Automotive Transmission

Automotive transmission repair can be an extremely involved process. Both manual and automatic transmissions use quite a few moving parts to bring the power created by an engine to the wheels. Like any other complex automotive system, a basic understanding will go a long way toward removing confusion.

Transmissions work by using a series of fixed gear ratios that allow the engine to produce the power needed to start from a stop and maintain cruising speed at a reasonable RPM. If engine power was routed directly to the wheels at a constant ratio, it would be very difficult to move a stationary vehicle. Additionally, there would only be a small percentage of acceleration where peak power could be produced.

Manual transmissions, in essence, are simply a series of rotating gears. One set of gears is mounted on a component called the countershaft. The countershaft is a rotating rod that connects to the input shaft (which connects to the clutch and engine flywheel assembly) to receive engine power. A final set of gears is located on the mainshaft. The mainshaft gears are in constant contact with the countershaft gears. Often, the input shaft and the mainshaft are concentric, rotate independently around one another and are referred to collectively as the mainshaft.

Each gear on the mainshaft is a different size and, therefore, a different ratio. Power is transferred at a fixed ratio when one of these gears is selected by the driver. When the shift is completed, the gear begins to transfer power through the mainshaft to the output shaft via the countershaft. The output shaft then sends power to the vehicle's differential which allows the drive wheels to spin. The mainshaft drives the countershaft, transferring engine power through the transmission. The gear ratio is changed when the driver moves the shifter inside the car.

Gear changes are kept smooth with a device called a synchronizer. Synchronizers are usually located on the mainshaft directly in between two gears. Even and odd numbered gears (gears 1 and 3 for example) will often share a synchronizer. In these situations, the synchronizer slides back and forth on the mainshaft with one side engaging a gear.

Automatic transmissions use a series of hydraulically controlled clutches to change the ratio produced by the internal planetary gears. A planetary gearset is a group of three types of rotating gears that are all encased in a common housing. Stopping the rotation of any one type of gear causes the ratio to change.

Rather than using a clutch, automatic transmissions use a fluid-filled item called a torque converter to allow the engine to run while the car is at a stop. The front portion of the torque converter is essentially bolted to the engine flywheel, while the back half is connected to the transmission. Torque converters use a series of internal fins to force transmission fluid into a flow strong enough to turn the transmission input shaft. A common explanation of this principal involves placing two electric fans face to face a few inches apart. If one fan is turned on while the other is left unplugged, the air from the working fan will cause the blades of the non-running fan to spin. The only difference is that a torque converter uses transmission fluid to transfer energy instead of air.

While this is a rather simplified view of both designs, it is critical to have a basic understanding of powerflow before any in-depth knowledge can develop. At the very least, it is always a nice to have a basic idea of how items so crucial to our everyday lives keep us moving.