There are two ways to change transmission fluid

Q. I have a question regarding the article you wrote advising readers to change transmission fluid every 30,000 miles. I have a Toyota Sienna and was told to flush the trans every time I change fluid.

Would you suggest changing and not flushing, or is it necessary to flush the transmission?

A. Great question! There are two ways to change the transmission fluid on most cars.

You can tap into the transmission cooler lines and replace the fluid with a machine commonly referred to as a flush machine. In reality you are not flushing the fluid but rather allowing the internal pump of the transmission to push the old fluid out and draw in all the new fluid. This method changes about 98 percent of the fluid.

The other method is to drop the transmission oil pan and change the filter. This method only changes about half the fluid, so you are mixing the clean fluid with the old, dirty fluid. You are getting the filter changed, which is a good thing.

If you start out performing regular fluid exchanges (flushes), you shouldn't need to change the filter until the third one. By doing this, you will be keeping the transmission fluid very clean and virtually eliminate the possibility of transmission failure, at least the kind that relates to fluid and wear.

Good luck with the Sienna!

• Douglas Automotive is at 417 W. Main St., Barrington, (847) 381-0454, and 123 Virginia Road, Crystal Lake, (815) 356-0440. For information, visit douglasautomotive.com. Send questions to underthehood@dailyherald.com.