I tried to drain the auto trans fluid and refill it, but it goes back to dirty for a short time, say 2 months. Is any specific procedure for changing the fluid? I heard someone saying that I need to wash the transmission by the new transmission fluid for a few times, how?
You don't say what model (really not that important) or How many miles (REALLY IMPORTANT!)
If it is a newer car and has had somewhat routine service, you can try having the transmission flushed. Some say, if the trans has been serviced regularly this is OK, but I still shy away, since they use a base of DexronII-Mercon with 'modifiers' for other types like Chrysler ATF+3 or +4. Not my idea of a good job...I have also personally seen two older Escort trannies go belly up after being flushed, having never had service before.
There are about 12-16 QTS of fluid. Changing out 3-4 at a time at least gets fresh fluid in there, and replenishes the friction modifiers and detergents.
If you're really nervous about it, drive for 3-7,000 miles, and do it again! It will take about 5-6 changes to change out about 50% of the tranny fluid.
A word: If your transmission fluid is BLACK, it is probably NOT a good idea to change too much of it at once. See that Black stuff? THAT is mostly what is providing the friction for the car to move!!!
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