Under the Hood: The essentials for changing transmission fluid

Q: I'm due for an automatic transmission fluid change for an '04 Accord V6 with 126,218 miles on it. The owner's manual recommends draining and filling the ATF Z1 four times, driving and moving the car through all gears between each drain and fill. I've contacted three mechanics, who told me only one drain and fill is necessary. One mechanic said you need to drain and fill four times only for the first change.

Which is correct, and why?

A: Opening the drain plug of your automatic transmission allows only about 40 percent (3.1 quart) of the fluid to escape. The remaining fluid is trapped in the torque converter and a multitude of internal places and passages. This makes a great case for draining/refilling several times (and running the transmission through its paces between) in order to turn the tide on the old fluid. The more times one repeats the process the smaller the percentage of old fluid remains. Draining an engine of oil when warm makes a lot of sense, but I don't see how it makes a big difference with low viscosity automatic transmission fluid. Honda has come out with a new ATF DW-1 fluid that offers improved cold transmission performance and improved fuel economy, compared to the original ATF-Z which has been discontinued. The new Honda specified fluid can be mixed with old in any ratio and isn't too pricey at about $7.50 per quart.