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    Ch10 Ch10 Presentation Transcript

    •  
    • OBJECTIVES
      • After studying Chapter 10, the reader should be able to:
          • Prepare for ASE Engine Performance (A8) certification test content area “B” (Ignition System Diagnosis and Repair).
          • Explain how ignition coils create 40,000 volts.
          • Discuss crankshaft position sensor and pickup coil operation.
          • Describe the operation of waste-spark or coil-on-plug ignition systems.
    • IGNITION SYSTEM OPERATION
      • All ignition systems apply battery voltage to the positive side of the ignition coil and pulse the negative side to ground, except for capacitor-discharge systems used on some vehicles. When the coil negative lead is grounded, the primary (low-voltage) circuit of the coil is complete and a magnetic field is created by the coil windings.
      (Continued)
      • When the circuit is opened, the magnetic field collapses and induces a high voltage in the secondary winding of the ignition coil, which is used to generate a high-voltage arc at the spark plug.
      • Electronic ignition uses a sensor such as a pickup coil or trigger to signal an electronic module that makes and breaks the primary connection of the ignition coil.
      (Continued)
        • 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.
    • IGNITION COILS
      • The heart of any ignition system is the ignition coil. The coil creates a high-voltage spark by electromagnetic induction. Many ignition coils for DI or some coil-on-plug (COP) systems contain two separate but electrically connected windings of copper wire.
      (Continued)
      • Other coils are true transformers in which the primary and secondary windings are not electrically connected.
      (Continued)
      • The center of an ignition coil contains a core of laminated soft iron (thin strips of soft iron). This core increases the magnetic strength of the coil. Surrounding the laminated core are approximately 20,000 turns of fine wire (approximately 42 gauge). These windings are called the secondary coil windings.
      (Continued)
      • Surrounding the secondary windings are approximately 150 turns of heavy wire (approximately 21 gauge). These windings are called the primary coil windings. The secondary winding has about 100 times the number of turns of the primary winding, referred to as the turn ratio (approximately 100:1).
      (Continued)
      • 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.
      (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 electronic ignition module (or igniter), which opens and closes the primary ignition circuit by opening or closing the ground return path of the circuit.
      (Continued)
      • 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. The polarity of an ignition coil is determined by the direction of rotation of the coil windings.
      (Continued)
      • The coil output voltage is directly proportional to the ratio of primary to secondary turns of wire used in the coil.
      • 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.
      (Continued)
        • Therefore, 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. The point at which a coil's maximum magnetic field strength is reached is called saturation.
      (Continued)
      • 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.
      (Continued)
        • Therefore, 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. Generating 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.
      (Continued)
      • How Ignition Coils Create 40,000 Volts
        • If the primary circuit is completed, current (approximately 2 to 6 A) can flow through the primary coil windings. This flow creates a strong magnetic field inside the coil.
      (Continued)
        • 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 (if the vehicle is so equipped), distributor cap, rotor, and spark plug wires to the spark plugs.
      (Continued)
      • Primary Ignition Circuit
        • Battery
        • Ignition switch
        • Primary windings of coil
        • Pickup coil (crank sensor) (module trigger)
        • Ignition module (igniter) (coil primary circuit switch)
      (Continued)
      • Secondary Ignition Circuit
        • Secondary windings of coil
        • Distributor cap and rotor (if the vehicle is so equipped)
        • Spark plug wires
        • Spark plugs
        • Married. This coil design is also called a tapped transformer design.
      (Continued)
        • Divorced. These windings are also called a true transformer design and are used by most waste-spark ignition coils to keep both the primary and secondary winding separated.
      (Continued)
    • Figure 10-1 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.
    • Figure 10-2 Typical air-cooled epoxy-filled E coil.
    • Figure 10-3 Cutaway of a General Motors Type II distributorless ignition coil. Note that the primary windings are inside of the secondary windings.
    • Figure 10-4 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.
    • Figure 10-5 A tapped (married) type of ignition coil where the primary winding is tapped (connected) to the secondary winding.
    • 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, including:
      (Continued)
        • Pickup coil (pulse generator).
          • 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).
        • Hall-effect switch.
          • This switch also uses a stationary sensor and rotating trigger wheel (shutter).
      (Continued)
          • Hall effect is the ability to generate a voltage signal in a 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.
          • 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 (another method uses a stationary sensor with a rotating magnet).
      (Continued)
          • 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, thereby creating an analog voltage signal. The Hall element contains a logic gate that converts the analog signal into a digital voltage signal, which triggers the switching transistor. The transistor transmits a digital square waveform at varying frequency to the ignition module or on-board computer.
      (Continued)
        • Magnetic crankshaft position sensors.
          • This 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).
      (Continued)
        • Optical sensors.
          • These use light from an LED and a phototransistor to signal the computer. An interruptor 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.
      (Continued)
      • Optical distributors use the light emitted from LEDs to trigger phototransistors. Most optical distributors use a shield between the distributor rotor and the optical interrupter ring.
      • If this shield is not replaced during service, the light signals are reduced and the engine may not operate correctly.
      (Continued)
    • Figure 10-6 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.
    • Figure 10-7 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.
    • Figure 10-8 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.
    • Figure 10-9 Shutter blade of a rotor as it passes between the sensing silicon chip and the permanent magnet.
    • Figure 10-10 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.
    • Figure 10-11 A typical magnetic crankshaft position sensor.
    • Figure 10-12 (a) Typical optical distributor. (b) A cylinder 1 slit signals the computer the piston position for cylinder 1. The 1-degree slits provide accurate engine speed information to the computer.
    • Figure 10-13 (a) An optical distributor on a Nissan 3.0-L V-6 shown with the light shield removed. A
    • Figure 10-13 (b) A light shield being installed before the rotor is attached. B
    • DISTRIBUTOR IGNITION
      • General Motors HEI Electronic Ignition
        • Some HEI models use an ignition coil inside the distributor cap and some use an externally mounted ignition coil. The operation of both styles is similar. The large-diameter distributor cap provides additional space between the spark plug connections to help prevent crossfire.
      (Continued)
        • HEI coils differ and can be identified by the colors of the primary leads. The primary coil leads can be either white and red or yellow and red. 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 therefore its polarity.
      (Continued)
      • Ford Electronic Ignition
        • The EEC IV system uses the thick-film-integration (TFI) ignition system. This system uses a smaller control module attached to the distributor, as well as an air-cooled epoxy E coil.
        • While most TFI modules were mounted on the side of the distributor, some Ford modules remotely mounted it away from engine heat.
      (Continued)
        • Under the distributor cap and rotor is a magnetic pickup assembly. This assembly produces a small alternating electrical pulse (approximately 1.5 volts) when the distributor armature rotates past the pickup assembly (stator). This low-voltage pulse is sent to the ignition module. The ignition module then switches (through transistors) off the primary ignition coil current.
      (Continued)
        • The coil current is controlled in the module circuits by decreasing coil-charging time depending on various factors determined by operating conditions.
        • Ford uses a Hall-effect sensor in the distributor on most TFI module equipped engines. The sensors were originally coated in a black plastic that would often become soft with age and break down electrically. The soft plastic sensor would also prevent proper connection to the TFI module
      (Continued)
      • DaimlerChrysler Distributor Ignition
        • The DaimlerChrysler system consists of a pulse generator unit in the distributor (pickup coil and reluctor).
        • DaimlerChrysler's name for their electronic ignition is electronic ignition system (EIS), and the control unit (module) is called the electronic control unit (ECU).
      (Continued)
        • The pickup coil in the distributor (pulse generator) generates the signal to open and close the primary coil circuit.
      (Continued)
    • Figure 10-14 An HEI distributor.
    • Figure 10-15 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, used red and yellow wire for the coil primary.
    • Figure 10-16 This unit uses a remotely mounted ignition coil.
    • Figure 10-17 Wiring diagram of a typical Ford electronic ignition.
    • Figure 10-18 Schematic of a Ford TFI-IV ignition system. The SPOUT connector is unplugged when ignition timing is being set.
    • Figure 10-19 Thick film-integrated type of Ford EI. Note how the module plugs into the Hall-effect switch inside the distributor. Heat-conductive silicone grease should be used between the module and the distributor mounting pad to help keep the electronic circuits inside the module coil.
    • Figure 10-20 A DaimlerChrysler 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 SYSTEMS
      • Waste-spark ignition is another name for distributorless ignition system (DIS) or electronic ignition (EI).
        • Each end of the secondary winding is connected to a cylinder exactly opposite the other in the firing order, which is called a companion cylinder or paired cylinder.
      (Continued)
        • This means that both spark plugs fire at the same time. When one cylinder (for example, 6) is on the compression stroke, the other cylinder (3) is on the exhaust stroke. 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 paired cylinder on its compression stroke. One spark plug of each pair fires straight polarity (from the top of the spark plug to the ground electrode) and the other cylinder fires reverse polarity (from the ground electrode to the center electrode).
      (Continued)
        • The coil polarity is determined by the direction the coil is wound (left-hand rule for conventional current flow) and cannot be changed.
        • When one cylinder is on compression, such as cylinder number 1, then the paired cylinder (number 4) is on the exhaust stroke. During the next rotation of the crankshaft, cylinder number 4 is on the compression stroke and cylinder number 1 is on the exhaust stroke.
      (Continued)
          • 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.
      (Continued)
        • Most vehicle manufacturers use a waste-spark system that fires the odd number 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.
      (Continued)
        • Some vehicle manufacturers equip their vehicles with platinum plugs with the expensive platinum alloy only on one electrode as follows:
          • 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.
      (Continued)
        • Replacement spark plugs use platinum on both electrodes and can, therefore, be placed in any cylinder location.
        • Waste-spark ignitions require a sensor (usually a crankshaft sensor) to trigger the coils at the correct time.
      (Continued)
    • Figure 10-21 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 12kV).
    • Figure 10-22 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.
    • Figure 10-23 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 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 the 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.
      (Continued)
      • Ford referred to this signal as spark output (SPOUT) and General Motors referred to this signal as electronic spark timing (EST). This signal is now referred to as the ignition control (IC) signal.
      (Continued)
      • 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.
      (Continued)
      • Bypass Ignition Control
        • A bypass ignition control is when the engine starts using the ignition module for timing control and then switches to the PCM for timing control after the engine starts. A bypass ignition is commonly used on General Motors engines equipped with distributor ignition (DI), as well as those equipped with waste-spark ignition.
      (Continued)
        • The bypass circuit includes four wires:
          • Tach reference (purple/white). This wire comes from the ignition control (IC) module and is used by the PCM as engine speed information.
          • Ground (black/white). This ground wire is used to ensure that both the PCM and the ignition control module share the same ground.
          • Bypass (tan/black). This wire is used to conduct a 5-volt DC signal from the PCM to the ignition control module to switch the timing control from the module to the PCM.
      (Continued)
          • EST (ignition control) (white wire). This is the ignition timing control signal from the PCM to the ignition control module.
      (Continued)
      • Diagnosing a Bypass Ignition System
        • One advantage of a bypass-type of ignition is that the engine will run without the computer because the module can do the coil switching and can, through electronic circuits inside the module, provide for some spark advance as the engine speed increases.
      (Continued)
      • 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.
        • Unlike a bypass ignition control circuit, it is not possible to separate the PCM from the ignition coil control to help isolate a fault.
      (Continued)
    • Figure 10-24 Typical wiring diagram of a V-6 distributorless (direct fire) ignition system. The computer applies 5 volts to the bypass wire when the engine starts. This signal changes the ignition timing function from the module to the computer (PCM).
    • COMPRESSION SENSING IGNITION
      • The electronics in the coil and the PCM can detect which of the two cylinders that are fired at the same time requires the higher voltage, which indicates the cylinder on the compression stroke. For example, a typical 4-cylinder engine equipped with a waste-spark ignition system will fire both cylinders 1 and 4.
      (Continued)
      • If cylinder number 4 requires a higher voltage to fire, as determined by the electronics connected to the coil, then the PCM assumes that cylinder number 4 is on the compression stroke. Engines equipped with compression-sensing ignition systems, such as Saturns, do not require the use of a camshaft position sensor to determine cylinder number.
    • COIL-ON-PLUG IGNITION
      • Coil-on-plug (COP) ignition uses one ignition coil for each spark plug. This system is also called coil-by-plug, coil-near-plug, or coil-over-plug ignition.
      (Continued)
      • 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.
      (Continued)
      • There are two basic types of coil-on-plug ignition including:
        • 2-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.
      (Continued)
        • 3-wire . This design includes an ignition module at each coil. The three wires include:
          • Ignition voltage
          • Ground
          • Pulse from the computer to the built-in module
      (Continued)
      • Most newer Ford, DaimlerChrysler, and many other engines use coil-over-plug-type ignition systems. Each coil is controlled by the PCM, which can vary the ignition timing separately for each cylinder based on signals the PCM receives from the knock sensor(s).
      (Continued)
      • Ford Triple Strike Ignition
        • Ford uses a coil-on-plug ignition system that is capable of firing three times instead of the normal single spark. This “triple strike” event only occurs when the engine is at idle speed and is used to improve combustion efficiency.
      (Continued)
        • As a result of the improved combustion, the engine produces fewer exhaust emissions and has a better idle quality. Above idle speed, it reverts back to single-strike ignition operation.
      (Continued)
    • Figure 10-25 A coil-on-plug ignition system.
    • Figure 10-25 (continued) A coil-on-plug ignition system.
    • Figure 10-26 A typical coil-on-plug (COP) ignition system on a V-8 with a separate coil for each cylinder.
    • Figure 10-27 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.
    • Figure 10-28 An ignition oscilloscope waveform, showing a Ford triple strike ignition system operation.
    • ION-SENSING IGNITION
      • 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 to sense the plasma inside the cylinder. The coil discharge voltage (10 to 15 kV) is electrically isolated from the ion-sensing circuit.
      (Continued)
      • The combustion flame is ionized and will conduct some electricity, which can be accurately measured at the spark plug gap. The purposes of this circuit include:
        • Misfire detection (required by OBD-II regulations)
        • Knock detection (eliminates the need for a knock sensor)
      (Continued)
        • 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
      (Continued)
      • 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, or a knock sensor, because both of these faults are achieved using the electronics inside the ignition control circuits.
    • IGNITION TIMING
      • Ignition timing refers to when the spark plug fires in relation to piston position.
      • The ignition in the cylinder takes a certain amount of time, usually 3 ms (0.003 or 3/1000 of a second). This burning time is relatively constant throughout the entire engine speed range.
      (Continued)
      • For maximum efficiency from the expanding gases inside the combustion chamber, the burning of the air-fuel mixture should end by about 10 。 after top dead center. If the burning of the mixture 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.
      (Continued)
      • This timing advance is determined and controlled by the PCM on most vehicles.
      (Continued)
    • Figure 10-29 Ignition timing marks are found on the harmonic balancers that are equipped with distributor ignition.
    • INITIAL TIMING
      • 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).
      (Continued)
      • Ignition timing does change as the timing chain or gear wears and readjustment is often necessary on high-mileage engines.
      (Continued)
    • Figure 10-30 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.
    • KNOCK SENSORS
      • Knock sensors are used to detect abnormal combustion, often called ping, spark knock, detonation, or simply knock. Whenever abnormal combustion occurs, a rapid pressure increase occurs in the cylinder, creating a noise. It is this vibration that is detected by the knock sensor.
      (Continued)
      • The signal from the knock sensor is used by the PCM to retard the ignition timing until the knock is eliminated, thereby reducing the damaging effects of the abnormal combustion on pistons and other engine parts.
      (Continued)
      • Inside the knock sensor is a piezoelectric element that generates a voltage when pressure or a vibration is applied to the unit. The knock sensor is tuned to the engine knock frequency, which is a range from 5 kHz to 10 kHz, depending on the engine design.
      (Continued)
      • The voltage signal from the knock sensor (KS) is sent to the PCM. The PCM retards the ignition timing until the knocking stops.
      (Continued)
      • Diagnosing the Knock Sensor
        • If a knock sensor diagnostic trouble code (DTC) is present, follow the specified testing procedure in the service information. A scan tool can be used to check the operation of the knock sensor, using the following procedure.
          • Step 1: Start the engine and connect a scan tool to monitor ignition timing and/or knock sensor activity.
      (Continued)
          • 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.
          • A knock sensor can also be tested using a digital storage oscilloscope.
      (Continued)
      • 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.
      (Continued)
    • Figure 10-31 A knock sensor can be tested using a digital storage oscilloscope by first disconnecting the sensor lead from the vehicle computer and connecting the probe to the sensor. Also be sure the probe ground is attached to a good clean ground. (Courtesy of Fluke Corporation)
    • Figure 10-32 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. This timing retard is accomplished by an output command from the computer to either a spark advance control unit or directly to the ignition module. (Courtesy of Fluke Corporation)
    • SPARK PLUGS
      • Spark plugs are manufactured from ceramic insulators inside a steel shell. The threads of the shell are rolled and a seat is formed to create a gas-tight 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.
      (Continued)
        • Type of seat . Some spark plugs use a gasket and others rely on a tapered seat to seal.
      (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 2500 ohms and 7500 ohms.
      (Continued)
      • Platinum Spark Plugs
        • Platinum spark plugs have a small amount of the precious metal platinum included onto the end of the center electrode, as well as on the ground or side electrode. Platinum is a grayish-white metal that does not rest with oxygen and therefore, will not erode away as can occur with conventional nickel alloy spark plug electrodes.
      (Continued)
      • 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.
      (Continued)
    • Figure 10-33 Parts of a typical spark plug.
    • Figure 10-34 The heat range of a spark plug is determined by the distance the heat has to flow from the tip to the cylinder head.