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ine service jobs require that you know the cylinder numbering and firing order. Some engines have cylinder numbering identification, firing order, and direction of ignition-distributor rotation cast into or imprinted on the intake manifold. The information is also in the manufacturer’s service manual. The complete firing order of a four-cycle engine represents two complete revolutions of the crankshaft. This is 720 degrees of crankshaft rotation. Most engines are “even firing “. This means, for example, that is an in-line six-cylinder engine a firing impulse occurs every 120 degrees of crankshaft rotation (720 ÷ 6 = 120). The firing order of this engine is 1-5-3-6-2-4. When piston number 1 is at TDC on the end of the compression stroke, piston number 6 is at TDC on the end of the exhaust stroke. To determine the two pistons that are moving up and down together ( piston pairs ), divide the firing order in half. Then place the second half under the first half : 1-5-3 6-2-4 The piston pairs for this inline six-cylinder engine are 1 and 6 , 5 and 2, 3 and 4 . 19-1 Introduction to gasoline fuel-injection systems Most 1980 and later cars have an electronic engine control (EEC) system. It controls the ignition and fuel-injection systems. The basic operation of electronic engine controls is described in chap 10. The fuel-injection system supplies the engine with a combustible air-fuel mixture. It varies the richness of the mixture to suit different operating conditions. When a cold engine is started, the fuel system delivers a very rich mixture. This has a high proportion of fuel. After the engine warms up, the fuel system “ leans out “ the mixture. It then has a lower proportion of fuel. For acceleration and high speed, the mixture is again enriched. There are two types of gasoline fuel-injection systems: Port fuel injection (PFI) which has an injection valve or fuel injector in each intake port (fig 19-1). Throttle-body fuel injection (TBI) in which one or two fuel injectors are located above the throttle valves ( fig 19-2). With either system, the electric fuel pump supplies the fuel injectors with fuel under pressure. As soon as the injector opens, fuel sprays out ( fig 19-3 ). An electric solenoid in the injection opens and closes the valve. The solenoid has a small coil of wire that becomes magnetized when the voltage is applied ( fig 19-4 ). The magnetism lifts the armature which raises the needle valve or pintle off its seat. Fuel sprays out as long as the pintle is raised. When the voltage stops, the coil loses its magnetism. The closing spring pushes the pintle back down onto its seat. This stops the fuel spray. Each opening and closing of the injector pintle is an injector pulse. Note : some injectors use a ball valve instead of a needle valve. Operation of the ball-type injector is basically the same as described above. 19-3 Electronic fuel injection Figure 10-19 shows the components of an electronic fuel injection (EFI) system. Most fuel-injection systems are electronically controlled. The controller is an electronic control module (ECM) or electronic control unit (ECU). It is also called an “ on-board computer“ because it is “on-board“ the car. Various components of the engine and fuel system send electric signals to the ECM (fig 19-5). The ECM continuously calculates how much fuel to inject. It then opens the fuel injectors so the proper amount of fuel sprays out to produce the desired air-fuel ratio. 19-6 Air and fuel metering The fuel system must accurately measure or meter the air and fuel entering the engine. This produces the proper air-fuel ratio to make a combustible mixture. A mixture that is too lean (not enough fuel in it) will not burn and produces excessive pollutants. A mixture that is too rich (excess fuel in it) will also produce excess pollutants. Figure 19-8 shows how mixture richness affects engine power. As the mixture becomes leaner, power falls off. The electronic engine control system includes the ECM and various sensing devices or sensors that report to it. A sensor is a device that receives and reacts to a signal. This may be a change in pressure, temperature, or voltage. Some sensors report the amount of air entering. The ECM then calculates for how long to open the injectors. 19-7 Operaion of fuel-injection systems Sensors that report to the ECM include ( fig 19-5) Engine speed. Throttle position. Intake-manifold vacuum or manifold-absolute pressure (MAP). Engine coolant temperature. Amount and temperature of air entering engine. Amount of oxygen in exhaust gas. Atmospheric pressure. The ECM continuously receives all this information or data. The ECM checks this data with other data stored in look-up tables in its memory. Then the ECM decides when to open the injectors and for how long (fig 19-9). For example, when the engine is idling, the ECM might hold the injectors open for only 0.003 second each time they open . The opening and closing of an injector is its duty cycle. How long the ECM signals the injector to remain open is the injector pulse width. Figure 19-9 shows how varying the pulse width varies the amount of fuel injected. Suppose more fuel is needed because the throttle has been opened for acceleration and more air is entering. Then the ECM increases the pulse width. This holds the injectors open longer each time they open to provide the additional fuel . Note: The system described above is a pulsed fuel-injection system. The injectors open and close (pulse). The continuous-injection system (CIS) is another type of fuel-injection system. It is used a few vehicles. The injectors are open continuously. Changing the pressure applied to the fuel varies the amount of fuel injected . 19-12 Indirect measurement of air flow Information about engine speed and engine load can be tell the ECM how much air is entering the engine. Using this information to regulate fuel feed is called speed-density metering. It is used in fuel-injection systems that do not directly measure mass air flow. The speed is the speed of the engine. The density is the density of the air or air-fuel mixture in the intake manifold. Throttle position (engine speed) and intake-manifold vacuum (engine load) measure air flow indirectly. Intake manifold vacuum is continuously measured by a sensor that changes vacuum (or absolute pressure) into a varying voltage signal. The ECM combines this with the TPS signal to determine how much air entering. Inputs from other sensor may cause the ECM to modify this calculation (fig 19-5 ). Engine speed (instead of throttle position) and intake-manifold vacuum can also tell the ECM how much air is entering the engine . 19-13 Measuring intake-manifold vacuum (manifold absolute pressure) Intake-manifold vacuum is measured in two ways ( fig 19-19 ): With a vacuum gauge. With a manifold absolute pressure (MAP) gauge. The two gauges are basically the same. Both have a flexible diaphragm that separates the two chambers in the gauge. The difference is that one chamber of the vacuum gauge is open to the atmosphere. One chamber of the absolute-pressure gauge contains a vacuum (fig 19-19). The vacuum gauge compares atmospheric pressure with intake-manifold pressure. In a naturally-aspirated engine, manifold pressure is less than atmospheric pressure. A vacuum gauge measures this partial vacuum in the intake-manifold . The manifold absolute-pressure (MAP) gauge compares the actual pressure in the intake manifold with a vacuum. This is more accurate than the vacuum gauge which compares intake manifold vacuum with atmospheric pressure. The vacuum gauge is less accurate because atmospheric pressure varies . Vacuum and pressure sensor are not constructed exactly like the gauges described above. But their operation is basically the same. Most electronic engine control systems include a manifold-absolute pressure (MAP) sensor (figs 10-19 and 19-20 ). It senses the pressure (vacuum) changes in the intake manifold. This information is sent as a varying voltage signal to the ECM . 19-14 Direct measurement of air flow Four methods of measuring air flow directly are vane, air-flow sensor plate, hot-wise induction, and heated film. Each continuously measures the actual amount of air flowing through the air-flow meter (fig 19-21). This information is then sent to the ECM . vane : The vane type air-flow meter is used in some pulsed fuel-injection systems such as the Bosch L system (fig 19-21). The spring-loaded vane is in the air-intake passage of the air-flow meter. Air flowing through forces the vane to swing. The more air, the farther the vane swings. A vane-position sensor works like the rotary throttle-position sensor. Depending on its position, it sends varying voltage signals to the ECM. This tells the ECM how much air is flowing through. The ECM then adjusts fuel flow to match . Air-flow sensor plate : The air flow sensor plate is used in mechanical continuous-injection systems (fig 19-14). The plate is in the intake-air passage of the air-flow meter. As air flow increases, the plate moves higher. This lifts a control plunger in the fuel distributor to allow more fuel flow to the injectors. The added fuel flow matches the additional air flow . Hot-wire induction : A platinum wire is in the path of the incoming air through the air-flow meter. The wire is kept hot by an electric current flowing through it. However, the air flow cools the wire. The more air that passes through the air-flow meter, the more heat that is lost from the wire . The system keeps the wire at a specific temperature by adjusting current flow. If more air flows through and takes more heat from the wire , the system sends more current through. This maintains the temperature. The amount of current required is therefore a measure of how much air is flowing through. The ECM reads this varying current as air flow . Heated film :The heated film consists of metal foil or nickel grid coated with a high-temperature material (fig19-22). Current flowing through the film heats it. Air flowing past the film cools it. Like the heated wire, the system maintains the film at a specific temperature. The amount of current required is a measure of air flow . 19-15 Atmospheric-pressure and air-temperature sensors Changing atmospheric pressure and air temperature change the density of the air. Air that is hot and at low atmospheric pressure is less dense. It contains less oxygen than an equal volume of cooler air under higher atmospheric pressure. When the amount of oxygen entering the engine varies, so does the amount of fuel that can be burned . Some systems include an atmospheric-pressure sensor. It is also called the barometric-pressure sensor or BARO sensor. It is similar to the MAP sensor. However, the barometric-pressure sensor reads atmospheric pressure. The air-temperature sensor (fig 19-23) is a thermistor. Its electrical resistance decreases as its temperature increases. Figure 19-21 shows its location in the vane-type air-flow meter. Both types of sensors send varying voltage signals to the ECM so it knows the atmospheric pressure and air temperature . 21-1 Purpose and types of carburetors: The carburetor (fig 21-1) is a mixing device that supplies the engine with a combustible air-fuel mixture. Figure 21-2 shows the three basic parts of a fixed-venturi carburetor.These are the air horn, the float bowl, and the throttle body . The venturi is a restricted space through which the air entering the engine must pass. The air movement produces a partial vacuum in the venturi. This is called venturi vacuum. The resulting pressure differential causes fuel to discharge from the fuel nozzle into the intake air (fig 21-3).This produces the air-fuel mixture for the engine. Some carburetors have a variable-venturi . These are described in 21-28 . 23-1 Diesel engines Diesel engines are similar to spark-ignition engines in construction. Both have pistons, with piston rings, moving up and down in cylinders. Both burn fuel in combustion chambers in the upper part of the cylinders. The high pressure produced by the burning fuel pushes the pistons down. This rotates the crankshaft and the rotary motion is carried through shafts and gears to the drive wheels. Diesel and spark-ignition engine are compared in 11-2 23-2 Diesel-engine operation Figure 23-1 shows the four piston strokes in a four-stroke-cycle diesel engine. Intake stroke : The diesel engine takes in air alone. No throttle valve impedes the airflow. In the spark-ignition engine, a mixture of air and fuel enters the engine cylinders on the intake stroke. The throttle valve controls the amount that enters. Compression stroke :In the diesel engine , the upward-moving piston compresses air alone. On the other hand, in the spark-ignition engine, the piston compresses the air-fuel mixture. Power stroke : In the diesel engine, a light oil called diesel fuel is sprayed (injected) into the compressed and hot air. The heat of compression ignites the fuel. In the spark-ignition engine, a spark at the spark plug ignites the compressed air-fuel mixture. Exhaust stroke : The exhaust stroke is the same for both engines. The exhaust valve opens and the burned gases flow out as the piston moves up the cylinder. 23-3 Diesel-engine characteristics The diesel engine has the following characteristics: No throttle valve (except some engines with the pneumatic governor described in 23-12 ). Compresses only air on the compression stroke. Heat of compression ignites fuel as it sprays into the engine cylinders. Has a high compression ratio of 16:1 to 22:1. Controls engine power and speed only by the amount of fuel sprayed into the cylinders. More fuel equals more power. Has glow plugs or an electric intake-manifold heater to make starting easier. 25-1 Heat in the engine The burning air-fuel mixture in the engine cylinders may reach 4000 oF (2200 oC) or higher. This means engine parts get hot. However, cylinder walls must not get hotter than about 500 oF (260 oC). Higher temperatures cause lubricating oil to break down and lose its lubricating ability. Other engine parts are also damaged. To prevent over-heating, the cooling system removes the excess heat (fig 15-14). This is about one-third of the heat produced in the combustion chambers by the burning air-fuel mixtrure . 25-2 Purpose of cooling system The cooling system (figs 11-23 and 25-1) keeps the engine at its most efficient temperature at all speeds and operating conditions. Burning fuel in the engine produces heat. Some of this heat must be taken away before it damages engine parts. This is one of the three jobs performed by the cooling system. It also helps bring the engine up to normal operating temperature as quickly as possible. In addition, the cooling system provides a source of heat for the passenger-compartment heater- and-air-coditioner. 27-1 The automotive electrical system The automotive electrical system (fig 27-1) does several jobs. It produces electric energy (electricity ) in the anternator. It stores electric energy in chemical form in the battery. And it delivers electric energy from these sources on demand to any other electrical component in the vehicle . The electric energy cranks the engine to start it, supplies the sparks that ignite the air-fuel mixture so the engine runs, and keeps the battery charged. These are the jobs performed by the battery, starting, charging, and ignition systems. Other electric and electronic devices and systems on the vehicle include : Electronic engine control systems and other electronic systems controlled by an electronic control module (ECM) or computer. These may include an electronic automatic transmission or transaxle, power train, brakes, traction control, steering, suspension, air conditioning, and other components that operate under varying conditions. Signaling and accessory systems. These include the lights, horn, instrument-panel indicators, service monitor systems, and other driver information systems. Also included are the heater and air conditioner, and the radio and tape player. Various motors that operate the seats, windows, door locks, trunk lid, and windshield wipers and washers . All these components use electric current and voltage. All may be computer controlled. And all are connected by insulated wires and the ground-return system. Chapter 10 describes basic electricity and the one-wire system. Chapter 19 describes electronic fuel injection and engine control system components. Separate chapters cover the battery, starting, charging, and ignition systems. Chapter 34 describes other electronic devices. 31-1 Purpose of ignition system The purpose of the ignition system (figs 11-21 and 31-1) is to ignite the compressed air-fuel mixture in the engine combustion chambers. This should occur at the proper time for combustion to begin. To start combustion, the ignition system delivers an electric spark that jumps a gap at the combustion-chamber ends of the spark plugs. The heat from this arc ignites the compressed air-fuel mixture. The mixture burns, creating pressure that pushes the piston down the cylinders so the engine runs. The ignition system may be either a contact-point ignition system or an electronic ignition system. This chapter describes the contact-point ignition system. Chapter 32 covers electronic ignition systems. Ignition system trouble-diagnosis and service are covered in chap. 33. 31-3 Producing the spark The ignition system consists of two separate but related circuits: the low-voltage primary circuit and the high-voltage secondary circuit. The ignition coil (fig 31-1) has two windings. The primary winding of few hundred turns of heavy wire is part of the primary circuit. The secondary winding of many thousand turns of fine wire is part of the secondary circuit. When the ignition key is ON and the contact points closed, current flows through the primary winding(fig 31-7). This produces a magnetic field around the primary windings in the coil . When the contact points open, current flow stops and the magnetic field collapses. As it collapses, it cuts across the thousands of turns of wire in the coil secondary winding. This produces a voltage in each turn. These add together to produce the high voltage delivered through the secondary circuit to the spark plug (fig 31-5). 31-7 Advancing the spark When the engine is idling, the spark is timed to reach the spark plug just before the piston reaches TDC on the compression stroke. At higher speeds, the spark must occur earlier. If it does not, the piston will be past TDC and moving down on the power stroke before combustion pressure reaches its maximum. The piston is ahead of the pressure rise which results in weak power stroke. This wastes much of the energy in the fuel . To better use the energy in the fuel, the spark takes place earlier as engine speed increases. This sprake advance causes the mixture to burn producing maximum pressure just as the piston moves through TDC. Most contact-point distributors have two mechanisms to control spark advance. A centrifugal-advance mechanism adjusts the spark based on the engine speed. A vacuum-advance mechanism adjusts the spark based on engine load. On the engine, both work together to provide the proper spark advance for the engine operating conditions. 31-8 Centrifugal advance The centrifugal advance mechanism advances the spark by pushing the breaker cam ahead as engine speed increases. Two advance weights, two weight springs, and a cam assembly provide this action. The cam assembly includes the breaker cam and an oval-shaped advance cam (fig 31-11). At low speed, the springs hold the weights in. As engine speed increases, centrifugal force causes the weights to overcome the spring force and pivot outward (fig 31-12). This pushes the cam assembly ahead. The contact points open and close earlier, advancing the spark . 31-9 Vacuum advance When the throttle valve is only partly open, a partial vacuum develops in the intake manifold. Less air-fuel mixture gets into the engine cylinders.Then the fuel burns slower after it is ignited. The spark must be advanced at part throttle to give the mixture more time to burn. The vacuum-advance mechanism (figs 31-8 and 31-13) advances spark timing by shifting the position of the breaker plate. The vacuum-advance unit has a diaphragm linked to the breaker plate. A vacuum passage connects the diaphragm to a port just above the closed throttle valve. When the throttle valve moves past the vacuum port, the intake-manifold pulls on the diaphragm. This rotates the breaker plate so the contact-points open and close earlier (fig 31-14). Any vacuum port above the throttle valve provides ported vacuum . 32.1 Types of electronic ignition systems By the early 1970s, most automotive engines using a contact-point distributor ( Chap. 31) could not meet exhaust-emission standards. Federal regulations required the ignition system to operate for 50.000 miles [ 80.465km] with little or no maintenance. Contact points cannot do this. They burn and wear during normal operation. This changes the point gap, which changes ignition timing and reduces spark energy. Misfiring and increased exhaust emissions result. Most 1975 and later automotive engines have an electronic ignition system (Fig. 32-1). It does not use contact points. Instead, transistors and other semiconductor devices (Chap. 10) act as an electronic switch that turns the coil primary current on and off. There are four basic types of electronic ignition systems: Distributor type with mechanical centrifugal and vacuum advance (Figs. 1-19 and 32-2). Distributor type with electronic spark advance ( Figs. 1-27 and 1-28). Distributor type with multiple ignition coils ( Figs. 1-8 and 1-13) . Distributor type with direct capactior-discharge (CD) ignition for each spark plug. 42.1 Purpose of the clutch The automotive drive train or power train ( 1-11) carries power from the engine to the drive wheels. In vehicles with a manual transmission or manual transaxle ( Chap. 43), the power flows through a clutch ( Figs. 1-19 and 42-1). This device couples and uncouples the manual transmission or transaxle and the engine. The clutch is usually operated by the driver’s foot. Some clutches have a power-assist device to reduce driver effort. Various electronic devices may be used so that the clutch operates automatically.(42-13). The clutch is located between the engine flywheel and the transmission or transaxle. Figure 42-1 shows the clutch location in a front-wheel-drive power train. This engine mounts longitudinally. Figure 42-2 shows the clutch location in a front-wheel-drive car with a transversely-mounted engine. Clutch layout in a car with front engine and rear-wheel drive is in Fig 42-3. Movement of a foot pedal operates the clutch (Figs 42-3 and 42-4). When the driver pushes the clutch pedal down, the clutch disconnects or disengages from the engine flywheel. No engine power can flow through to the transmission or transaxle. When the diver releases the clutch pedal, the clutch engages. This allows power to flow through. Note : to avoid needlessly repeating the phrase transmission or transaxle, following references generally are to the transmission. This may indicate a separate transmission or the transmission section of a manual transaxle. Transaxle is used when the reference applies only to a transaxle . 42-2 Functions of the clutch : The clutch has four functions: It can be disengaged (clutch pedal down). This allows engine cranking and permits the engine to run freely without delivering power to the transmission. While disengaged (clutch pedal down), it permits the driver to shift the transmission into various gears. This allows the driver to select the proper gear( first ,second ,third ,fourth ,fifth ,reverse ,or neutral) for the operating condition. While engaging (clutch pedal moving up), the clutch slips momentarily. This provides smooth engagement and lessens the shock on gears, shafts, and other driver train parts. As the engine develops enough torque to overcome the inertia of the vehicle, the drive wheels turn and the vehicle begins to move . When engaged ( clutch pedal up ), the clutch transmits power from the engine to the transmission. All slipping has stopped. 43-1 Purpose of transmission or transaxle : There are three reasons for having a transmission or transaxle in the automotive power train or drive train. The transmission or transaxle can: Provide the torque needed to move the vehicle under a variety of road and load conditions. It does this by changing the gear ratio between the engine crankshaft and vehicle drive wheels. Be shifted into reverse so the vehicle can move backward . Be shifted into neutral for starting the engine and running it without turning the drive wheels. There are two basic types of transmissions and transaxles: manual and automatic. Manual transmissions and transaxles are shifted manually, or by hand. Automatic transmissions and transaxles shift automatically, with no help from the driver . 43-2 Difference between transmissions and transaxles The manual transmissions (Figs , 42.1and 43.1) is an assembly of gears, shafts, and related parts. There are contained in a metal case or housing filled with lubricant (43.16). A manual transmissions is used in some front–wheel-drive vechicles (Fig , 42.1) and in front-engine rear- wheel-drive vehicles (Fif,43.2). It is positioned between the clutch (Chap , 42 ) and the driveshaft ( Chap , 45) that carries engine power to the drive wheels. The engine, clutch, transmission, and driveshaft are all in a single line . The manual transaxle ( Figs,42.2 and 43.3 ) is also an assembly of gears and shafts. It attaches to a front-mounted tranverse engine and drives the front wheels (Fig , 43.4). Rear-engine cars use engine-mounted transaxle to drive the rear wheels. A few front-engine cars drive the rear wheels through a rear-mounted transaxle. The transaxle includes a final drive and a differential (front differential in Fig . 43.3). There devices are not found in the transmission.The final drive is a set of gears that provides the final speed reduction or gear ratio (43.4) between the transmission and the drive wheels. The differential permits the drive wheels to rotate at different speeds when the vehicle turns from straight ahead. Both are described in Chap 45. Some transaxles include a viscous coupling and a center differential (Fig , 43.3). There are used in four-wheel-drive and all-wheel-drive power trains (Chap.46). 43-3 Manual transmissions and transaxles Older transmissions are three-speed units. They have three forward gear-ratios or speeds. These are first or low, second, and third or high. They also have reverse and neutral. Four-speed transmissions and transaxles have been widely used. They provide first, sencond, third, and fourth. They also have reverse and neutral. Many transmissions and transaxles are five speeds with a fifth forward gear. Fourth gear in some four-speed units and fifth gear in five-speed units is overdrive. The output shaft tunrns faster than, or overdrives, the input shaft (Fig,43.1). This allows a lower engine speed to keep the vehicle moving at its desired road speed. Better fuel economy and reduced engine wear result, with less noise and vibration. Some cars have a six-speed manual transmission (Fig , 43.1 ) or transaxle. Both fifth gear and sixth gear are overdrive ratios. However, these may not be usable during city driving in heavy traffic. The different gear ratios are nececssary because the engine develops relatively little power at low engine speeds. The engine must be turning at a fairly high speed before it can deliver enough torque to start the vehicle moving. This means the transmission or transaxle must in first gear to start out. After the vehicle is moving, progressively higher gears are selected (second, third, fourth,fifth ) to suit operating conditions. Usually, the vehicle is in top gear after reaching highway speed . Moving the gearshift lever (Fig , 42.3, and 43.1) makes the shift which changes the gear ratio( 42.3). In some vehicles, the gearshift lever is on the steering column ( 43.13). In others, it is on the floor or in a center console( Fig 34.29). 45-3 Universal joints A universal joint allows driving torque to be carried through two shafts that are at an angle with each other. Figure 45.3 shows a simple cardan universal joint. It is a double-hinged joint made of two Y-shaped yokes and a cross-shaped member or spider. One yoke is on the driving shaft, and the other is on the driven shaft. The four arms of the spider or trunnions are assembled in needle bearings in the two yokes (Fig , 45.4). The driving-shaft-and-yoke force the spider to rotate. The other two trunnions of the spider then cause the driven yoke to rotate. When the two shafts are at an angle with each other, the needle bearings permit the yokes to swing around on the trunnions with each revolution. There are several types of universal joints. The simplest is the spider-and-two-yoke design (Fig.45-3 and 45-4). However, this is not a constant-velocity universal joint. If the two shafts are at an angle, the driven shaft speeds up and slows down slightly, twice per revolution. The greater the angle, the geater the speed varies. This can cause a pulsating load that wears the bearings and gears in the drive axle. Contant-velocity universal joints or CV joints eliminate this unwanted speed change. Figure 45-1 shows a two-piece drive line with the sections connected through a double-cardan universal joint at the center. The double-cardan joint is one type of constant-velocity universal joint. It basically is two simple universal joints assembled together (Fig.45-5). They are linked by a centering ball and socket which splits the angle between the two shafts. This cancels any speed variation because the two joints operate at the same angle (half the total). The acceleration of one joint is canceled by the deceleration of the second joint. Later sections describe other types of universal joints. 45-12 Functions of rear-drive axle The rear-drive axle or rear axle is often suspended from the body or frame of the vehicle by leaf springs attached to the axle housing. Vehicles with other types of springs position the rear axle with control arms (Fig. 45-1). A rear axle performs several functions. These include: Changing the direction of driveshaft rotation by 90 degrees to rotate the axle shafts. Providings a final speed reduction between the drive-shaft and the axle shafts through the final-drive gears (Ø45-14). Providing differential action (Ø45-18) so one wheel can turn at a different speed than the other, if necessary. Providing axle shafts or halfshafts to drive the rear wheels. Acting as a thrust and torque-reaction member during acceleration and braking (Chap. 52). 45-14 Final-drive gears The final drive is the gear set that transmits torque received from the transmission output shaft to the differential. The gear set is made up of a smaller driving gear or pinion gear and a larger driven gear or ring gear (Figs. 45-13 and 45-14). The smaller gear in a gear set is always the pinion gear. Rear-drive axles use hypoid gears (Figs. 43-6 and 45-14). Hypoid gears have teeth cut in a sprial form, with the pinion gear set below the centerline of the ring gear. This lowers the driveshaft, which allows a lower floor pan and driveshaft tunnel. It also allows more teeth to be in contact to carry the load. The ring gear is three to four times large than the pinion gear (Figs. 45-14). When the pinion turns the ring gear, it reduces the speed of the axle shafts while increasing the torque applied to them. The pinion gear connects to the rear end of the drive-shaft (Figs. 45-13), and is assembled into the front of the axle housing or differential carrier. The ring gear attaches to the differential case. The differential side gears are splined to the inner ends of the axle shafts. Rotation of the ring gear rotates the differential case (Ø45-18). NOTE: The final-drive gears described above are bevel or hypiod gears (Figs. 45-6). They change the direction of power flow by 90 degrees so rotation of the driveshaft rotates the axle shafts (Figs 45-14). In a transxale, the final-drive gears are usually hellcal gears (Figs. 43-3 and 43-15). These are used because the pinion gear and ring gear are on parallel shafts. Figure 43-6 shows both types of gears. 45-19 Differential operation Figure 45-17 shows the basic parts of a differential. Figure 45-13 shows an assembled differential. When the car is on a straight, level road and both tires have equal traction, there is no differential action. (Traction is the adhesive or pulling friction of a tire on the road). The ring gear, differential case, differential pinion gears, and differential side gears all turn as a unit. The pinion gears do not rotate on the pinion shaft, but rather turn both side gears and axle shafts at the same speed. When the vehicle enters a curve, the resistance of the inner tire to turning begins to increase. It now has a shorter distance to travel (Fig. 45-18). The outer tire must travel a greater distance. The differential pinion gears are applying the same torque to each side gear. However, the unequal loads from the tires cause the pinion gears to begin rotating on the pinion shaft. They walk around the slower-turning inner-wheel side gear. This increases the speed of the outer-wheel side gear by the same amount. Figure 45-18 shows differential action in a typical turn. The differential case speed is 100 percent. The rotating pinion gears carry 90 percent of this speed to the slower-turning inner wheel. The rotating pinion gears carry 110 percent of the speed to the faster-turning outer wheel. The differential described above is a standard or open differential. It delivers the same torque to each wheel. If one tire begins to slip and spin, the open differential divides the rotary speed unequally. The tire with good traction slows and stops. This may also stop the vehicle or prevent it from moving. 46-1 Four-wheel drive (4wd) A vehicle with four-wheel drive (4WD) has a drive train that can send power to all four wheels (Fig. 46-1). This provides maximum traction for off-road driving. It also provides maximum traction when the road surface is slippery, or covered with ice or snow. Some vehicles have a four-wheel-drive system that engages automatically or remains engaged all the time. Other vehicles have a selective arrangement that permits the driver to shift from two-wheel drive to four-wheel drive, and back, according to driving conditions. The instrument panel or console may include an indicator light or display to show when the vehicle is in four-wheel drive. Many four-wheel-drive vehicles are basically light-duty trucks. They have rear-wheel drive (Chap. 45) with auxiliary front-wheel drive. A two-speed gearbox (Ø43-6) or transfer case (Ø46-3) engages and disengages the front axle,while providing high and low speed ranges. Other vehicles use the front axle as the main-drive axle. To get four-wheel drive, the transfer case engages the rear axle which then serves as the auxiliary-drive axle. Four-wheel-drive vehicles usually have high ground clearance, oil-pan and underbody protection, and tire treads suitable for off-road use. 46-2 All-wheel drive (AWD) Some passenger vehicles have all-wheel drive (AWD). This is a version of four-wheel drive used in vehicles primarily for on-road use. It provides improved traction, especially on slippery or snow-covered road surfaces. A two-speed transfer case is not used, so there is no low range for off-roading. Figure 46-2 shows an AWD car that normally drives both front and rear axles equally. When the wheels on one axle slip, the system automatically transfers torque to the other axle which has better traction. Other AWD vehicles have front-wheel drive with auxiliary rear-wheel drive, or rear-wheel drive with auxiliary front-wheel drive. Some AWD vehicles have a singel-speed transfer case. Others have the gearing to drive the auxiliary axle built into the transmission or transaxle. 46-3 Purpose of the transfer case The typical transfer case attaches to the rear of the transmission in place of the extension housing (Figs 46-1 and 46-3). Engine power flows through the transmission output shaft to the transfer-case input shaft. If the vehicle has part-time four-wheel drive, the driver selects either two-wheel or four-wheel drive. Gearing in the transfer case then sends power to only the rear axle (two-wheel drive) or to both front and rear axles (four-wheel drive). Some vehicles have full-time four-wheel drive. The transfer case remains in four-wheel drive and the front axle engages automatically as soon as the rear wheels begin to spin. Automotive transfer cases are classified as single-speed or two-speed. The single-speed transfer case can divide the power and deliver it to either axle or both axles. In addition, the two-speed transfer case has a low range and a high range. The driver can select either two-wheel drive or four-wheel drive in high range. Neutral, or low range with four-wheel drive (Fig.46-4). Figure 46-5 shows the power-flow through a two-speed transfer case as the shift lever is moved to the different positions. The four modes of transfer case operation are obtained by moving two sliding gears. These are splined to the transfer-case output shafts for the front and rear axles. High range in the transfer case provides direct drive, or a gear ratio of 1:1. Low range usually produces a gear reduction of about 2.5:1. This reduces vehicle speed while greatly increasing the low-speed torque available. A single-speed transfer case usually has s 1:1 ratio . 49-1 Purpose of the suspension system The suspension system (Fig. 49-1) is located between the wheel axles and the vehicle body or frame. Its purpose is to: Support the weight of the vehicle. Cushion bumps and holes in the road. Maintain traction between the tires and the road. Hold the wheels in alignment. The suspension system allows the vehicle to travel over rough surfaces with a minimum of up-and-down body movement. It also allows the vehicle to corner with minimum roll or tendency to lose traction between the tires and the road surface. This provides a cushioning action so road shocks have a minimal effect on the occupants and load in the vehicle. Road shocks are the actions resulting from the tires moving up and down as they meet bumps or holes in the road. 49-2 Components of suspension system The suspension system components include the springs and related parts that support the weight of the vehicle body on the axles and wheels. The springs and the shock absorbers (Fig. 49-1) are the two main parts. The springs support the weight of the vehicle and its load, and absorb road shocks. The shock absorbers help control or dampen spring action. Without this control, spring oscillation occurs. The springs keep the wheels bouncing up and down after they pass bumps or holes. Shock asborbers allow the basic spring movement, but quickly dampen out the unwanted bouncing that follows. These ride control components_springs and shock absorbers_may be mechanically or electronically controlled. Following sections describe both types. NOTE: In describing springs and absorbers, jounce or compression is the condition when the wheel moves up. Rebound is the condition when the wheel moves down. AUTOMOTIVE SPRINGS 49-3 Types of springs Four types of springs are used in automotive suspension systems. These are coil, leaf, torsion bar, and air (Fig 49-2). COIL SPRING The coil spring is made of a length of round spring-steel rod wound into a coil (Fig 49-3). Figure 49-1 shows front and rear suspension systems using coil springs. Some coil springs are made from a tapered rod (Fig. 49-3). This gives the springs a variable spring rate (Ø49-5). As the spring is compressed, its resistance to further compression increases. LEAF SPRING Two types of leaf springs are single-leaf and multileaf springs (Fig. 49-4). These have several flexible steel plates of graduated length, stacked and held together by clips. In operation, the spring bends to absorb road shocks. The plates bend and slide on each other to permit this action. Single-leaf springs are described in Ø49-13 TORTION BAR The torsion bar is a straight rod of spring steel, rigidly fastened at one end to the vehicle frame or body. The other end attaches to an upper or lower control arm (Fig. 49-5). As the control arm swings up and down in response to wheel movement, the torsion bar twists to provide spring action. AIR SPRING The air spring (Fig 49-6) is a rubber cylinder or air bag filled with compressed air. A plastic piston on the lower control arm moves up and down with the lower control arm. This causes the compressed air to provide spring action. If the load in the vehicle changes, a valve at the top of the air bag opens to add or release air. An air compressor connected to the valve keeps the air springs inflated. 49-4 Sprung and unsprung weight The total weight of the vehicle includes the sprung weight and the unsprung weight. The sprung weight is the weight supported by springs. The unsprung weight is the part not supported by springs. This includes the weight of drive axles, axle shafts, wheels, and tires. The unsprung weight is kept as low as possible. The roughness of the ride increases as unsprung weight increases. To take an extreme example, suppose the unsprung weight equals the sprung weight. As the unsprung weight moves up and down, due to the wheels meeting road bumps and holes, the sprung weight would move up and down the same amount. For this reason, the unsprung weight should be only a small part of the total weight of the vehicle. 49-5 Spring rate The softness or hardness of a spring is its spring rate. This is the load required to move a spring a specified distance. The rate of a spring that compresses uniformly (a linear-rate spring) is the weight required to compress is 1 inch [25.4 mm]. If 600 pounds [272 kg] compresses the spring 3 inches [76 mm], then 1200 pounds [544 kg] will compress it 6 inches [152 mm]. Variable-rate springs do not move or deflect at a constant or linear rate. The coil spring in Fig 49-3 is one type of variable-rate spring. Winding the coils from a tapered rod provides the variable rate. The spring rate varies from an initial 72.2 pounds per inch [1.29 kg/mm] to 163.5 pounds per inch [2.92 kg/mm]. Other variable-rate coil springs have the coils closer together at the top than at the bottom, or are wound in a cone or barrel shape. 50-1 Purpose of the steering system The steering system (Figs. 49-18 and 49-22) allows the driver to control the direction of vehicle travel. This is made possible by linkage that connects the steering wheel to the steerable wheels and tires. The steering system may be either manual or power. When the only energy source for the steering system is the force the driver applies to the steering wheel, the vehilcle has manual steering. Power steering uses a hydraulic pump or electric motor to assist the driver’s effort. Most vehicles have power steering to make parking easier. The basic operation is the same for both manual and power steering. As the driver turns the steering wheel, the movement is carried to the steering gear (Fig. 50-1). It changes the rotary motion of the steering wheel into straightline or linear motion. The linear motion acts through steering linkage or tie rods attached to the steering-knuckle arms (Ø49-19) or steering arms. The steering knuckles then pivot inward or outward on ball joints (Ø49-20). This moves the wheels and tires to the left or right for steering. 52-1 Automotive brakes Figure 52-1 shows the brake system in an automobile. It has two types of brakes: The service brakes, operated by a food pedal, which slow or stop the vehicle. The parking brakes, operated by a food pedal or hand lever, which hold the vehicle stationary when applied. Most automotive services brakes are hydraulic brakes. They operate hydraulically by pressure applied through a liquid. The service or foundation brakes on many medium and heavy-duty trucks and buses are oprated by air pressure (pneumatic). These are air brakes. Many boat and camping trailers have electric brakes. All these braking system depend on friction( 52-2) between moving parts and stationary parts for their stopping force. 52-15 Types of disc brakes The disc brake (fig 52-17) has a metal disc or rotor instead of a drum. It uses a pair of flat, lined shoes or pads that are forced against the rotating disc to produce braking. The pads are held in a caliper (figs 52-17 and 52-18) that straddles the disc. The caliper has one or more pistons, with a seal and dust boot for each. During braking, hydraulic pressure behind each piston in fig. 52-17 pushes it outward. This forces the pad into contact with the disc. The resulting frictional contact slows and stops the disc and wheel. There are three types of disc brakes. Figure 52-17 shows a fixed-caliper disc brake. The other two are the floating-caliper and sliding-caliper. Each differs in how the caliper mounts and operates. Note: All three types of disc brakes work in the same general way. However, vehicle manufacturers have used many variations of each. Typical examples are described below. Refer to the vehicle service manual for information about the brakes on a specific vehicle. fixed-caliper disc brake: A fixed caliper (figs 52-17 and 52-19A) has pistons on both sides of the disc. Some use two pistons, one on each side. Others use four pistons with two on each side. The caliper is rigidly attached to a steering knuckle or other stationary vehicle part. Only the pistons and pads move when the brakes are applied. floating-caliper disc brake: A typical floating caliper (fig 52-19B and 52-20) has only one piston, located on the inboard side of the disc. The caliper moves or “floats” on rubber bushings on one or two steel guide pins. The bushings allow the caliper to move slightly when the brakes are applied. Some floating calipers have two pistons on the inboard side of the disc. Applying the brakes causes brakes causes brake fluid to flow into the caliper (fig 52-21). This pushes the piston outward so the inboard shoe is forced against the disc. At the same time, the pressure pushes against the caliper with an equal and opposite force. This reaction causes the caliper to move slightly on the bushings, bringing the outboard shoe into contact with the disc. The two pads clamp the disc to produce the braking action. sliding-caliper disc brake: Figure 51-12 shows a sliding-caliper disc brake. It is similar to the floating-caliper brake. Both calipers move slightly when the brakes are applied. However, the sliding caliper slides on machined surfaces on the steering-knuckle adapter or anchor plate. No guide pins are used. 53-1 Purpose of antilock braking Tires skid when they slow or decelerate faster than the vehicle. One way to help prevent skidding is to keep the brakes from locking. This is the purpose of the antilock-braking system (ABS). During normal braking, the antilock-braking system (fig.53-1) has no affect on the service brakes. However during hard or severe braking, the antilock-braking system prevent wheel lockup. The system allows the brakes to apply until the tires are almost starting top skid. Then the antilock-braking system can vary or modulate the hydraulic pressure to the brake at each wheel. This “pumping the brakes” keeps the rate of wheel deceleration below the speed at which the wheels can lock. 53-2 Operation of the antilock-braking system Figure 53-1 shows a vehicle equipped with a vacuum brake booster (52-32) and four-wheel antilock brakes.The brake lines from the master cylinder connect to a hydraulic unit or actuator. Lines from the actuator connect to the wheel brakes. The actuator is controlled by the ABS control module. Wheel-speed sensors (fig.53-1 and 53-2) at each wheel continuosly send wheel-speed information to the ABS control module. There is ABS action until the stoplight switch signals the control module that the brake pedal has been depressed. When the control module senses a rapid drop in wheel speed, it signals the actuator to adjust or modulate the brake pressure to that wheel.This prevents wheel lockup. 53-9 Purpose of traction control Any time a tire is given more torque than it can transfer to the road, the tire loses traction and spins. This usually occurs during acceleration. To prevent unwanted wheelspin, some vehicles with ABS also have a traction-control system (TCS). When a wheel is about to spin. The traction-control system (Fig.53-10) applies the brake at that wheel. This slows the wheel until the chance of wheel spin has passed. 53-10 Operation of traction-control system The antilock-braking system and traction-control system share many parts. The wheel-speed sensors report wheel speed to the ABS/TCS control module (Fig. 53-10). When a wheel slows so quickly that it is about to skid, the ABS holds or releases the brake pressure at that wheel. If wheel speed increases so quickly that the wheel is about to spin, the TCS applies the brake at that wheel. This slows the wheel and prevent wheel spin. The TCS can also reduce engine speed and torque if braking alone does not prevent wheelspin. When this is necessary, the ABS/TCS control module signals the engine control module. It then retards the spark and reduces the amount of fuel delivered by the fuel injectors.