The steps involved in changing transmission fluid can vary from vehicle to vehicle, and the biggest difference is whether the vehicle has manual or automatic transmission. Changing manual transmission fluid is typically a straight forward process of draining and then refilling the transmission with the proper type of oil. In most automatics, changing transmission fluid typically involves draining the pan, replacing the filter, and then flushing out the system. The flushing component of changing transmission fluid may be accomplished with the use of a fluid exchanger or through more primitive means. It is often possible for a vehicle owner to change their own transmission fluid at home, though the process is typically more complex than other do-it-yourself operations, like changing motor oil.
Manual transmission fluid is typically not subjected to the same type of heat as automatic transmission fluid (ATF), but it still must be changed at regular intervals. The most difficult step in the process is usually lifting the car up and locating the drain plug on the transmission. Drain plugs are sometimes difficult to find, and if a vehicle has a differential or transfer case it is important not to drain either of those. Sometimes the fluid will be drained and filled from the same location, in which case both a suction device of some kind will be needed to remove the old fluid as well as a bottle with a long neck or nozzle to refill the reservoir.
Changing automatic transmission fluid is typically a more complex operation. If a fluid exchanger is available, the first step will typically be to remove much of the ATF from the system. In the absence of such a device, the fluid may be drained from the pan, either through the plug or during pan removal. Once the transmission pan has been removed, the filter can be replaced. Most vehicles have filters that are located inside the pan, but some use external filters.
After the transmission filter has been replaced, the system is typically flushed. Most fluid exchanger units can accomplish this process with relatively little input from the technician, though there are more labor intensive ways to get the job done. Without a fluid exchanger, the lines between the transmission and the cooler are typically removed and positioned over a drain pan. The transmission is then filled with ATF and then the engine is started. As long as the ATF is replaced at the same rate it is pumped out of the cooler line, the system can be flushed without doing any damage.
An alternate method of changing transmission fluid involves simply draining and refilling the system, then driving the vehicle. If this process is done repeatedly, most of the fluid will be cycled out, leaving relatively little contaminated ATF behind. This is typically the least efficient method of changing transmission fluid, though it can take comparatively little time if the pan has an easily accessible drain plug.