Transmission fliud changes - general question

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I have three cars: #1 - '98 Plymouth Grand Voyager, 3.3L V6, 190,000 miles; #2 - 2000 Toyota Echo, 1.5L inline 4, 175,000 miles; and #3 - 2001 Saturn L200, 2.2L inline 4, 37,000 miles ... all of them with automatic transmissions. Here's what I'm wondering: my Plymouth van went through its first trans at 80,000 and its second one at 160,000. That vehicle gets a whole lot of around-town driving. The owner's manual said to change the fluid every 25,000 but I did it every 15,000 at the dealer's recommendation. Obviously it didn't help much. My Toyota, however, gets mostly highway miles. The owner's manual says that's 'non-severe' driving and that the transmission fluid doesn't have to be changed at all. The published maintenance schedule ends at 100,000 miles - after that I guess I'm just supposed to repeat the scheduled maintenance cycle - but it doesn't even say to change the fluid at that point. I'm not sure I feel comfortable 'ignoring' the fluid altogether for that many miles (I do check the fluid color and level every 5,000 miles or so). My Saturn (actually my teenaged son's car) also has a similar 'extended' service interval on the transmission - although I think the interval on that one is more like 50,000 miles. We haven't had that car but a couple of months. So ... what's going on? Has there been that much improvement in fluids and/or transmission design recently that more frequent changes aren't necessary? Any advice from you more experinced folks out there?

geezer.

posted by  geezer

every 15,000??? sounds excessive..... but then 25,000 seems a bit excessive to me..... lol.

posted by  lunatic987

this is done as a band-aid for some cars with overdrive trannys and undersized tranny oil coolers and crappy fluid.

Due to the heat generated in some of these trannys, the viscosity thins and the friction modifiers break down thus the frequent changes.

posted by  tbaxleyjr

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