Dear Tom and Ray:

Hi, guys! I have a 2006 Toyota RAV4 that was due recently for the 60,000-mile maintenance. Toyota recommends that the transmission fluid be checked, but I told the independent shop that since it had never been changed (I bought it with about 20,000 miles), they should go ahead and change it. They called me later that day and stated that although they had flushed it with 12 quarts of Toyota fluid, it was still coming out brown, and recommended flushing it with another 12 quarts at just over another 100 dollars. I said OK, as I hope to keep the vehicle for many more years. I have used this vehicle only as a commuter car, no off-roading. Does it seem reasonable that it would require this second flush? -- Rick

TOM: It shouldn't. It sounds like they did an old-fashioned "drain and refill," Rick.

RAY: Basically, transmissions have two separate reservoirs: One houses the torque converter, and the other houses the guts of the transmission -- the valves, gears, clutches, etc.

TOM: When the engine is running, the fluid from these reservoirs mixes together. But when the engine stops, some fluid comes to rest in each place.

RAY: So if you drain only one of the reservoirs -- which is all you can do when you open the drain plug or drop the pan from under the car -- you end up changing only a little more than half of the fluid. The other half of the fluid, which was unreachable in the torque converter, remains untouched.

TOM: So when you run the engine again, the new fluid you just added will mix with the old fluid that was in the torque converter, and the mix will look dirty again. That's when your mechanic called you and asked you to authorize another fluid change.

RAY: We avoid that problem by using a system that changes 100 percent of the fluid under pressure. We have a machine that taps into the transmission's own circulation system to force out every bit of the old fluid, including what's in the torque converter, and replaces it all with new stuff.

TOM: Without seeing the state of your fluid right now, we can't tell you whether you just had a normal mixture of new and old fluid, or if the fluid is really dark brown, indicating that something is wrong with your tranny.

RAY: Now that you've done two flushes, my advice would be to drive it as is for a few weeks, and then take it back and ask them to take a look at your transmission fluid. If they come away gagging and holding their shop rags to their noses, there may be something wrong.

TOM: But if they say it looks fine, just normally dirty, then just keep driving. And next time, at 120,000 miles, find a shop that can do the powered flush for you.