The ignition system includes those parts and wiring required to generate and distribute a high voltage to the spark plugs.
A fault anywhere in the primary (low-voltage) ignition circuit can cause a no-start condition. A fault anywhere in the secondary (high-voltage) ignition circuit can cause engine missing, hesitation, stalling, or excessive exhaust emissions.
IGNITION SYSTEM OPERATION
Ignition systems include components and wiring necessary to create and distribute 40,000 volts or more. All systems apply battery voltage to the coil positive side, and pulse the negative to ground. When the coil negative lead is grounded, the primary (low-voltage) circuit of the coil is complete and a magnetic field is created around the coil windings. When the circuit is opened, the magnetic field collapses and induces a high-voltage spark in the secondary winding of the ignition coil. Early ignition systems used a mechanically opened set of contact points. Electronic ignition uses a sensor, such as a pickup coil and reluctor (trigger wheel) to signal an electronic module that makes and breaks the primary connection of the ignition coil.
NOTE: Distributor ignition ( DI ) is the term specified by the Society of Automotive Engineers (SAE) for an ignition system that uses a distributor. Electronic ignition ( EI ) is the term specified by the SAE for an ignition system that does not use a distributor.
The ignition coil creates a high-voltage spark by electromagnetic induction. Many contain two separate, electrically connected windings of copper wire.
Other coils are true transformers in which primary and secondary windings are not electrically connected. Figure 56–1 Internal construction of an oil-cooled ignition coil. Notice that the primary winding is electrically connected to the secondary winding. The polarity (positive or negative) of a coil is determined by the direction in which the coil is wound. Continued
The center of a coil contains a core of laminated soft iron which increases magnetic strength of the coil. Surrounding the core are about 20,000 turns of fine wire (approximately 42 ga.), called the secondary coil windings. Surrounding the secondary windings are approximately 150 turns of heavy wire (approximately 21 ga.), called the primary coil windings. The secondary winding has about 100 times the number of turns of the primary winding, referred to as the turns ratio (approximately 100:1). In many coils, these windings are surrounded with a thin metal shield and insulating paper and placed into a metal container to help retain the magnetic field produced in the windings. Many coils contain oil to help cool the ignition coil.
Figure 56–3 Cutaway of a General Motors Type II distributorless ignition coil. Note that the primary windings are inside of the secondary windings.
Other coil designs, such as those used on GM’s high energy ignition ( HEI ) systems, use an air-cooled, epoxy-sealed E coil . The E coil is so named because the laminated, soft-iron core is E-shaped, with the coil wire turns wrapped around the center “finger” of the E and the primary winding wrapped inside the secondary winding.
Figure 56–2 Typical air-cooled epoxy-filled E coil. Continued
The positive terminal of the coil attaches to the ignition switch, which supplies current from the positive battery terminal. The negative terminal is attached to an ignition control module ( ICM or igniter ), which opens and closes the primary ignition circuit by opening or closing the ground return path of the circuit. The labeling of positive ( + ) and negative ( – ) of the coil indicates that the positive terminal is more positive (closer to the positive terminal of the battery) than the negative terminal of the coil. This condition is called the coil polarity , and must be correct to ensure that electrons will flow from the hot center electrode of the spark plug on DI systems.
Continued The polarity of an ignition coil is determined by the direction of rotation of the coil windings .
Self-Induction When current starts to flow into a coil, an opposing current is created in the windings of the coil. This opposing current generation is caused by self - induction and is called inductive reactance . Inductive reactance is similar to resistance because it opposes any changes (increase or decrease) in current flow in a coil. When an ignition coil is first energized, there is a slight delay of approximately 0.01 second before the ignition coil reaches its maximum magnetic field strength, called saturation .
Mutual Induction In an ignition coil there are two windings, a primary and a secondary winding. When a change occurs in the magnetic field of one coil winding, a change also occurs in the other coil winding. If the current is stopped from flowing (circuit is opened), the collapsing magnetic field cuts across the turns of the secondary winding and creates a high voltage in the secondary winding. This generation of an electric current in both coil windings is called mutual induction . The collapsing magnetic field also creates a voltage of up to 250 volts in the primary winding.
How Ignition Coils Create 40,000 Volts All ignition systems use electromagnetic induction to produce a high-voltage spark from the ignition coil. Electromagnetic induction means a current can be created in a conductor (coil winding) by a moving magnetic field. The magnetic field in a coil is produced by current flowing through the primary windings. Current for the winding is supplied through the ignition switch to the positive terminal of the coil. The negative terminal is connected to the ground return through an electronic ignition module (igniter). If the primary circuit is completed, current can flow through the primary coil windings and create a strong magnetic field in the coil.
When the primary coil winding ground return path connection is opened, the magnetic field collapses and induces a voltage of from 250 to 400 volts in the primary winding of the coil and a high-voltage (20,000 to 40,000 volts ) low-amperage (20 to 80 mA) current in the secondary coil windings. This high-voltage pulse flows through the coil wire, distributor cap, rotor, and spark plug wires to the spark plugs. For each spark that occurs, the coil must be charged with a magnetic field and discharged. The ignition components that regulate the current in the coil primary winding by turning it on and off are known collectively as the primary ignition circuit . The components necessary to create and distribute the high voltage produced in the secondary windings of the coil are called the secondary ignition circuit . See Figure 56–4.
Figure 56–4 Typical primary and secondary electronic ignition using a ballast resistor and a distributor. To protect the ignition coil from overheating at lower engine speeds, many electronic ignitions do not use a ballast resistor but use electronic circuits within the module. Continued
The ignition circuits include the following components:
Primary Ignition Circuit
Primary windings of coil
Pickup coil (crank sensor)
Ignition module (igniter)
Secondary Ignition Circuit
Secondary windings of coil
Distributor cap and rotor (if the vehicle is so equipped)
Spark plug wires
An ignition coil contains two windings, a primary winding and a secondary, and these windings can be either connected at one end or kept separated.
What is a “Married” and “Divorced” Coil Design? Married — also called a tapped transformer design. The primary winding is electrically connected to the secondary. This commonly used in older distributor type ignition system coils, as well as many coil-on-plug designs. The inductive kick, called flyback voltage, created when the primary field collapses is used by the PCM to monitor secondary ignition performance. Figure 56–5 A tapped- (married) type ignition coil where the primary winding is tapped (connected) to the secondary winding. Divorced — Also called a true transformer design, used by most waste-spark ignition coils to keep both the primary and secondary winding separated.
IGNITION SWITCHING AND TRIGGERING
For any ignition system to function, the primary current must be turned on to charge the coil and off to allow the coil to discharge, creating a high-voltage spark. This turning on and off of the primary circuit is called switching . The unit that does the switching is an electronic switch, such as a power transistor. This power transistor can be located in any of the following locations:
In the ignition control module (ICM)
In the PCM (computer)
NOTE: On some coil-on-plug systems, the ICM is part of the ignition coil itself and is serviced as an assembly.
The device that signals switching of the coil on and off is called the trigger . It is typically a pickup coil in some distributor-type ignitions and a crankshaft position sensor (CKP) on electronic (waste-spark and coil-on-plug) and many distributor-type ignitions. There are three types of devices used for triggering, including the magnetic sensor, Hall-effect switch, and optical sensor.
PRIMARY CIRCUIT OPERATION
To get a spark out of an ignition coil, the primary coil circuit must be turned on and off. This primary circuit current is controlled by a transistor (electronic switch) inside the ignition module or (igniter) that in turn is controlled by one of several devices:
Pickup coil ( pulse generator ) Most manufacturers use the rotation of the distributor shaft to time the voltage pulses. The magnetic pulse generator is installed in the distributor housing. The pulse generator consists of a trigger wheel (reluctor) and a pickup coil. The pickup coil signal triggers the transistor inside the module and is also used by the computer for piston position information and engine speed (rpm).
See Figures 56–6 and 56–7.
Figure 56–6 Operation of a typical pulse generator (pickup coil). At the bottom is a line drawing of a typical scope pattern of the output voltage of a pickup coil. The module receives this voltage from the pickup coil and opens the ground circuit to the ignition coil when the voltage starts down from its peak (just as the reluctor teeth start moving away from the pickup coil). Continued
Figure 56–7 The varying voltage signal from the pickup coil triggers the ignition module. The ignition module grounds and ungrounds the primary winding of the ignition coil, creating a high-voltage spark.
The pickup coil consists of an iron core wrapped with fine wire, in a coil at one end and attached to a permanent magnet at the other end. The center of the coil is called the pole piece.
Hall-effect switch —A Hall - effect switch also uses a stationary sensor and rotating trigger wheel (shutter). Unlike a magnetic pulse generator, the Hall-effect switch requires a small input voltage to generate an output or signal voltage.
Figure 56–8 Hall-effect switches use metallic shutters to shunt magnetic lines of force away from a silicon chip and related circuits. All Hall-effect switches produce a square wave output for every accurate triggering. Continued
Hall effect is ability to generate a voltage signal in semiconductor material (gallium arsenate crystal) by passing current through it in one direction and applying a magnetic field to it at a right angle to its surface. If the input current is held steady and the magnetic field fluctuates, an output voltage is produced that changes in proportion to field strength.
Continued Most Hall-effect switches in distributors have a Hall element or device, a permanent magnet, and a rotating ring of metal blades (shutters) similar to a trigger wheel. Some blades are designed to hang down, typically Bosch and Chrysler systems; others may be on a separate ring on the distributor shaft, typically GM and Ford Hall-effect distributors.
Figure 56–9 Shutter blade of a rotor as it passes between the sensing silicon chip and the permanent magnet.
When the shutter blade enters the gap between the magnet and the Hall element, it creates a magnetic shunt that changes the field strength through the Hall element.
This analog signal is sent to a Schmitt trigger inside the sensor itself, which converts the analog signal into a digital signal. A digital (on or off) voltage signal is created at a varying frequency to the ignition module or onboard computer. See Figure 56–10. Continued
Figure 56–10 Some Hall-effect sensors look like magnetic sensors. This Hall-effect camshaft reference sensor and crankshaft position sensor have an electronic circuit built in that creates a 0- to 5-volt signal as shown at the bottom. These Hall-effect sensors have three wires: a power supply (8 volts) from the computer (controller); a signal (0 to 5 volts); and a signal ground. Continued
Magnetic crankshaft position sensor Uses the changing strength of the magnetic field surrounding a coil of wire to signal the module and computer. This signal is used by the electronics in the module and computer as to piston position and engine speed (rpm). See Figures 56–11 and 56–12.
Optical sensors Use light from a LED and a phototransistor to signal the computer. An interrupter disc between the LED and the phototransistor has slits that allow the light from the LED to trigger the phototransistor on the other side of the disc. Most optical sensors (usually located inside the distributor) use two rows of slits to provide individual cylinder recognition (low-resolution) and precise distributor angle recognition (high-resolution) signals. See Figure 56–13
Figure 56–11 A magnetic sensor uses a permanent magnet surrounded by a coil of wire. The notches of the crankshaft (or camshaft) create a variable magnetic field strength around the coil. When a metallic section is close to the sensor, the magnetic field is stronger because metal is a better conductor of magnetic lines of force than air. Continued
Figure 56–12 A typical magnetic crankshaft position sensor. Continued
Figure 56–13 (a) Typical optical distributor. (b) Cylinder I slit signals the computer the piston position for cylinder I. The I-degree slits provide accurate engine speed information to the computer.
Optical distributors use light emitted from LEDs to trigger phototransistors. Most optical distributors use a shield between the distributor rotor and the optical interrupter ring. Sparks jump the gap from rotor tip to distributor cap inserts. This shield blocks the light from the electrical arc from interfering with the detection of the light from the LEDs.
Optical Distributors Do Not Like Light - Part 1 Figure 56–14a An optical distributor on a Nissan 3.0 L V-6 shown with the light shield removed.
Optical Distributors Do Not Like Light - Part 2 If this shield is not replaced during service, the signals are reduced and the engine may not operate correctly. This can be difficult to detect because nothing looks wrong during an inspection. Remember all optical distributors must be shielded between the rotor and the interrupter ring. Figure 56–14b A light shield being installed before the rotor is attached.
Crankshaft position sensor
When diagnosing a no-start or intermittent missing condition, check the operation of the tachometer. If the tachometer does not indicate engine speed (no-start condition) or drops toward zero (engine missing), then the problem is due to a defect in the primary ignition circuit. The tachometer gets its signal from the pulsing of the primary winding of the ignition coil. The following components in the primary circuit could cause the tach to not work when the engine is cranking. The Tachometer Trick
No tach reading means the problem is in the primary ignition circuit.
Tachometer reading okay means the problem is in the secondary ignition circuit or is a fuel-related problem.
Ignition module (igniter)
Coil primary wiring
If the vehicle is not equipped with a tachometer, connect a handheld tachometer to the negative terminal of the coil. Remember the following:
GM HEI Electronic Ignition Standard equipment on GM vehicles. Some use a coil in the distributor cap and some models use an externally mounted ignition coil.
Continued Figure 56–15 An HEI distributor. The large-diameter distributor cap provides additional space between the spark plug connections to help prevent crossfire. Defective HEI coils must be replaced with the exact replacement style.
Figure 56–16 A typical General Motors HEI coil installed in the distributor cap. When the coil or distributor cap is replaced, check that the ground clip is transferred from the old distributor cap to the new. Without proper grounding, coil damage is likely. There are two designs of HEI coils. One uses red and white wire as shown, and the other design, which has reversed polarity, uses red and yellow wire for the coil primary.
Most HEI distributors use 8-mm diameter spark plug wires that use female connections to the distributor cap towers.
Continued HEI coils differ and can be identified by the colors of the primary leads.
Figure 56–17 This uses a remotely mounted ignition coil.
The correct color of lead coil must be used for replacement. The colors of the leads indicate the direction in which the coil is wound, and its polarity.
Ford Electronic Ignition All function similarly, though over the years the system has been called by various names. The EEC IV system uses the thick-film-integration (TFI) ignition system. This system uses a smaller control module attached to the distributor and uses an air-cooled epoxy E coil. See Figure 56–18. TFI means all electronics are manufactured on small layers built up to form a thick film. Construction uses pastes of different electrical resistances deposited on a thin, flat ceramic material by a process similar to silk-screen printing, connected by tracks of palladium silver paste. The chips that form the capacitors, diodes, and integrated circuits are soldered directly to the palladium silver tracks. The thick-film manufacturing process is highly automated.
Figure 56–18 Wiring diagram of a typical Ford electronic ignition. Continued
Operation of Ford Distributor Ignition Ford DI systems function in basically the same way regardless of year and name. Under the distributor cap and rotor is a magnetic pickup assembly, which produces a small alternating electrical pulse when the distributor armature rotates past the pickup assembly (stator). This low-voltage pulse is sent to the ignition module. The ignition module switches off the primary ignition coil current (through transistors). When the ignition coil primary current is stopped quickly, a high-voltage “spike” discharges from the coil secondary winding. The coil current is controlled in the module circuits by decreasing dwell (coil-charging time).
See Figure 56-19.
Figure 56–19 Schematic of a Ford TFI-IV ignition system. The SPOUT connector is unplugged when ignition timing is being set. Continued
Chrysler Distributor Ignition The first domestic manufacturer to produce electronic ignition as standard equipment. The Chrysler system consists of a pulse generator unit in the distributor (pickup coil and reluctor). Chrysler’s name for their electronic ignition is electronic ignition system ( EIS ), and the control unit (module) is called the electronic control unit ( ECU ). The pickup coil in the distributor (pulse generator) generates the signal to open and close the primary coil circuit. See figure 56–20.
Figure 56–20 A Chrysler electronic ignition distributor. This unit is equipped with a vacuum advance mechanism that advances the ignition timing under light engine load conditions.
WASTE SPARK IGNITION
Waste - spark ignition is another name for distributorless ignition system ( DIS ) or electronic ignition ( EI ). Waste-spark ignition was introduced in the mid-1980s and uses the onboard computer to fire the ignition coils. First used on some Saabs and General Motors engines. A four-cylinder engine uses two ignition coils and a six-cylinder engine three coils. Each is a true transformer in which the primary winding and secondary winding are not electrically connected. Each end of the secondary winding is connected to a cylinder exactly opposite the other in the firing order, which is called a paired cylinder . See Figure 56–21.
Figure 56–21 A waste-spark system fires one cylinder while its piston is on the compression stroke and into paired or companion cylinders while it is on the exhaust stroke. In a typical engine, it requires only about 2 to 3 kV to fire the cylinder on the exhaust strokes. The remaining coil energy is available to fire the spark plug under compression (typically about 8 to 12 kV).
This means both spark plugs fire at the same time. When one cylinder (eg. 6) is on the compression stroke, the other cylinder (3) is on exhaust.
The spark that occurs on the exhaust stroke is called the waste spark , because it does no useful work and is only used as a ground path for the secondary winding of the ignition coil. Continued
The voltage required to jump the spark plug gap on cylinder 3 (the exhaust stroke) is only 2 to 3 kV and provides the ground circuit for the secondary coil circuit. The remaining coil energy is used by the cylinder on compression stroke. One spark plug of each pair always fires straight polarity and the other cylinder always fires reverse polarity. Spark plug life is not greatly affected by the reverse polarity. If there is only one defective spark plug wire or spark plug, two cylinders may be affected.
Coil polarity is determined by the direction the coil is wound (left-hand rule for conventional current flow) and cannot be changed. Each spark plug for a particular cylinder always will be fired either with straight or reversed polarity, depending on its location in the engine and how the coils are wired.
Figure 56–22 The left-hand rule states that if a coil is grasped with the left hand, the fingers will point in the direction of current flow and the thumb will point toward the north pole. Continued When a cylinder is on the compression stroke, such as number 1, the paired cylinder (number 4) is on the exhaust stroke.
During the next rotation of the crankshaft, cylinder 4 is on the compression stroke and number 1 on the exhaust stroke
Continued NOTE: With a distributor-type ignition system, the coil has two air gaps to fire: one between the rotor tip and the distributor insert (not under compression forces) and the other in the gap at the firing tip of the spark plug (under compression forces). A DIS also fires two gaps: one under compression (compression stroke plug) and one not under compression (exhaust stroke plug). Cylinder 1 —Always fires straight polarity, one time, requiring 10 to 12 kV and one time, requiring 3 to 4 kV. Cylinder 4 —Always fires reverse polarity, one time, requiring 10 to 12 kV and one time, requiring 3 to 4 kV.
On odd-numbered cylinders (1, 3, 5), the platinum is on the center electrode.
On even-numbered cylinders (2, 4, 6), the platinum is on the ground electrode.
Waste-spark ignition systems fire two spark plugs at the same time. Most vehicle manufacturers use a waste-spark system that fires the odd-numbered cylinders (1, 3, and 5) by straight polarity (current flow from the top of the spark plug through the gap and to the ground electrode). The even-numbered cylinders (2, 4, and 6) are fired reverse polarity, meaning that the spark jumps from the side electrode to the center electrode. Some vehicle manufacturers equip their vehicles with platinum plugs with the expansive platinum alloy only on one electrode as follows: Replacement plugs use platinum on both electrodes (double platinum) and can, therefore, be placed in any cylinder location. Odds Fire Straight Continued
Waste-spark ignitions require a sensor (usually a crankshaft sensor) to trigger the coils at the correct time. See Figure 56–23. The crankshaft sensor cannot be moved to adjust ignition timing. Ignition timing is not adjustable. The slight adjustment of the crankshaft sensor is designed to position the sensor exactly in the middle of the rotating metal disc for maximum clearance. Some engines do not use a camshaft position sensor, but rather double Hall-effect crankshaft sensors and again, ignition timing is not adjustable.
Figure 56–23 Typical Ford EDIS 4-cylinder ignition system. The crankshaft sensor, called a variable-reluctance sensor (VRS), sends crankshaft position and speed information to the EDIS module. A modified signal is sent to the computer as a profile ignition pickup (PIP) signal. The PIP is used by the computer to calculate ignition timing, and the computer sends a signal back to the EDIS module as to when to fire the spark plug. This return signal is called the spark angle word (SAW) signal.
IGNITION CONTROL CIRCUITS
Ignition control ( IC ) is OBD-II terminology for the output signal from the PCM to the ignition system that controls engine timing. Previously, each manufacturer used a different term to describe this signal. Ford referred to this signal as spark output ( SPOUT ); GM referred to it as electronic spark timing ( EST ). This signal is now referred to as the ignition control (IC) signal. The signal is usually digital output sent to the ignition system as a timing signal. If the system is equipped with an ignition module, this signal is used by the ignition module to vary timing as engine speed and load changes.
If the PCM directly controls the coils, such as most coil-on-plug ignition systems, then this IC signal directly controls the coil primary and there is a separate IC signal for each ignition coil. The IC signal controls the time that the coil fires; it either advances or retards the timing. On many systems, this signal controls the duration of the primary current flow in the coil, which is referred to as the dwell .
BYPASS IGNITION CONTROL
A bypass ignition is common on GM distributor ignition (DI) and waste-spark ignition engines. The engine starts using the ignition module for timing, and switches to the PCM after engine start.
Continued Figure 56–24 Typical wiring diagram of a V-6 distributorless (direct fire) ignition system.
The bypass circuit-includes four wires:
Tach reference (purple/white) This wire comes from the ignition control (IC) module; used by the PCM as engine speed information.
Ground (black/white) Used to ensure that both PCM and ignition control module share the same ground.
Bypass (tan/black) Conducts a 5-volt DC signal from PCM to the ignition control module to switch the timing control from the module to the PCM.
EST (ignition control) (white wire) The ignition timing control signal from the PCM to the ignition control module.
NOTE: It is this bypass wire that is disconnected before the ignition timing can be set on many GM engines equipped with distributor ignition.
Diagnosing a Bypass Ignition System An advantage of a bypass ignition is that the engine will run without the computer because the module can do coil switching and can, through circuits in the module, provide some spark advance as engine speed increases. This is a safety feature that helps protect the catalytic converter if the ignition control from the PCM is lost. If there is a problem, use a digital meter and check for the presence of 5 volts on the tan bypass wire. If there is not 5 volts present with the engine running, then the PCM or the wiring is at fault.
Up-Integrated Ignition Control Most coil-on-plug and many waste-spark-type ignition systems use the PCM for ignition timing control. This type of ignition control is called up - integrated because all timing functions are interpreted in the PCM, rather than being split between the ignition control module and the PCM. The ignition module, if even used, contains the power transistor for coil switching. The signal as to when the coil fires, is determined and controlled from the PCM. Unlike a bypass ignition control circuit, it is not possible to separate the PCM from the ignition coil control to help isolate a fault.
Some waste-spark ignition systems, such as on Saturns, use the voltage required to fire the cylinders to determine cylinder position. It requires higher voltage to fire a plug under compression than when the spark plug is fired on the exhaust stroke. The electronics can detect which of the two cylinders fired at the same time requires higher voltage, indicating the cylinder on compression. Engines equipped with compression - sensing ignition systems do not require use of a camshaft position sensor to determine cylinder number.
Coil-on-plug ( COP ) ignition is also called coil - by - plug , coil - near - plug , or coil - over - plug ignition. One ignition coil is used for each spark plug.
Continued Figure 56–25 A coil-on-plug ignition system. This system eliminates spark plug wires, often sources of electromagnetic interference ( EMI ) that can cause problems to some computer signals. See Figure 56–26.
Figure 56–26 A typical coil-on-plug (COP) ignition system on a V-8 with a separate coil for each cylinder. Continued
Figure 56–27 Individual coils with modules shown on the General Motors 4.2-L inline 6-cylinder light-truck engine. Note the aluminum cooling fins (heat sink) on top of each assembly. The vehicle computer controls the timing of the spark. Ignition timing also can be changed (retarded or advanced) on a cylinder-by-cylinder basis for maximum performance and to respond to knock sensor signals.
There are two basic types of coil-on-plug ignition, including:
Two-wire This design uses the vehicle computer to control the firing of the ignition coil. The two wires include ignition voltage feed and the pulse ground wire, which is controlled by the computer. All ignition timing and dwell control are handled by the computer.
Three-wire This design includes an ignition module at each coil. The three wires include:
Pulse from the computer to the built-in module
Many V-8s use a coil-near-plug system. Short secondary ignition spark plug wires are used to connect the output terminal of the ignition coil to the spark plug.
Newer Chrysler engines use coil-over-plug-type ignition systems. Each coil is controlled by the PCM, which can vary ignition timing separately for each cylinder based on signals the PCM receives from the knock sensor(s).
Ignition systems produce a high-voltage pulse necessary to ignite a lean air-fuel mixture. If you disconnect a spark plug wire when the engine is running, this high-voltage spark could cause personal injury or damage to the ignition coil and/or ignition module. Never Disconnect a Spark Plug Wire When the Engine is Running! If the knock sensor detects that a spark knock has occurred after firing cylinder 3, then the PCM will continue to monitor cylinder 3 and retard timing on just this one cylinder if necessary to prevent engine-damaging detonation.
In an ion - sensing ignition system, the spark plug itself becomes a sensor. The ignition control (IC) module applies a voltage of about 100 to 400 volts DC across the spark plug gap after the ignition event to sense the plasma inside the cylinder. See Figure 56–28. The coil discharge voltage (10 to 15 kV) is electrically isolated from the ion-sensing circuit. The combustion flame is ionized and will conduct some electricity, which can be accurately measured at the spark plug gap.
Figure 56–28 A DC voltage is applied across the spark plug gap after the plug fires and the circuit can determine if the correct air–fuel ratio was present in the cylinder and if knock occurred. Continued
The purpose of this circuit includes:
Misfire detection (required by OBD-II regulations)
Knock detection (eliminates the need for a knock sensor)
Ignition timing control (to achieve the best spark timing for maximum power with lowest exhaust emissions)
Exhaust gas recirculation (EGR) control
Air–fuel ratio control on an individual cylinder basis
Ion-sensing ignition systems still function the same as conventional coil-on-plug designs, but the engine does not need to be equipped with a camshaft position sensor for misfire detection, or a knock sensor because both of these faults are achieved using the electronics inside the ignition control circuits.
Ignition timing refers to when the spark plug fires in relation to piston position. When spark occurs depends on engine speed, and must be advanced as the engine rotates faster. The ignition in the cylinder takes about 30 ms (3/1000 second). This time is relatively constant throughout the engine speed range. For maximum efficiency from expanding gases in the combustion chamber, burning of the air–fuel mixture should end by about 10° after top dead center (ATDC). If burning is still occurring after that point, the expanding gases do not exert much force on the piston because it is moving away from the gases (the gases are “chasing” the piston).
To achieve the goal of having the air–fuel mix completely burned by the time the piston reaches 10° after top dead center, the spark must be advanced (occur sooner) as engine speed increases. Timing advance is controlled by the PCM on most vehicles.
Figure 56–29 Ignition timing marks are found on the harmonic balancers that are equipped with distributor ignition. Continued
If the engine is equipped with a distributor, it may be possible to adjust the base or the initial timing . The initial timing is usually set to fire the spark plug between zero degrees (top dead center or TDC) or slightly before TDC (BTDC).
Ignition timing changes as the timing chain or gear wears. Adjustment is often necessary on high-mileage engines. Waste-spark and coil-on-plug ignitions can’t be adjusted. Figure 56–30 The initial timing is where the spark plug fires at idle speed. The computer then advances the timing based on engine speed and other factors. Continued
Knock sensors detect abnormal combustion, called ping , spark knock , or detonation . When this occurs, rapid pressure increase occurs in the cylinder, creating vibration in the block, detected by the knock sensor. The PCM uses the signal from the knock sensor ( KS ) to retard ignition timing until knock is eliminated, reducing damaging effects of the abnormal combustion on pistons and other engine parts. Inside the sensor is a piezoelectric element that generates a voltage when pressure or a vibration is applied. The sensor is tuned to a range from 5 kHz to 10 kHz, depending on engine design. The signal from the knock sensor is sent to the PCM, which retards ignition timing until the knocking stops.
Diagnosing the Knock Sensor If a knock sensor diagnostic trouble code (DTC) is present, follow specified testing procedure. A scan tool can be used to check the operation of the knock sensor, using the following procedure:
Continued Step #1 Start the engine and connect a scan tool to monitor ignition timing and/or knock sensor activity. Step #2 Create a simulated engine knocking sound by tapping on the engine block or cylinder head with a soft-faced mallet. Step #3 Observe the scan tool display. The vibration from the tapping should have been interpreted by the knock sensor as a knock, resulting in a knock sensor signal and a reduction in the spark advance.
Figure 56–31 A typical waveform from a knock sensor during a spark knock event. This signal is sent to the computer which in turn retards the ignition timing. The timing retard is accomplished by an output command from the computer to either a spark advance control unit or directly to the ignition module.
A knock sensor can be tested using a digital storage oscilloscope.
Replacing a Knock Sensor If replacing a knock sensor, be sure to purchase the exact replacement needed, because they often look the same, but the frequency range can vary according to engine design, as well as where it is located on the engine. Always tighten the knock sensor using a torque wrench and tighten to the specified torque to avoid causing damage to the piezoelectric element inside the sensor.
NOTE: Some engine computers are programmed to ignore knock sensor signals when the engine is at idle speed to avoid having noise from a loose accessory drive belt, or other accessory, interpreted as engine knock. Always follow the manufacturer’s recommended testing procedure.
Spark plugs are made from ceramic insulators inside a steel shell.
Figure 56–32 Parts of a typical spark plug. The threads of the shell are rolled. A seat is formed to create a gastight seal with the cylinder head. Continued
The physical difference in spark plugs includes:
Reach This is the length of the threaded part of the plug.
Heat range The heat range of the spark plug refers to how rapidly the heat created at the tip is transferred to the cylinder head. A plug with a long ceramic insulator path will run hotter at the tip than a spark plug that has a shorter path because the heat must travel further.
Type of seat Some spark plugs use a gasket and others rely on a tapered seat to seal.
See Figure 56-33
Figure 56–33 The heat range of a spark plug is determined by the distance the heat has to flow from the tip to the cylinder head. Continued
Resistor Spark Plugs Most spark plugs include a resistor in the center electrode, which helps to reduce electromagnetic noise or radiation from the ignition system. The closer the resistor is to the actual spark or arc, the more effective it becomes. The value of the resistor is usually between 2,500 ohms and 7,500 ohms.
Platinum Spark Plugs A small amount of the precious metal platinum welded onto the end of the center electrode gives platinum spark plugs their name. Platinum is a grayish-white metal that does not react with oxygen and will not erode away as can occur with conventional nickel alloy spark plug electrodes. Platinum is also used as a catalyst in catalytic converters where it is able to start a chemical reaction without itself being consumed.
Iridium Spark Plugs Iridium is a white precious metal and is the most corrosion-resistant metal known. Most iridium spark plugs use a small amount of iridium welded onto the tip of a small center electrode 0.0015 to 0.002 inch (0.4 to 0.6 mm) in diameter. The small diameter reduces the voltage required to jump the gap between the center and the side electrode, thereby reducing possible misfires. The ground or side electrode is usually tipped with platinum to help reduce electrode gap wear.
All inductive ignition systems supply battery voltage to the positive side of the ignition coil and pulse the negative side of the coil on and off to ground to create a high-voltage spark.
If an ignition system uses a distributor, it is a distributor ignition (DI) system.
If an ignition system does not use a distributor, it is called an electronic ignition (EI) system.
A waste-spark ignition system fires two spark plugs at the same time.
A coil-on-plug ignition system uses an ignition coil for each spark plug.