What is done when trans fluid is flushed compared to just changed? Can you flush yourself or are unusual equip/tools involved? Would two changes be better than one flush since flushing seems to cost about twice as much?
What is done when trans fluid is flushed compared to just changed?
Your wallet is lightened. Best bet most of the time is to change the fluid AND the filter. Most of the time they don’t change the filter when they flush.
The issue is, if you do it yourself, when you drain the fluid, or pull the pan, you are only getting 30-60% of the fluid out. Automatic transmissions use fluid in areas like valve bodies, coolers, and torque converters that wont empty when you empty the fluid by dropping the pan/pulling the plug.
Also, some transmissions, (a lot of 90’s FWD Nissans for example), are not made to ever have the filter changed (you can change it, but it pretty much requires a rebuild).
If the fluid is discolored and you want to do it yourself, drain, refill, and drive a few times. Without a machine, it is hard to do an actual flush, but it is doable.
You can disconnect the cooler lines and use one to put clean fluid in and the other to pump the old out, but I don’t recommend it.
Te best thing you can do is to always keep the fluid clean.
A transmission fluid exchange can get around 95%+ of the old fluid out. There are some home methods that will get the job done. See Bobistheoilguy.com for more info. A pan drop and filter change should still be done at the same time.
For my Subaru, it takes three to four fluid changes (about 12 quarts for this trans; for yours could be 16 qts or more) and refills to get close to about 87%, in comparison. I do something along that line to keep the fluid replenished, but also realize that it is not as good as a properly done fluid exchange.
The “flush” that is most harmful is a high pressure back flush method that can be harmful to your transmission. A machine that uses the car’s engine to pump the fluid in while old fluid is leaving does not appear to damage transmissions. Biggest problem is if the tech is not trained to do the job properly (like at quick change places).