The transmission is a drivetrain component in your vehicle that transmits energy from your engine to the drive wheels. In short, it is a vehicle’s gearbox. A large, metal gearbox that sends rotational power through a drive shaft to another, smaller gearbox called a differential. From there, that power is transmitted through axle shafts to the wheels.
Much like shifting gears on a bicycle, a transmission changes gears to allow your engine to operate with the most effective amount of power to match the speed you are traveling. Transmissions come in manual versions (where you shift the gears manually) and automatic versions (where a computer control module determines when the transmission should shift). Most vehicles on the road today come with automatic transmissions. The transmission on a front-wheel-drive vehicle is known as a transaxle, as it serves the purpose of both transmission and differential. Some transmissions, rather than relying on gears to match engine speed to travel speed, use a pair of internal pulleys connected by a belt or chain, similar to that of a snowmobile. These transmissions are known as Continuously Variable Transmissions, or CVTs.
A transmission rebuild, also known as a transmission overhaul, is a service that includes disassembly of the internal components of a transmission (gears, shafts, clutches, fluid pumps, valves, bearings, sensors, and the like), inspection of those components for wear, cleaning, and replacement of components that are worn or damaged.
The job of rebuilding or overhauling an automatic transmission is extremely complex and requires a good deal of precision. That only comes with significant training and experience.
In order to rebuild your transmission, a technician will first separate the transmission from the engine and remove it from your vehicle. On some vehicles, the engine must also be removed in order to access the transmission. Once the transmission is out of the vehicle, it will be completely disassembled. Hundreds of parts will be carefully separated, organized, cleaned, and inspected. The technician will look for signs of excessive wear, cracks, warpage, and corrosion. Measurements are made with precision tools - dial indicators, feeler gauges, and micrometers. Components normally subjected to wear are replaced with components in a master rebuild kit. Other worn or damaged parts are also replaced. The transmission is reassembled and reinstalled in your vehicle and filled with fresh transmission fluid.