spark duration must be sufficient to allow
a complete burn. With the leaner mixtures on modern vehicles, the flame front does
not self-propagate under certain conditions.
Spark Timing
Spark timing refers to the importance of spark ignition occurring at a specific point
in the four-stroke engine cycle in order to maximize the development of peak
combustion power. In theory, the peak combustion power of the burning air/ fuel
mixture should be delivered to the piston just as it passes Top Dead Center (TDC),
at the beginning of the power stroke. Engineers use the rule of Minimum advance
timing, Best Torque (MBT). In function, the mixture will not develop peak
combustive power at the same time as the spark occurs. In order to develop
combustive power, the air/fuel mixture needs to be ignited and burn for a short
period of time. This brief burn will not occur unless the spark occurs before TDC.
Ignition Systems
Spark timing must be precise, but the necessary timing changes with engine speed
and load. Several terms exist to define the timing change of the spark. Advancing
the spark is to deliver the spark sooner, and retarding the spark is delaying the
spark. Advancing the spark will cause the spark to occur sooner in the compression
stroke before its TDC.
There are reference marks located on a pulley to indicate the position of the
number on piston. Vehicle manufacturers use these marks to set specified initial or
base ignition timing. When the marks are aligned at TDC, the piston in cylinder
number one is at TDC of its compression stroke. The additional marks or numbers
on a scale indicate the number of degrees of crankshaft rotation before TDC
(BTDC) or after TDC (ATDC). Most engine manufacturers specify the initial timing
at a point between TDC and 20° BTDC. A change in timing from 10° BTDC to 20°
BTDC is an advance of 10°. A change in timing from 20° BTDC to 10° BTDC is a
retard of 10°.
Combustion timing is also critical to diesel engine operation. Instead of a spark, the
diesel engine injects fuel at the proper time. The diesel engine’s temperature rises
enough from higher compression to auto-ignite the injected fuel.
Spark Plug Wires
Spark plug wires carry the high secondary voltage from the coil or distributor cap to
the spark plugs. They are made of silicone rubber, with a fiber core that acts as a
resistor to reduce secondary current. This cuts down on radio and television
interference (TVRS), and reduces spark plug wear. The insulated boots at the end
of the wires strengthen the connections to the plug and repel dust and moisture, as
well as prevent voltage loss.
Typically, the resistance of a spark plug wire should be less than 30,000 ohms.
Refer to the OEM service manual for specifications on each model. High resistance
can cause a misfire and possible coil damage. The wire can be damaged if it is not
carefully removed from the plug. Twist the boot to loosen, or use a removal tool
such as J21350 or equivalent to remove wires. If the boot is equipped with
aluminum heat shields, verify the shield is properly seated when the boot is
Ignition Systems
Spark Plugs
The spark plug provides an air gap through which the secondary voltage arcs and
ignites the air/fuel mixture in the engine (Figure 5-16). The basic plug consists of a
ceramic insulator and a pair of electrodes. Most plugs have a resistor, which, like
the wires, reduces current in the secondary system. Spark plugs must be the
correct size, reach, and heat range for a specific application. Always use the spark
plug specified for a particular engine.
Figure 5-16 Spark Plug
Spark plug electrodes are subjected to extreme heat, pressure, and corrosion. As a
result, they should be included in normal maintenance. Arcing tends to deteriorate
the electrodes over time, so a higher spark voltage is required to jump the gap.
Fouling may provide an alternate path for the spark, which causes misfires. Wetting
of the plug may also short out the electrodes. Cracked insulators, carbon tracking,
burned electrodes, and improper torque can all lead to undesirable performance
and premature failure.
Ignition Systems
Extended Life Spark Plug
The extended life spark plug (Figure 5-17) has a nickel-plated shell to improve
resistance to corrosion for the life of the vehicle. It uses a copper core center
electrode to improve resistance to low speed carbon fouling. Platinum tips are used
on the center electrode and side electrode to prevent spark erosion (which
contributes to gap growth). The tip also minimizes ignition demand voltage due to a
smaller surface area.
Figure 5-17 Extended Life Spark Plug
Ignition Systems
Spark Plug Heat Range
Spark plugs are designed to accommodate different heat ranges. The performance
objective is to accommodate different engines and various types of driving. The
reason for a built-in heat range is that the spark plug tip must operate at a high
enough temperature to prevent fouling, and yet remain cold enough to avoid preignition. (Pre-ignition can happen when anything in the combustion chamber
remains hot enough to ignite the fuel/air mixture before the timed spark occurs.)
Heat range, then, is the measure of spark plug capability to transfer heat received
from the engine combustion chamber to the cylinder head (Figure 5-18).
Figure 5-18 Spark Plug Heat Range
The rate of heat transfer, whether fast or slow, is a product of spark plug design
and identifies the difference between a "hot" spark plug and a "cold" spark plug.
The length of the lower insulator and the conductivity of the center electrode
primarily determine heat range.
A "hot" spark plug transfers heat more slowly and operates at a higher temperature
than a cold spark plug. The "cold" spark plug has a faster rate of heat transfer and
operates at a cooler temperature when installed in the same engine and operated
under the same conditions. Therefore, a "colder" spark plug may be best suited for
a "full load" or continuous high-speed highway-type driving. A "hotter" spark plug
may be better for prolonged idling or city type stop and go traffic.
Ignition Systems
A spark plug with an "ideal heat range" operates between the pre-ignition and
fouling temperatures and delivers the best all round performance. The spark plug
normally provided in new engines is designed to function in the proper heat range
for most operating conditions. The length of the insulator tip, its configuration, and
the conductivity of the center electrode determine the heat range of the spark plug.
The length of the insulator tip, its configuration and the conductivity of the center
electrode mostly determine the heat range of the spark plug. Figure 5-19 shows the
difference between a hot and cold spark plug insulator tip length.
Figure 5-19 Hot and Cold Spark Plug Tips
Ignition Systems
Triggering Devices
As discussed earlier in this lesson, there are four categories of triggering devices:
the Permanent Magnet (PM) generator (also known as Variable Reluctance
Sensor), Hall effect switch, Optical Pickup, and Magneto Resistive (MR).
Permanent Magnet (PM) Generator
The PM generator uses the principle of induction to develop an Alternating Current
(AC) signal. In the crank sensor (Figure 5-20), wire coils around a permanent
magnet. By rotating a reluctor which has notches cut into it at precise locations, the
magnetic field moves back and forth across the wire winding. This produces an AC
voltage signal in the wire. The ends of the wires are connected to either the Ignition
Control Module (ICM) or the Powertrain Control Module (PCM). The signal is
converted to an on/off reference and used as the base triggering for the primary
circuit. For the Crankshaft Position (CKP) sensor to function, it must have a
.050mm (.020in) air gap between the sensor and the reluctor.
Figure 5-20 Crank Sensor
Ignition Systems
Pick-Up Coil/Pole Piece
On electronic ignition (distributorless) systems, the sensor is mounted in the block
or the front cover and is non-adjustable. On a distributor system, the pickup coil
operates similarly. A magnetic field increases and decreases as the teeth of the
timer core and the pole piece move in and out of alignment (Figure 5-21). This
induces an AC flow through the pickup coil, which is the triggering signal to the
Figure 5-21 Timer Core
Law of Induction
Electricity creates magnetism, and magnetism creates electricity. In other words,
current flowing through a conductor creates a small magnetic field around the
conductor. Conversely; any time a magnetic field is allowed to cut through a
conductor; current flow is produced in the conductor. It is this principle that is used
in PM generators.
Ignition Systems
Hall Effect Switch
The Hall effect switch (Figure 5-22) is an electronic device that produces a
voltage signal controlled by the presence or absence of a magnetic field in an
electronic circuit. In other words, a regulated signal voltage from the ignition
module is passed through a semiconductor wafer in the Hall switch. A permanent
magnet, mounted beside the semiconductor, induces Hall voltage across the
semiconductor. The CMP is positioned so that metal blades or “vanes” of an
interrupter ring mounted on the harmonic balancer pass between the
semiconductor and the permanent magnet. When one of these metal vanes passes
between the magnet and semiconductor, the magnetic field is interrupted and Hall
voltage drops off.
Figure 5-22 Hall Effect Switch
Ignition Systems
Figure 5-23 shows the magnetic flux leaving the north pole of the permanent
magnet, passing through the semi-conductor wafer, and entering the south pole.
The wafer is encapsulated with a steel jacket on one side to attract the magnetic
flux, thus generating a very low voltage that is proportional to the strength of the
magnetic flux density and the current passing through the wafer from switched
ignition feed. The current stays fairly constant at .017 amps (17 milliamps) through
the wafer.
Figure 5-23 Hall Effect Switch - “On”
Ignition Systems
As the distributor shaft rotates in Figure 5-24, a steel blade (one blade for each
cylinder of the engine) passes between the magnet and the semi-conductor wafer.
The low reluctance of the steel blade provides a path for the magnetic flux which
bypasses the Hall effect wafer (for a review of the Hall effect, refer to the next
subsection). Since the wafer is now shielded from the magnetic field, the voltage
output from the semi-conductor drops to near zero.
As the Hall voltage drops off, the Hall voltage becomes amplified and then routes to
the base of a transistor, which controls the ground on the signal voltage from the
ignition module. The Hall effect switch is used for a Crankshaft Position Sensor
(CKP) as well as a CMP on some engines. The Hall effect switch sends out square
wave signals to the ignition module and the computer (along with input from other
sensors) to maintain correct spark and fuel injection timing.
Figure 5-24 Hall Effect Switch - “Off”
Ignition Systems
The Hall Effect
When a magnetic field is introduced perpendicular to a current flowing through a
solid conductor, a measurable voltage is induced at right angles to the main current
flow at the sides of the conductor (Figure 5-25).
Figure 5-25 Hall Effect
Ignition Systems
Ignition systems have undergone many changes in recent years. The obsolete
mechanically-driven distributors with breaker-point design have been replaced with
the Electronic Ignition (EI) system shown in Figure 5-26.
Figure 5-26 Distributorless Electronic Ignition System Schematic
Ignition Systems
Though there are many variations of ignition systems in current use, according to
the Society of Automotive Engineers (SAE) standard J1930, all fall into two
Distributor Ignition (DI) system
Electronic Ignition (EI) system
Distributor Ignition
Distributor Ignition systems have been used since the 1970s. They have been
constantly refined and improved. Timing is computer-controlled and better meets
the changing engine conditions.
The most common type of DI is the High-Energy Ignition (HEI) system. Advantages
of this type of system include:
The ability to fire leaner air/fuel mixtures with high spark plug firing
Extended spark plug life
Fewer moving parts
The signal for ignition timing can be controlled by a computer
Coil primary current is controlled electronically by solid state circuitry
Ignition Systems
As shown in Figure 5-27, the following components make up the typical distributor
Distributor assembly
Pickup coil
Ignition module
Ignition coil
Cap and rotor
Figure 5-27 Distributor System (Exploded View)
Ignition Systems
Electronic Ignition (EI)
The Electronic Ignition (EI) system (also called distributorless ignition system)
replaces the DI and eliminates many mechanical parts.
The components that make up a typical EI system are listed as follows
(Figure 5-28):
Ignition module
Ignition coils
CKP sensor
CMP sensor
Interrupter or reluctor
Figure 5-28 Typical Electronic Ignition System
The EI system has the following advantages over the distributor ignition system:
Fewer moving parts
Compact mounting
Remote mounting capability
Elimination of mechanical timing adjustments
Less maintenance
No mechanical load on engine
Increased available coil saturation (dwell time)
More coil cool down time between firing events
EI operates on what is called the waste spark principal. Regardless of the number
of engine cylinders, one ignition coil supplies voltage to a pair of spark plugs. Each
end of the coil secondary circuit is connected to one spark plug. The corresponding
Ignition Systems
pistons for these plugs reach TDC at the same time, but on opposite strokes of the
combustion stroke when the other piston is on the exhaust stroke.
The terminology that is used when referring to the EI system is determined by the
actual vehicle’s application of the system. The five EI systems currently used are
listed as follows:
Computer Controlled Coil Ignition (C3I)
Direct Ignition System (DIS)
Integrated Direct Ignition (IDI)
Up-Integrated Direct Ignition (UIDI)
Integrated Coil Electronics (ICE)
EI Secondary Operation
In an El system, a spark plug is attached to each end of the ignition coil secondary.
Each coil of the system fires the plugs in two companion cylinders (Figure 5-29).
These are cylinders that reach TDC at the same time. The cylinder at TDC on the
compression stroke is referred to as the event cylinder, while the cylinder at TDC
on the exhaust stroke is the waste cylinder. When the coil discharges, both plugs
fire at the same time to complete the series circuit.
Figure 5-29 Companion Cylinder Schematic
Ignition Systems
Since the polarities of the ignition primary and secondary windings are fixed, one
plug always fires in a forward direction, while the other always fires in reverse
(Figure 5-30). This arrangement requires somewhat more energy than conventional
systems. Coil design, saturation time and primary current flow on El systems are
able to produce the necessary energy to accomplish this.
Figure 5-30 Electronic Ignition Coil Design
Since both plugs in companion cylinders fire at the same time, it is not necessary
for the module to recognize which cylinder is on which stroke. Because of lower
pressure in the cylinder on the exhaust stroke, its plug requires less voltage to
produce an arc. Therefore, most of the available voltage is used to fire the plug in
the cylinder that is on the compression stroke.
Ignition Systems
C3I (Computer Controlled Coil Ignition)
The EI system known as C3I uses Hall effect switches for crank and cam position
with the interrupter ring mounted to the harmonic balancer, an ignition module, and
a coil pack assembly (Figure 5-31).
Figure 5-31 Computer Controlled Coil Ignition Components
There are two coil pack designs, Type I and Type II (Figure 5-32).
Figure 5-32 Type I and Type II Coil Pack
The cam sensor signal identifies cylinder sequence for injector firing on Sequential
Fuel Injection systems, as well as the “sync” signal for the ignition module.
C31 Type I and II Fast Start 3800 Engine
The GM C31 Fast Start system on the 3800 and 1993 3300 engines use a dual
crankshaft sensor and a separate cam sensor. Advantages of the Fast Start
system are faster start-up, walk-home protection in the event of cam sensor
malfunction, and a precise measurement of crankshaft sensor signals.
Ignition Systems
On the Fast Start systems, the dual crank sensor is mounted on the front of the
engine beside the harmonic balancer/crankshaft pulley. The cam sensor is
mounted on the timing cover beside the cam sprocket. The arrangement of the
interrupter rings on the harmonic balancer is different than on the other GM V6
First, the outside ring has 18 evenly sized and evenly spaced interrupter blades to
produce 18 pulses per crankshaft revolution. These pulses are known as the 18X
signal. The inside ring has three interrupter blades with gaps (or windows) of 10°,
20° and 30°. These gaps, in turn, are spaced 100°, 90° and 110° apart
respectively. These pulses are referred to as the 3X signal. With this interrupter
ring arrangement, the ignition module can identify the proper cylinder pair to fire
within as little as 120° of crankshaft rotation. The module can also fire any cylinder
pair reaching TDC first without waiting for the cam or sync signal. The 1-4 cylinder
pair reaches TDC 75° after the trailing edge of the 10-degree window. TDC of the
6-3 pair is 75° after the trailing edge of the 20° window. TDC of the 2-5 cylinders
occurs 75° after the trailing edge of the 30° window. The trailing edges of the
windows are each 12° apart.
Direct Ignition System (DIS)
Another EI system is the Direct Ignition System (DIS). This system uses a magnetic
crankshaft sensor, a reluctor ring cast into the crankshaft, an ignition module, and a
coil pack (Figure 5-33). Notches in the reluctor ring are spaced to provide crank
position information, as well as information to synchronize coil firing.
Figure 5-33 Direct Ignition System
The system is used on current GM 3800 engines and 3.8L Sequential Fuel Injected
(SFI) engines of the past. The past 3.0-liter, 3.8-liter, 3300, and 3800 V6 engines
Ignition Systems
use Hall effect switches for both the crankshaft signal and the camshaft signal.
The CKP sensor on the 3.0-liter engine is located adjacent to the crankshaft
harmonic damper. Two concentric rings on the back of the damper pass on each
side of the Hall effect magnet. The inner ring has three evenly spaced vanes and
windows, which send identically timed signals of the same duration. The outer ring
has only one window. This single pulse acts as the synchronized signal to set up
the logic for triggering the correct ignition coil.
Integrated Direct Ignition (IDI)
Another type of ignition system is the Integrated Direct Ignition (IDI) (Figure 5-34).
The system is similar to the DIS in operation. The module, coils, and spark plugs
are contained in one assembly.
Figure 5-34 Integrated Direct Ignition System
Ignition Systems
Up-Integrated Direct Ignition (UDI)
Another variation to the IDI system is the Up-Integrated Direct Ignition (UIDI)
(Figure 5-35). The main difference between UIDI and IDI is that all timing control is
performed by the PCM/ECM
Figure 5-35 Up-Integrated Direct Ignition System Schematic
In an up-integrated ignition system, all ignition coil timing is controlled by the
PCM/ECU. The triggering signal from either the CKP sensor or the pickup coil is a
direct input to the PCM. The PCM processes the triggering signal along with other
inputs, and provides the ignition control module with an on/off signal. Based on the
on/off signals from the PCM, the ignition control module turns the coils on and off,
providing secondary voltage for the spark plugs to fire. The main function of the
ignition control module in an up-integrated system is to turn the coils on and off
based on IC signals from the PCM and limit primary current flow (in some
In some cases, the ignition control module still receives the CKP sensor signal. The
ignition control module, in these cases, is just a processor for the CKP sensor
signal. It passes the signal to the PCM, sometimes converting it from an AC signal
to a DC signal, but has no control of ignition timing.
Ignition Systems
Bypass Systems
In a bypass system, the ICM processes the triggering device signal. Because the
triggering device signal can be either an AC signal from a PM Generator, or a DC
signal from a Hall effect switch or optical pickup, the module sometimes must
convert the signal from an AC signal to a DC signal. The ignition control module
provides an ignition control (IC) reference signal to the PCM based on the signal
from the triggering device. The ignition module controls the primary coil current by
completing the ground path for the ignition coil primary current. Several circuits
within the module and the external triggering devices determine the timing and the
sequencing of the coil drivers.
Bypass Mode
Another function of the ICM is to control the bypass mode of the ignition system. In
a bypass type of system, there are two modes of operation: bypass and IC. Bypass
can be thought of as ICM-controlled timing and is normally used when the engine is
cranking and running below a certain RPM, or during a default mode due to a
system failure. In IC mode, the PCM controls the timing instead of the ignition
control module.
During the bypass mode of operation, base timing may not be fixed. Depending on
the application, some timing advance may be engineered into the ignition control
module during this mode.
The pickup coil or CKP sensor provides the signal necessary for the ignition coil to
fire when the engine is cranking below the “run threshold” (usually between 400
RPM and 600 RPM, depending on the engine).
Integrated Coil Electronics
Another type of IC system is the Integrated Coil Electronics (ICE) (Figure 5-36).
The system uses a magnetic crank sensor and the engine controller controls
timing. The two coils and ignition module are contained in a single assembly.
Figure 5-36 Integrated Coil Electronics
Ignition Systems
High Voltage Switch (HVS) Distributor Ignition System
The ignition system used on late model truck applications, called the High Voltage
Switch (HVS) DIS, features a high-energy ignition coil and ignition coil driver
module (Figure 5-37). Each engine application of this enhanced ignition system has
a unique distributor where some are non-adjustable and others adjustable. They
are adjustable to eliminate the chance of crossfire only, not for timing adjustment.
Figure 5-37 Integrated Coil Electronics
The HVS distributor appears similar to a typical distributor, but key operational
features make it very different. The HVS distributor does not provide engine
position information for spark delivery. Therefore, rotating the HVS distributor does
not change ignition base timing. The VCM contains the base timing information
within its calibration.
The ignition coil driver module is mounted with the high energy coil (Figure 5-37).
The Vehicle Control Module (VCM) controls the coil driver module. The coil driver
module, in turn, controls current through the primary windings of the coil. Note that
the coil driver module has no backup (bypass) mode.
Base timing is not adjustable because the crank sensor, not the distributor,
determines base timing. This makes it the main sensor for fuel and spark. As a
result, the engine will not run without a CKP sensor signal, because the ignition coil
driver module does not have system trigger information. The crank sensor is
located on the front of the engine in the timing cover.
Ignition Systems
The HVS DIS uses crankshaft and camshaft position signals as inputs to the VCM.
The VCM then uses the IC circuit to signal the coil driver module to control spark
The CKP sensor signal is used to determine engine position and speed.
The CMP sensor signal (.5X signal) identifies piston position. The CMP is
used to sequence the fuel injectors and detect misfire for OBD II
The VCM uses the IC signal to control advance and retard based upon
engine load, atmospheric pressure, RPM, and engine temperature.
Since the distributor has no influence on base timing, turning it will not modify base
timing in any way. However, the distributor on eight cylinder applications is
adjusted to eliminate the chance of crossfire at an adjacent terminal. Distributor
terminal crossfire can be evident by poor performance, as the control module will
reduce the operating window for spark advance and retard.
Magneto Resistive Sensors
A CKP sensor supplies trigger information for ignition timing. The CKP sensor is
located in the timing chain cover. The CMP sensor is located in the distributor
base, and is used to sequence the fuel injectors and for on-board misfire
diagnostics. The CKP sensor (Figure 5-38) on late model General Motors trucks is
a Magneto-Resistive (MA) sensor that generates a digital signal. The MA sensor is
similar in operation to a Hall effect switch. Both sensors require a magnetic field to
operate, have three wires, and output a digital signal.
Figure 5-38 Crankshaft Position Sensor on GM Trucks
Ignition Systems
A permanent magnet is located inside the sensor end nearest the crankshaft
reluctor wheel (Figure 5-39). The magnet is positioned between two magnetic
reluctance pickups, MA1 and MA2. The magnetic field changes in the area of MA1
and MA2 as the reluctor wheel passes. Each tooth of the reluctor wheel reaches
MA1 first, then MA2. Both MA1 and MA2 produce identical voltage signals, but the
MA2 signal is just a fraction of a second later than the MA1 signal because of its
location to the approaching reluctor wheel.
Figure 5-39 Crankshaft Reluctor Wheel
Both the CKP sensor and the reluctor wheel should be handled carefully. Any dents
or other imperfections in the wheel can cause excessive CKP sensor noise. A
damaged reluctor wheel or CKP sensor may cause improper operation of on-board
diagnostics, such as the misfire diagnostic.
Ignition Systems
Signals from MA1 and MA2 cause a differential amplifier to produce the MA
differential output (Figure 5-40). This signal is used to switch a Schmidt trigger on
and off .The sensor output is then like that of a Hall effect switch. One difference
from most Hall effect switches is that the VCM does not supply a pulled up signal
wire for the sensor to toggle to ground. Instead, the MA sensor pulls up the signal
wire to 5 volts and toggles it to ground.
Figure 5-40 Magneto-Resistive Sensor Differential Output
Ignition Systems
Another ignition system that uses a magneto resistive CKP sensor is the coil-nearplug system (Figure 5-41) used by several OEMs. The coil-near- plug system
consists of the following components/ circuits:
Four, six, or eight ignition coils/modules
Four, six, or eight IC circuits
CMP sensor
Camshaft reluctor wheel
CKP sensor
Crankshaft reluctor wheel related connecting wires
Figure 5-41 Magneto-Resistive Sensor Differential Output
Ignition Systems
Crankshaft Position (CKP) Sensor and Reluctor Wheel
The dual magneto resistive Crankshaft Position (CKP) sensor is located in the
right rear of the engine, behind the starter. The CKP sensor works with a 24X
reluctor wheel (Figure 5-42). The reluctor wheel is mounted on the rear of the
crankshaft. The 24X reluctor wheel has two different width notches that are 15°
apart. This pulse width encoded pattern allows cylinder position identification within
90° of crankshaft rotation. In some cases, cylinder identification can be located in
45° of crankshaft rotation. The reluctor wheel also has dual track notches that are
180° out of phase. The dual track design allows for quicker starts and accuracy.
Figure 5-42 24X Reluctor Wheel
The CKP sensor signal must be available for the engine to start. The CMP sensor
signal is not needed to start and operate the engine. The PCM cannot determine
when a particular cylinder is on either compression or exhaust stroke by the 24X
signal. The CMP sensor determines the engine’s current stroke. If the CMP sensor
fails, the system will attempt synchronization by firing one of two companion
cylinders, and look for an increase in the RPM. An increase in the RPM signal
indicates that the correct cylinder was fired, and the engine has started. If the PCM
does not detect an increase RPM signal, the opposite cylinder will re-sync. A
slightly longer cranking time may be a symptom of this condition.
Ignition Systems
Camshaft Position (CMP) Sensor
The Camshaft Position (CMP) sensor (Figure 5-43) is mounted through the top of
the engine block at the rear of the intake valley cover. The CMP sensor is used in
conjunction with a 25X CKP sensor reluctor wheel. The reluctor wheel is located at
the rear of the camshaft. The CMP sensor is used to determine whether a cylinder
is or compression or the exhaust stroke. As the camshaft rotates, the reluctor
wheel interrupts a magnetic field (produced by a magnet within the sensor). The
CMP sensor internal circuitry detects this and produces a square-wave signal,
which is used by the PCM. The PCM/ECU uses this signal in combination with the
CKP sensor 24X signal to determine crankshaft position and stroke.
Figure 5-43 Camshaft Position Sensor
Ignition Systems
Ignition Coils/Modules
The coil-per-plug system has as many ignition coils/modules as there are cylinders
usually mounted in the valve covers (Figure 5-44). The secondary ignition wires are
short compared with a distributor ignition system wire. The coils/modules are fired
sequentially. There is an IC circuit for each ignition coil/module.
Figure 5-44 Ignition Coils
The ignition control circuits are connected to the PCM/ECU. All timing decisions are
made by the PCM/ECU, which triggers each coil/module individually. The ignition
coil modules have the following circuits attached:
Ignition Feed Circuit
Ignition Control Circuit
Ground Circuit
Reference Low Circuit
The ignitions feed circuits are fused separately for each bank of the engine. The
two fuses also supply the injectors for that bank of the engine.
This system puts out exceptionally high ignition energy for pit firing. Because the
ignition wires are shorter, lesser is lost to ignition wire resistance. Furthermore, no
energy is lost to the resistance of a waste spark system. Figure 5-45 shows a
typical schematic for a Coil-Near-Plug system.
Ignition Systems
Figure 5-45 Coil-Near-Plug System Schematic
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