You can't flush the system yourself without the equipment. All you can do is a drain and fill with a filter replacement. That gets about half of the fluid replaced which is sufficient for general maintenance. When your mechanic flushes the system, they typically add a real strong detergent first, then drive it a little before doing the flush. That procedure gets all the old fluid out along with the detergent. The equipment is expensive, and many systems run through twice as much fluid as the system holds. A lot of new fluid washes right through the transmission and cooler and on out to the waste side. That wasted fluid adds to the cost.

In most cases, flushes are not necessary. On the other end of the extreme, I regularly use my '88 Grand Caravan, (non-computer-controlled transmission), to pull a tandem axle enclosed trailer that's bigger than the van, and I've replaced the filter and fluid once in 230,000 miles and 23 years. That's abuse, not neglect, but it shows what those old tough transmissions were capable of. Too bad they don't build 'em like that any more.

Answered by caradiodoc (expert)
17,655 answers provided