Photoillustration: Harold A. Perry; images: BMW & Thinkstock GETTING UP TO SPEED ON TRANSMISSION DIAGNOSIS BY KARL SEYFERT Regulations for emissions and fuel economy have become increasingly stringent, making electronically controlled, multispeed transmissions a necessity. And it all started with the torque converter clutch. Electronic transmission controls were first introduced on a widespread basis in the late 1970s, as EPA regulations for emissions and fuel economy grew more stringent. The first electronically controlled transmissions included simple systems that controlled only the operation of the torque converter clutch (TCC). The rest of the transmission s operation remained under the valve body s hydraulic control. The ability to lock the torque converter in the higher gears eliminated nearly all of the power losses inherent in automatic transmissions equipped with hydraulic torque converters. This was considered a big step forward at the time. But these early systems were pretty crude by today s standards and even then, many customers complained about their operating characteristics. Some torque converter clutches produced a noticeable driveline shock when they engaged, while others were responsible for a juddering sensation if they remained engaged for too long as engine and road speed dropped. Things have certainly progressed in the intervening years. Electronic transmission controls today are every bit as sophisticated as the electronic systems that control the engine management and other vehicle systems. Unlike the standalone control systems of the past, today s are now fully integrated. The engine management system knows what the transmission control unit is doing and the operation of both systems is optimized to achieve the best possible fuel economy and engine performance, while producing the lowest possible emissions. The best way to achieve the seemingly conflicting goals of high fuel economy, high performance and low emissions is to ensure that the engine is always operating in its most efficient rpm range. This is why vehicles equipped with multispeed manual transmissions used to be able to deliver better fuel economy than similar vehicles equipped with three- or four-speed automatic transmissions. Semitractors, with more than a dozen different gear ratios to choose from, can efficiently move extremely heavy loads over very long distances because their engines are always operating at maximum efficiency. More gear ratio choices equals better fuel economy, higher performance and lower emissions. Passenger vehicle designers have applied these lessons to current automatic transmission design. Today we see torque converter automatic transmission designs with six, seven and eight speeds in widespread use. Ford and GM recently announced they ll collaborate on the design of nine- and 10-speed automatic transmissions. ZF is working on a ninespeed torque converter automatic, which will feature a gear ratio spread of 9.84:1, and Hyundai will offer a 10-speed torque converter automatic in the next-generation Genesis. Will they stop at 10? Other manufacturers have turned to continuously variable transmission (CVT) designs. CVTs use a belt and pulley system to deliver the advantage of a seemingly limitless number of potential gear ratios. Consequently, the engine always operates at the most efficient rpm, regardless of load or vehicle speed. CVT designs offer a gear ratio spread in the range of 7:1 and are said to reduce internal friction while minimizing engine noise at higher vehicle speeds. Some manufacturers have turned to dual-clutch transmission (DCT) designs. These transmissions take the advantages of manual transmissions and combine them with electronic controls, which allow fully automatic operation. Semiautomatic operation is also an option if the driver wishes to select his own gears via a control lever or steering wheel paddles. DCTs require no torque converter, which results in a significant 30 June 2013
weight savings. These transmissions effectively shift into Neutral when the vehicle is standing still, so the engine isn t fighting against an inefficient torque converter. This saves additional fuel. Volkswagen recently announced it will produce a 10-speed dual-clutch transmission with an extremely wide gear ratio spread of 10:1. This design is said to be suitable for a torque range of more than 369 ft.-lbs. Six- and sevenspeed DCTs are already in production. What does all of this transmission innovation mean to service technicians? Put simply, the more complicated something becomes, the more complicated and difficult it gets to diagnose and repair it when something goes wrong. It s always easier to fix something if you first have an understanding of how it s supposed to work. The remainder of this article will be devoted to explaining how the major transmission designs described above actually work, and to covering diagnostic strategies as well. Multispeed Automatic Transmissions The ZF eight-speed-automatic transmission (8HP) is claimed to achieve an additional 6% fuel saving over its sixspeed designs, which also reduces CO 2 emissions. The 8HP is a completely new transmission concept with four gear sets and five shift elements, a higher overall gear ratio, a variable oil pump, a new torque converter and optimized hydraulic and transmission control. An optional stop-start function is also offered. Idling at a standstill, the engine automatically switches off; when engaged again, the engine automatically starts up. Even at standstill times of just 10 seconds, switching off the engine cuts average consumption and CO 2 emissions noticeably. The start-stop function is enabled by the development of the hydraulic impulse oil storage (HIS). It supplies the hydraulic oil that the transmission s shift elements need for starting. When the engine is switched off, it allows for a quick start as is required with the start-stop function. Only 350 milliseconds after starting the engine, the vehicle is ready for setting off. With the start-stop function of the hydraulic impulse oil storage, it s possible to reduce fuel consumption by another 5%. There are only five shift elements multidisc clutches and brakes in the June 2013 31
heart of the transmission and only two are open in each gear. The fewer open shift elements there are, the fewer transmission parts there are rotating relative to one another. This results in a significant reduction of drag losses in the transmission. The development engineers were also able to increase the gear-meshing efficiency with the new transmission concept. Energy is lost in some gears when power is transmitted through gear wheels. In the 8HP, these losses are below 2% throughout a further factor helping to reduce consumption. Continuously Variable Transmissions The continuously variable transmission is a stepless gear transmission whose merits include lower fuel consumption and seamless acceleration. As opposed to manual or automatic transmissions, the CVT can automatically select the most suitable transmission gear ratio without any steps. Vehicles with CVT can run on the most efficient engine rpm (i.e., with the best fuel combustion) for regular velocity. The technology delivers seamless performance without shift changes when accelerating and decelerating. On recent Nissan CVT designs, the low-to-high gear ratio range is expanded, delivering a significant reduction in friction and up to 10% improvement from the previous generation. The CVT adjusts the width of two pulleys and changes the arc radius of the steel belt running between them in order to control the transmission gear ratio. When driving at slow speeds, the arc radius of the belt for the engine shaft pulley gets narrower. At this point, manual or automatic transmission vehicles are in a lower gear (e.g., 1st or 2nd gear). At higher speeds, the arc radius of the belt on the drive pulley gets wider. At this point, manual or automatic transmission vehicles would be in a higher gear, such as 5th or 6th. The steel belt serves to bridge the two pulleys as they change the diameter. If the gear ratio at low gear is lowered even more, acceleration improves. This is similar to starting in 1st gear with more power than starting in 2nd. Vehicle response improves at lower speed and on takeoffs. The ZF eight-speed-automatic transmission (8HP) has just five shift elements multidisc clutches and brakes and only two are open in each gear. The fewer open shift elements there are, the fewer transmission parts to rotate relative to one another. This results in a significant reduction of drag losses in the transmission. The control system of BMW s eight-speed automatic transmission is claimed to predict and adapt gearshift commands dynamically according to the driving situation. The control unit gathers inputs from the driver, scans the topology of the road ahead and relates all of it to traffic data to anticipate the ideal gear for the circumstances. Using an interface that includes data from the traction and stability control systems, navigation, radar sensors and cameras, the system takes into account things like surface friction coefficient, radius of a corner, cars ahead and more. Photo courtesy BMW Photo courtesy ZF On the other hand, when the gear ratio runs at a higher gear, the engine rotation decreases, meaning there s better fuel combustion and less noise. In a similar fashion on a manual transmission vehicle, the engine rotates at a slower speed when in 5th gear at highway speed than in 4th gear. In addition to the acceleration, the fuel combustion and engine noise levels can be improved by making the low and high gear ratio range (called transmission gear ratio range) larger. Through the use of a narrower pulley axle and newly developed belt, the New Generation XTRONIC CVT has expanded the transmission gear ratio range from 6.0:1 to 7.0:1. It also includes adaptive shift control, which interprets the driver s intentions from the acceleration and steering inputs to provide optimum shift control. Coordinated control between the engine and transmission delivers optimal fuel consumption and driving performance. Dual-Clutch Transmissions A dual-clutch transmission can be considered two traditional manual transmissions, each with its own clutch operating in parallel and alternating shifts. The Ford PowerShift is a six-speed transmission with one clutch acting on gears 1, 3 and 5 and the other used for 2, 4 and 6. As 1st gear is engaged, the 2-4-6 clutch is disengaged and the 2nd gear cogs are engaged. At the appropriate time, the 1-3-5 clutch is disengaged and the other is engaged. While in 2nd gear, the other side moves from 1st to 3rd. The process is repeated with quick, smooth shifts and none of the efficiency losses normally associated with torque converter-equipped transmissions. The PowerShift transmission is a joint development 34 June 2013
The CVT adjusts the width of two pulleys and changes the arc radius of the steel belt running between them in order to control the transmission gear ratio. When driving at slow speeds, the arc radius of the belt for the engine shaft pulley gets narrower; at higher speeds, the arc radius of the belt on the drive pulley gets wider. among Ford, Getrag and LuK and was first introduced in Europe. Both Volvo and Ford have used wet clutches and hydraulic actuation on their previous DCT designs. However, the new Power- Shift gearbox used in the Ford Fiesta and Focus uses dry clutches and electric motor/solenoid actuation. The Power- Shift transmission is claimed to improve fuel efficiency by as much as 10% over a conventional automatic transmission. Volkswagen refers to its dual-clutch transmission by the acronym DSG, which stands for a direct-shift gearbox (Direkt-Schalt-Getriebe). The DSG is an electronically controlled dual-clutch multiple-shaft manual gearbox, in a transaxle design without a conventional clutch pedal, and with full automatic or semimanual control. The first actual dual-clutch transmissions derived from Porsche s in-house development for its 962 racing cars in the 1980s. Like the Ford design, a DSG consists of two separate manual gearboxes (and clutches), contained within one housing and working as one unit. By using two independent clutches, a DSG can achieve faster shift times, and eliminates the weight and efficiency losses Photo courtesy Nissan produced by a torque converter in a conventional automatic transmission. Volkswagen has also introduced a seven-speed DSG. In comparison to the six-speed DSG, there s no wet clutch in the new transmission. The clutch is not immersed in an oil bath rather, a dry clutch is used. Under normal, progressive and linear acceleration and deceleration, the DSG shifts in a sequential manner, (1st > 2nd > 3rd > 4th > 5th > 6th). The sequence is reversed for deceleration. However, the DSG can also skip the normal sequential method, by missing out adjacent gears, and shift two or more gears. This is most apparent if the vehicle is being driven at sedate speeds in one of the higher gears with a light throttle opening, and the accelerator pedal is then pressed down, engaging the kick-down function. During kick-down, the DSG will skip gears, shifting directly to the most appropriate gear, depending on speed and throttle opening. This kick-down may be engaged by any increased accelerator pedal opening, and is completely independent of the additional resistance Dual-clutch transmissions combine some of the best attributes of manual and automatic transmissions. The two clutches allow two gears to be selected at once. When the transmission shifts, it needs to open only one clutch and close another, saving time. Quick, smooth shifts mean none of the efficiency losses normally associated with torque converter-equipped transmissions. Photo courtesy Ford In addition to quicker reaction times, dual-clutch transmissions are typically lighter than traditional automatic transmissions because they do not need a torque converter. They re also more efficient, providing better fuel economy than conventional automatic or manual transmissions. With companies like Ferrari switching to transmissions of this design, the end of conventional manual transmissions could be very near. to be found when the pedal is pressed fully to the floor, which will activate a similar kick-down function when in Manual operation mode. Porsche AG offers a series production of two variants of the longitudinally installed, ZF Friedrichshafen AG designed and built 7DT. These are wet-clutch versions of the previously race-only Porsche Doppelkupplungsgetriebe (PDK) transmission. Both variants use seven forward speeds, and it s believed the PDK transmission will replace the outgoing conventional Tiptronic automatic transmissions. However, the eight-speed ZF 8HP may continue to be used in some applications. Like all DCTs, the Porsche PDK transmission is fundamentally two separate manual transmissions in one. The 1st, 3rd, 5th, 7th and Reverse gears are available on one shaft, and 2nd, 4th and 6th gears are available on the other shaft. Porsche claims improvements in CO 2 emissions of around 15% when comparing DCT installations against its former automatic transmission, half of which can be directly attributed to the DCT. When compared directly with a manual transmission, a 16% improvement can be directly attributed to the DCT. Photo courtesy Ford 36 June 2013
Diagnosis Electronic transmission control (ETC) systems are controlled by microprocessors, also referred to as control modules. The ETC module constantly monitors inputs from various sensors. Module programming then processes the input information and makes output decisions for optimal transmission control. The operation of the ETC system is well integrated with vehicle systems by means of the vehicle s communications network. ETC offers better shift control, better transmission fluid pressure control, programmable software, bidirectional scanner controls, adaptive learning and self-testing. While system components will vary by manufacturer and individual vehicle, all ETC systems contain the same basic sensor and solenoid types. However, there are variations in the electrical operation of these systems. Most domestic vehicles use the ground side of their circuits for circuit control, while most Asian vehicles use the power side of their circuits for control. Main ETC module inputs include: a gearshift lever position sensor, engine and transmission temperature sensors, a MAF or MAP sensor, a transmission input shaft speed sensor, an output shaft rpm/vehicle speed sensor, Under normal, progressive acceleration and deceleration, the DSG shifts in a sequential manner, using all six forward gears. The sequence is reversed for deceleration. However, under lighter loads, the DSG can bypass the normal sequential method, by skipping over adjacent gears and shifting two or more gears at a time. a throttle position sensor, a brake position sensor. Depending on the vehicle, additional inputs may include: AWD/4WD, ABS/Traction Control, manual switch controls, engine management. Limp-in strategies are triggered by In 2008, Volkswagen added a seventh gear to its DSG transmissions, and in 2009 a new model was introduced with greater power and torque-handling capacity. This exploded view reveals the DSG s internal similarity to a conventional manual transmission. The unit at the right handles shift execution. Since VW s DSG transmission design was introduced, more than 3.4 million of the units have been produced at the company s Kassel, Germany, plant. Photo courtesy Volkswagen Photo courtesy Volkswagen a major failure either electrical or mechanical. Limp-in mode usually limits the transmission function to one gear and boosts system pressures to maximum. The strategies vary by make, model and year. For example, Chrysler systems power the shift solenoids through a module-controlled relay. Limp-in mode is accomplished by disabling the relay and removing power from the system. On systems from other manufacturers, limp-in mode may be accomplished by removing power or ground from certain actuator components. The actual limp-in function depends on the type of control circuit used by the ETC system. An illuminated MIL is usually the first thing a customer notices when an ETC problem occurs and limp-in mode is triggered. The customer s complaint is often a lack of power from a stop. At this point, the first thing the technician must determine is whether the problem is electrical or mechanical. Most ETC systems are very good at detecting faults in transmission operation, and a failure that causes a limp-in condition will normally result in a diagnostic trouble code (DTC) that will provide a diagnostic direction. ETC systems also have the ability to adapt for component wear. The system will raise or lower line pressure to meet various adaptive calculations. For example, an ETC system may compare actual shift times to desired shift times. The difference between the two is reported as shift time error. This is accomplished by comparing input and output shift sensor data. If the shift time is too long or too short, the controller can compensate by adjusting line pressure. As with all diagnostic work, it pays to invest the time in observing and recording known-good readings on systems that are operating properly. This will make it much easier to spot out-of-spec data when it presents itself on a vehicle that s experiencing a problem. This article can be found online at www.motormagazine.com. 38 June 2013