ATF Color

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Jun 17, 2008
United States
I know that motor oil color is a poor indicator of oil health, as so many on BITOG say over and over, but it seems many people don't feel the same about ATF color(?). I have read more than one post where it was said to change AFT if it was discolored. Or is this also a myth, and ATF color means little, just like with motor oil? Opinions?
If it's not red, it's bad, think mighty hard about changing some. Then changing some more over a period of several months until it is red. Even if it is red, if you have ~30k on most fluids it's time for a refresh. IMO, ATF color is indicative of when to cautiously change it.
I once spent money to have some Amsoil synthetic ATF analyzed after 50k miles of service in my sister's minivan. This was probably one of the first vehicles I had used it in, and it was certainly an application that got lots of abuse and hard service, although it did have a decent size factory aux. ATF cooler installed. The fluid did not look bright red anymore, in fact it did not have much red left in it. Also, it did not smell new and was starting to get an oxidized smell to it. I do not have the numbers from that analysis anymore, but they said it was fine for continued use. We had already changed it again, flushing the whole unit out to get new fluid throughout. It has been changed once more since then, and this van currently has well over 300k miles on the original transmission. But, getting back to your questions, I look at the color and smell of the fluid to get an idea of how it is doing. The smell tells me more since I seem to be able to smell oxidized ATF a mile away. Comes from the days in tech school rebuilding burnt GM TH350's. ATF holds up much better usually when there is adequate or even upgraded ATF cooling. Consistently higher running temps degrade fluid faster. Higher quality fluids resist this abuse better. Most factory-installed ATF fluids are much higher quality than fluids from 10 years ago. Another thing to consider about ATF is the mostly inadequate filtration from the factory design. It would help to have some type of filter in one of the cooler lines to help always keep the fluid clear of smaller wear particles.
You can learn a lot about a fluid from its color and smell. But, you can only verify it with a UOA.
ATF used to go brown and smell when it was tired, but those symptoms are not valid anymore. They can go brown or tan very quickly, and they smell foul when new.
I try to keep the fluid in any AT I work on (esp. the ones I own) a nice cherry red color. Generally if it has gotten a little brown or smells burnt/oxidized then I should have already changed it. A better rule of thumb to determine your transmission's condition is to see if there is any gray matter (clutch material or tiny bits of metal) on the dipstick when you wipe it with a white rag. Also most AT's have a magnetic drain bolt and you should not be seeing too much stuck to the magnet, especially larger metal flakes- which would be a bad sign if it persists throughout several drain/fills/flushes. Then there is your tranny filter (if equipped), you don't want to see lots of big metal flakes there either. I guess what I'm saying is you can go by the fluid, but better to gauge the load you are putting on your tranny and service accordingly. I don't think I have ever let an AT go more than 15k between service intervals. Some cars can go longer but it's cheap insurance for an expensive machine.
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