Auto Repair Help

by Lance Wright

The first thing to understanding manual transmissions is to understand the idea of torque. Torque is the twisting force that an engine produces. The torque that an engine generates, is the energy that propels a vehicle. An engine is only able to produce torque over a limited RPM range. At low engine speeds, torque is minimal. As engine speed is increases, torque increases at a steady rate until it peaks at high RPM. After that, torque falls off quickly. A manual transmission is used to allow the available torque in an engine to be transferred to the driving wheels, across a wide range of vehicle speeds.Manual Transmission

As stated earlier, engine torque is low at low engine speeds. Because of this, an engine is not able to generate enough torque to accelerate a vehicle from a standing stop. The transmission provides a means of increasing the torque generated by the engine before it is transferred to the rear wheels. Torque is multiplied inside the transmission by using a small gear to rotate a large gear. The number of revolutions that one gear makes to complete one revolution is expressed in a ratio. If a gear were to rotate four times in order to complete one revolution of the driven gear, that ratio would be called 4 to 1. At this ratio torque multiplication would be very high. The vehicle would be able to accelerate smoothly from a standing stop, but vehicle speed would be limited since the engine speed would quickly reach maximum limit. However, as vehicle speed increases, the torque demand falls off. Each higher gear selected will have a closer gear ratio, that multiplies torque less and less. When in high gear, torque multiplication is zero, the input speed of the transmission matches the output speed. When in overdrive, the input speed of the transmission is slower than the output speed. Engine torque is actually reduced, but since the vehicle will be moving at high speed, torque demand is very low.

The transmission components consist of a bell housing, transmission case, and extension housing. The bell housing is used to enclose the flywheel, clutch release bearing, and clutch disengagement actuator or release lever, as well as provide an attachment area to the engine (refer to a manufacturer’s auto repair manual covering your vehicle for an assembly diagram of the manual transmission). The transmission case contains a mainshaft, counter shaft, reverse idler shaft, and shift levers. The extension housing contains the output shaft as well as the speedometer drive gear or the vehicle speed sensor.

ClutchThe clutch consist of a spring loaded diaphragm, referred to as a cover or pressure plate, and a disc with a splined hub. When troubleshooting manual transmission issues, the pressure plate assembly is typically the first component inspected during an auto repair procedure involving the manual transmission. The clutch cover is bolted to the flywheel and secures the clutch disc between its face and the surface of the flywheel. The clutch cover face and the flywheel surface a machined to a smooth surface. The clutch cover uses spring pressure to compress the clutch disc between the cover face and the flywheel surface. This allows the engines rotating motion to directly drive the transmission input shaft through the splined hub in the center of the clutch disc. The clutch cover and flywheel are the driving members and the clutch disc is the driven member.

Clutch disengagement is achieved by removing the clamping force that the clutch cover places on the clutch disc. The clutch plate is retracted by using a clutch release bearing to depress the levers that retract the clutch plate inside the clutch cover. The clutch release bearing is placed over the input shaft housing of the transmission. It is often referred to as a throwout bearing. The clutch release bearing is moved back and forth on the input shaft housing by an actuator called the clutch release lever. The clutch release bearing is designed to allow the face of the bearing that depresses the clutch diaphragm to rotate, while the portion of the bearing that slides on the input shaft housing remains motionless.

Movement of the clutch release bearing is controlled by the driver by depressing the clutch pedal. There are two basic clutch release control systems: hydraulic and mechanical release. The hydraulic systems uses a master cylinder, connected to the clutch pedal that actuates a single slave cylinder mounted on the transmission bell housing. When the clutch pedal is depressed it moves a piston inside the master cylinder that creates hydraulic pressure that operates the slave cylinder. A piston contained in the slave cylinder moves a rod that actuates the clutch release lever. The clutch release lever slides the throwout bearing forward, depressing the clutch cover diaphragm. With the mechanical release system, a series of linkage rods and pivots are used to control the action of the clutch release lever. Some mechanical release systems use a cable to transmit the movement of the clutch pedal to the clutch release lever. Both mechanical systems are more subject to wear and require more maintenance than the hydraulic release system.

The main section of the manual transmission is called the case. It contains the components that provide the different gear ratios needed for the operation of the vehicle, as well as the reverse function. The mainshaft is the central component of the transmission case. The mainshaft is a two piece design that consists of an input and output shaft. The input shaft is able to rotate independently of the output shaft. This action allows a neutral position that enables an engine to operate without its rotation being transmitted to the rear wheels. Contained on the transmission mainshaft are gears of different sizes. The gears can be slid back and forth on the mainshaft to mesh with corresponding gears on the transmission countershaft. It is the meshing of gears from the transmission mainshaft to the transmission countershaft that causes the rotation of the engine to be transmitted from the transmission input shaft to the transmission output shaft. By meshing gears of different sizes, distinct forward speeds are provided. Synchronizers are mounted on the mainshaft to ease shifting into different forward speeds while driving. Reverse is made possible by use of an additional shaft called the reverse idler shaft. When the transmission is placed in reverse, the reverse idler shaft provides a gear path that causes the output shaft to rotate in the opposite direction of the input shaft.

The manual transaxle is a manual transmission that also contains the final drive that connects power to the driving wheels. It contains many components that are similar to longitudely mounted manual transmission, with the exception a differential assembly and driving axles. To diagnose and repair a manual transmission, always refer to the proper repair guide, preferable the repair manual published by the manufacturer of your vehicle’s year and model.

(Lance owned his own auto repair shop for 30 years before retiring in 2006.)

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One Response to “Manual Transmission”

  1. kibrom says:

    it is nice go head

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