If an automatic transmission system is to work correctly, the hydraulic fluid must be pushed through a series of interconnected parts to create a pressure differential. Much of this work takes place within the torque converter mechanism, where you'll find a series of clutches and gears that need to be well lubricated for the process to take place as expected. Yet this can be one of the most at-risk areas of the transmission system when it comes to long-term damage, and you will need to pay close attention to its condition if you want to avoid issues in the future. In particular, you need to ensure that all of the "old" fluid is taken out of these inner reaches before adding any new lubricant during service. Why is this crucial?
Understanding the System
As an automatic system cannot work without elevated pressure levels, the fluid will often have to deal with high temperatures, components that rotate at high speed and narrow apertures. In these environments, a certain amount of friction is to be expected, and tiny particles of metal will invariably break away from certain surfaces and find their way into the lubricant. As the impeller turns and pushes the lubricant to the outside of the casing, some of these particles break free. Other contaminants can find their way through a filter into the sump, but "rogue" particles will stay inside the system.
The drain plug is removed during a routine automatic transmission service, and the old lubricant will be evacuated. Yet this process relies on the force of gravity and little else, so some of the old fluid may remain within the torque converter due to its design.
The Better Approach
It's important for a mechanic to flush the entire system through to get rid of the old fluid and contamination. Otherwise, it'll simply remain in place and mix into the new lubricant once added. If this process is repeated during subsequent service visits, the lubricant will not be as pure as possible, which may lead to the risk of damage.
Insist on a Flush and Full Clean
So, if it is time you take your vehicle in for an automatic transmission service, ask your technician whether they will perform a full flush and clean operation. This is the best approach if you want your transmission to last as long as possible without the risk of failure.