Transmission Fluid Flush Tips
Change transmission fluid every 50,000 km. For optimal protection, change the Transmission Fluid and filter every 50,000 km or 100,000 km if the fluid is synthetic. Check your manufacturer's specifications if you aren't sure which type your vehicle requires.
Why ATF Wears Out
An automatic transmission creates a lot of internal heat through friction: the friction of the fluid churning inside the torque converter, friction created when the clutch plates engage, and the normal friction created by gears and bearings carrying their loads.
It doesn't take long for the automatic transmission fluid (ATF) to heat up once the vehicle is in motion. Normal driving will raise fluid temperatures to 175 degrees F., which is the usual temperature range at which most fluids are designed to operate. If fluid temperatures can be held to 175 degrees F., ATF will last almost indefinitely - up to 150,000 km. But if the fluid temperature goes higher, the life of the fluid decreases. Even under normal driving conditions, fluid temperatures can fluctuate which causes issues.
At elevated operating temperatures, ATF oxidizes, turns brown and takes on a burnt smell. As heat destroys the fluid's lubricating qualities and friction reducing characteristics, varnish begins to form on internal parts (such as the valve body) which interferes with the operation of the transmission. If the temperature gets above 250 degrees F., rubber seals begin to harden, which leads to leaks and pressure losses. At higher temperatures the transmission begins to slip, which only aggravates overheating even more. Eventually the clutches burn out and the transmission fails. This is a major repair that can be prevented with regular maintenance and care
According to the automotive industry professionals, 90% of ALL transmission failures are caused by overheating. Most of these cases can be prevented by replacing worn out fluid.
On most vehicles, the automatic transmission fluid is cooled by a small heat exchanger inside the bottom or end tank of the radiator. Hot ATF from the transmission circulates through a short loop of pipe and is thus "cooled." Cooling is a relative term here, however, because the radiator itself may be running at anywhere from 180 to 220 degrees F.!
Any number of things can push ATF temperatures beyond the system's ability to maintain safe limits: towing a trailer, mountain driving, driving at sustained high speeds during hot weather, stop-and-go driving in city traffic, "rocking" an automatic transmission from drive to reverse to free a tire from mud or snow, etc. Problems in the cooling system itself such as a low coolant level, a defective cooling fan, fan clutch, thermostat or water pump, an obstructed radiator will also diminish ATF cooling efficiency. In some cases, transmission overheating can even lead to engine coolant overheating!
ATF Fluid Types
What kind of automatic transmission fluid should you use in your transmission? The type specified in your owner's manual or printed on the transmission dipstick.
For older Ford automatics and certain imports, Type "F" is usually required. Most Fords since the 1980s require "Mercon" fluid, which is Ford's equivalent of Dexron II.
For General Motors, Chrysler and other imports, Dexron II is usually specified.
NOTE: Some newer vehicles with electronically-controlled transmissions require Dexron IIe or Dexron III fluid. GM says its new long-life Dexron III fluid can be substituted for Dexron II in older vehicle applications.
CAUTION: Using the wrong type of fluid can affect the way the transmission shifts and feels. Using Type F fluid in an application that calls for Dexron II may make the transmission shift too harshly. Using Dexron II in a transmission that requires Type F may allow the transmission to slip under heavy load, which can accelerate clutch wear.
Changing The Fluid
Changing transmission fluid is a job that's best left to the professionals. It is a large and complex job that is made much easier with the right tools including flush equipment and replacement fluid on hand.
A typical fluid change will require anywhere from 3 to 6 quarts of ATF depending on the application, a new filter and a pan gasket (or RTV sealer) for the transmission pan. The pan must be thoroughly cleaned prior to reinstallation. This includes wiping all fluid residue from the inside of the pan and scraping all traces of the old gasket from the pan's sealing surface. When the new filter is installed, it must be mounted in the exact same position as the original and any O-rings or other gaskets have been properly positioned prior to tightening the bolts. Tighten the bolts to the manufacturer's recommended specs.
When refilling the transmission with fresh fluid, be careful not to allow any dirt or debris to enter the dipstick tube. Using a long-neck funnel with a built-in screen is recommended.
CAUTION: Do not overfill the transmission. Too much fluid can cause the fluid to foam, which in turn can lead to erratic shifting, oil starvation and transmission damage. Too much fluid may also force ATF to leak past the transmission seals.