Why do I need to flush my Transmission Fluid?
The number one reason automatic transmissions fail is because of fluid breakdown.
There are many things that happen to the fluid as it breaks down but I would like to touch on a few of them.
Transmission fluid is a multi-grade, about a 5W20 on a motor oil scale. This means that when it is cold, the oil will flow like a 5 weight oil would at that temperature. When the oil is hot, it will flow like a 20 weight oil. This is achieved with viscosity enhancers. To put it simply, they are small particles that expand with heat making the oil thicker. When the oil cools down, they shrink back to their original size.
This works very well when the oil is new but after a while, the particles start to break down (shear) and are no longer able to reach the 20 weight viscosity rating that the transmission was designed for. As this oil starts to break down, the transmission has to work harder in order to do the same job. And as the transmission works harder, the fluid breaks down faster to a point that the oil is a straight 5 weight. At this point, the transmission tends to shift harder with more of a "jerk" between shifts. Because this deterioration in shift quality occurred over a period of time, the driver often doesn't notice the change.
For a vehicle in cool climates that makes lots of short trips, the problem is condensation. For instance, the family vehicle that runs the kids to school and back, to the store and back. Every time it is shut off, condensation forms inside the transmission as it cools. When it is started again, the condensation (water) is washed into the transmission fluid. Short trips do not allow the transmission enough time to get hot enough to burn the moisture off. As a result, the water molecules emulsify in with the oil molecules, which quickly breaks down the fluid.
Additives in transmission fluid and motor oils prevent foaming. Without these additives, oxidization would happen at a much faster rate. Heat, oxygen and water create sludge. Sludge is deadly for an engine or transmission. These anti foaming agents only work so long before they get used up and longer do their job. Pull out a dipstick on a hot engine or transmission with old oil in it and you can see all the little bubbles, that oil should be changed.
When transmissions fail, people tend to blame the manufacturer or the driving habits or "it was just plain worn out" when in actual fact, all they needed was to change their fluid when they were supposed to.