158 Manual Drive Trains and Axles Copyright by Goodheart-Willcox Co., Inc. Introduction This chapter explains the construction and operation of manual transmissions used on rear-wheel drive vehicles. At one time, such transmissions were the only kind used in passenger cars. Today, automatic transmissions and manual and automatic transaxles are installed in many vehicles. Although manual transmissions are installed in a smaller percentage of vehicles, you must still be knowledgeable about their construction and operation. Manual transaxles are similar in design and operation to manual transmissions, and many of the same principles apply. Purpose of the Manual Transmission The manual transmission multiplies engine torque or reduces engine rpm to match varying forward operat- ing conditions. The transmission uses different gear ratios to achieve these results. (Review Chapter 4 for previous discussions of gear ratio.) The manual transmission also provides a way to back up a vehicle. Transmissions use idler gears—gears located between drive and driven gears—for this purpose. Idler gears cause the transmission output shaft to rotate in the opposite direction of the input shaft. In this way, the idler gears allow a vehicle to be reversed. Idler gears do not change the gear ratio. (Review Chapter 4 for previous discussion on idler gears.) Transmissions make the most of engine torque. Engines provide relatively little torque at a low rpm. Without a transmission, high engine rpm would be needed to pro- vide enough torque to move the car from rest, and initial acceleration would be jerky and unacceptable. In a moving vehicle, the internal combustion engine develops maximum torque over only a very narrow rpm range. This narrow range occurs at relatively high engine speeds— several thou- sand revolutions per minute. The transmission is needed to operate the vehicle in this range under many speed and load conditions. To use available engine torque in the most efficient manner, transmissions use several different gear ratios— first gear, second gear, third gear, and such. The lower trans- mission gears multiply (increase) torque while they reduce speed at the gearbox output. As vehicle speed increases, less torque multiplication is needed. A higher gear having less gear reduction is selected. Output shaft speed increases as a result, allowing engine rpm to be reduced for maximum fuel economy. (Review Chapter 4 for previous discussion on gears ratio.) At highway, or cruising, speeds, torque multiplication is not a requirement, and direct drive is selected. A direct drive gear provides no gear reduction. Transmission output shaft speed again increases. Engine speed may be reduced to operate within its maximum power range again. Some vehicles have one or more overdrive gears for highway travel. Overdrive gears have a gear ratio of less than one and allow the engine to run even slower and use less fuel. Operating the Manual Transmission In a manual transmission, such as the one shown in Figure 8-1, the driver selects the gears. First, the driver depresses the clutch pedal to disengage the engine from the transmission. Then, the driver moves the hand-operated trans- mission gearshift lever to select the proper gear. After the gear is selected, the driver carefully releases the clutch pedal while depressing the throttle pedal. This action smoothly couples the power between the engine and transmission. The vehicle is then in gear, and power is transmitted to the rear wheels. In the accelerating mode, as the vehicle gains speed, the gearshift lever is moved to the next higher gear. This operation, called upshifting, brings the vehicle back into its peak operating range for higher speeds. Before moving the lever, the clutch pedal must be depressed and, at the same time, the throttle pedal let up. After the gearshift lever is moved, the clutch pedal is released while the throttle pedal is pushed down again. In the decelerating mode, the same basic operation applies however, the gearshift lever is moved into lower gears. This operation, called downshifting, brings the vehicle back into the peak operating range for lower speeds. In general, lower gears are used when the load is heavy or when vehicle speed is low. If an increased load, such as a hill, is encoun- tered, the driver may downshift from high gear, for example, down to third gear or second gear. When the vehicle is at a stop, the clutch pedal can be held down or the gearshift lever can be moved to the Neutral (no gear) position. Either action interrupts the power flow to the drive wheels. This summarizes the basic external operation of the manual transmission. Once the driver learns to coordinate the movements of the gearshift lever, the clutch pedal, and the throttle pedal, manual transmission operation becomes an unconscious effort. Construction and Internal Operation To prepare for more specific details of transmission construction and operation, refer back to Figure 8-1 and study the parts of the transmission. Learn to identify and locate the fundamental components. These components are presented in detail in this section: • Manual transmission shafts—support gears and directly or indirectly transfer rotation from the clutch disc to the drive shaft assembly. • Transmission gears—transmit power and provide a means of changing torque, speed, and direction at the gearbox output.