Originally Posted by Back40
Why would replacing 1/2 the ATF be preferable over a low pressure flush?
Original Post posed a question which there are many opinions on. There is no right answer. Circumstances vary. Although I am skeptical of what some people believe to be fact. They can continue to hold onto their beliefs. I will follow my own course.
My own opinion is that replacing only half the fluid is not preferable over a complete fluid exchange. Drain & fill 3 or 4 times, and you will have almost 100% fresh fluid. Drain the transmission from the return line until clean fluid is coming out, and you will have almost 100% fresh fluid. Fresh fluid is always preferable to mixing old & new fluid. Yes, fresh fluid with it's fresh additive package and fresh detergents, will clean off some of the build-up which the contaminated fluid was not removing. So now your fresh fluid is working, there are more contaminants in the fluid. Would you rather have those contaminants still stuck in your transmission's moving parts? What you need to do now is change the fluid again. Drain out the contaminants which your fresh fluid cleaned off the transmission. Now you will have clean fluid with a clean transmission. Or would you rather have contaminated fluid with sludge, varnish, and metal floating around?
Imagine a cup. You drink everything from that cup. You never wash it. You pour the next beverage into it, while a little of the last drink is still in the bottom. Coffee, tea, hot chocolate, beer, milk, juice, soda.......the inside of that cup will have everything stuck to it. Then suddenly, some soapy water got in. All that stuff stuck to the side of the cup is free, and now floating around in the cup. Do you continue to drink from that cup? Do you add the next refreshment in with that soapy water, and all the crud swilling around? Or do you actually wash it out before you drink from it again?
On another forum, a member plans to drain his transmission pan from the drain plug. Add 3 quarts of fresh fluid. Fill those empty bottles with contaminated fluid. Then however much contaminated fluid is left over, he will pour back into the transmission. His belief is that this method will provide a true level of transmission fluid fill, since the transmission does not have a dipstick. He could also measure the amount drained, and refill with the same amount. The transmission also has a fill plug on the side. Run the engine until the transmission is at operating temperature, open the fill plug, add transmission fluid until it reaches the fill plug level. But this guy is going to pour 1.25 quart of contaminated fluid back into the transmission, right into the 3 quarts of fresh fluid.
The transmission in question calls for a 4 liter fill, it's in the owners manual.. This transmission is serviced by draining and filling 3 times, as per the service manual. I do the triple drain & fill, and fill with 4 liters.