Because of the way an internal combustion engine breathes, it can produce torque, or twisting force, only within a narrow speed range. Most modern engines must turn at about 2,500-3,000 rpm to produce their peak torque. By 4,500 rpm they are producing so little torque that continued increases in engine speed produce no power increases.
The manual transaxle and clutch are employed to vary the relationship between engine speed and the speed of the wheels so that adequate engine power can be produced under all circumstances. The clutch allows engine torque to be applied to the transaxle input shaft gradually, due to mechanical slippage. The car can then be started smoothly from a full stop.
The transaxle changes the ratio between the rotating speeds of the engine and the wheels by the use of gears. 4-speed or 5-speed transaxles are most common. The lower gears allow full engine power to be applied to the rear wheels during acceleration at low speeds.
The clutch drive plate is a thin fiber disc, the center of which is splined to the transaxle input shaft. Both sides of the disc are covered with a layer of material which is similar to brake lining and is capable of allowing slippage without roughness or excessive noise.
The clutch cover or pressure plate is bolted to the engine flywheel and incorporates a diaphragm spring which provides the pressure to engage the clutch. The driven disc is sandwiched between the pressure plate and the smooth surface of the flywheel when the clutch pedal is released, thus forcing it to turn at the same speed as the engine crankshaft.
The mainshaft passes all the way through the transaxle, from the clutch to the driveshaft. This shaft is separated at one point, so that front and rear portions can turn at different speeds.
Power is transmitted by a countershaft in the lower gears and reverse. The gears of the countershaft mesh with gears on the mainshaft, allowing power to be carried from one to the other. All the countershaft gears are integral with that shaft, while several of the mainshaft gears can either rotate independently of the shaft or be locked to it. Shifting from one gear to the next causes one of the gears to be freed from rotating with the shaft and locks another to it. Gears are locked and unlocked by internal dog clutches which slide between the center of the gear and the shaft. The forward gears usually employ synchronizers - friction members which smoothly bring gear and shaft to the same speed before the toothed dog clutches are engaged. The clutch is operating properly if:
- It will stall the engine when released with the vehicle held stationary.
- The shift lever can be moved freely between first and reverse gears when the vehicle is stationary and the clutch disengaged and the engine running at normal idle speed.
Clutch pedal free-play adjustment is important. There should be a small but noticeable amount of play in the clutch pedal before it begins to release the clutch. This play or looseness should be easily felt at the top of the pedal travel. (As you begin to push the clutch pedal towards the floor, the first inch or so of travel should have no effect on the clutch engagement). Inadequate free-play wears all parts of the clutch releasing mechanisms and may cause slippage. Excessive free-play may cause inadequate release and hard shifting of gears.
Some clutches use a hydraulic system in place of mechanical linkage. In these systems, moving the clutch pedal develops pressure in a column of fluid rather than pulling on a cable. If the clutch fails to release, fill the clutch master cylinder with fluid to the proper level and pump the clutch pedal to fill the system with fluid. Bleed the system in the same way as a brake system. If leaks are located, tighten loose connections or overhaul the master or slave cylinder as necessary.
Manual transaxles used in Nova/Prizm cars are mated to their engine families. Please refer the manual transaxle application chart in Routine Maintenance .