I need a little help understanding how water can get into an automatic transmission housing under normal driving conditions. OK, well, wet driving conditions in the Daytona area of Florida where it has been pretty rainy recently.

My sister has a Toyota pickup truck with an automatic. It is a relatively new vehicle, maybe two or three years old. She took it to the dealership for routine service, which included changing the transmission fluid. They reported to her that the fluid was contaminated with water. They did not suggest the transmission was trashed but did tell her that was very bad. Well, a few weeks later the transmission began shifting "differently" and she took it back. They drained the fluid and told her they found more water. They noted the transmission would not last long if it kept getting water in it. She swears other than running through some puddles (she agrees they are pretty big ones, but lots of cars run through them) the truck is not being subjected to any water exposure other than rain.

So, do automatics have vents or something that could be out of position and collecting water to add to the transmission? Is there some defect allowing water to get into the fluid? Did the shop just fail to drain the whole transmission? I am not an automatic transmission savvy type since I avoid them at nearly all costs. Thanks for the help, Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)