Purcell: We'll keep you moving

Transmission Flush

Another fluid that few motorists rarely check is the automatic transmission fluid (ATF) in their automatic transmissions or transaxles. If the dipstick reads low, the transmission is probably leaking. A low fluid level can cause delayed engagement, slipping and shift problems, and may contribute to premature transmission failure.

On most vehicles, the fluid level should be checked when the fluid is hot with the engine idling, the parking brake set and the transmission in Park. If fluid is needed, add only enough ATF to bring the level up to the full mark. Do not overfill because doing so can cause the fluid to become aerated, which may affect transmission operation.

You should also check the condition of the fluid. Some discoloration and darkening is normal as the fluid ages, but if the ATF is brown or has a burnt smell, it is badly oxidized and needs to be changed. Varnish on the dipstick is another indication of worn-out fluid.

You can also do a "blotter test" to check for worn-out fluid. Place a few drops of ATF on a paper towel and wait 30 seconds. If the spot is widely dispersed and red or light brown in color, the fluid is in satisfactory condition. But if the spot does not spread out and is dark in color, the ATF is oxidized and should be changed.

When adding ATF, use the type specified by the vehicle manufacturer. Honda, Mercedes-Benz and others all have their own specs for ATF. There is no such thing as a "universal" ATF that works in all transmissions. Some fluids meet a variety of specifications but cannot meet them all because of the different friction additives that are required.

Many transmission experts say that most transmission problems can be prevented by changing the ATF and filter regularly for preventive maintenance. How often depends on how the vehicle is driven. For some vehicles, this might be every 30,000 miles or two years.

The harder the transmission works, the hotter the fluid runs. The life of the fluid drops quickly once its temperature gets up above about 200 degrees F.

ATF also becomes contaminated with normal wear particles from the clutch plates, bushings and gears. The filter will trap most of this debris before it can cause problems.