Don't 'flush' transmission fluid

Last week I referred to the proper technique for changing the transmission fluid as an exchange, rather than drain-and-fill. It prompted this response from a reader.

Q. According to my Honda Dealer, you should not flush the transmission fluid, but only drain it every 30,000 miles. Flushing it will break the seals. Is my dealer correct?

A. What I was referring to in my last article was a transmission fluid exchange. This system uses the transmission's own fluid pressure to pump the old fluid out, while at the same time pushing the new fluid in. There is no external pump or pressure put on the transmission or any of its seals.

This technique is better because 98 percent of the fluid gets changed. There are some cars where there is no way to hook a unit like this up. In those cases, a drain-and-fill method is the best you can do.

This transmission fluid change is sometimes referred to as a flush, but a fluid exchange is more accurate. I hope this clarifies things.

• 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.