ATF thermal stability

[NMBurb02]

At what temperatures do various auto transmission fluids begin to experience accelerated thermal breakdown, such as at rates greater than intended for normal or even severe use fluid change intervals?

The reason I ask is that the Dexron-VI in the GP normally varies between 203F and 206F, but has hit 231F in 104F heat while driving at 70 mph with a few consecutive hills and has stayed at 213F for extended periods during pretty smoothly flowing 65 mph at 95F+ ambient. The owners manual states that 180-200F is the normal operating temp, which is where the fluid needs to be when checked for fluid level. The normal drain and fill interval is 100k miles and severe is 50k. I have given up on trying to lower the fluid temps and was planning on changing every 25k due to those temps, but am now thinking that might be excessive as the manual for the Burb says the normal operating fluid temp is 100-265F and the high transmission fluid temp warning will not display until it hits 275F, and that is running Dex-III and specifies the same drain and fill intervals!

I wasn't so anal about checking the temps in winter and spring, so I don't know how hot/cool it runs with ambient temps below 90F. Should I be able to run 50k with these summertime temps?

 

[901Memphis]

I would just do a drain and fill every 30k and call it a day. 50k is fine, but a little too long for me.

You could always ad a transmission cooler if you want to drop the temps a little.

 

[JHZR2]

Most reaction rates align with Arrhenius, which means that the rates relate to the exponential e^(-E/RT) type of characteristic.

Usually that means that rates can double roughly each 10 degrees celsius. But remember that the rate may be slow, the reaction may be impeded or a lot of AO additive may be in there that depletes at some rate that may or may not behave in such a manner, etc. Lots to it.

In general, ATFs have been designed with this king of thing in mind, and while as a rule, cooler is better, the design attributes for the fluid and service intervals suggested account far beyond any elementary handwaving that we can do, and instead uses FTIR and other advanced techniques to identify side reactions occurring in time.

 

[NMBurb02]

Originally Posted By: JHZR2
Most reaction rates align with Arrhenius, which means that the rates relate to the exponential e^(-E/RT) type of characteristic.

Usually that means that rates can double roughly each 10 degrees celsius. But remember that the rate may be slow, the reaction may be impeded or a lot of AO additive may be in there that depletes at some rate that may or may not behave in such a manner, etc. Lots to it.

In general, ATFs have been designed with this king of thing in mind, and while as a rule, cooler is better, the design attributes for the fluid and service intervals suggested account far beyond any elementary handwaving that we can do, and instead uses FTIR and other advanced techniques to identify side reactions occurring in time.

Wow...that went waaay over my head. The only part that I understood was the first 2/3 of the last sentance and going off of that, I'll stick with 50k, if I even get that far before it gets totalled or sold as it has 140k miles and only 1k on the current fluid (full line off exchange and filter change).

 

[NMBurb02]

Originally Posted By: 901Memphis
You could always ad a transmission cooler if you want to drop the temps a little.

It has a stock cooler that is integrated in the radiator and I had considered adding a supplemental inline cooler, but don't really see enough of a return on the cost and time commitment for doing such. I think the temp issues are associated with the radiator/transmission cooler as it was pretty beat up (looked like the PO ran into a pile of sand at a pretty good clip) and I figure I may end up replacing it sometime down the road if it becomes necessary for some other reason.

 

[Nyquist]

I had a 2007 Malibu with the 3.5L LZ4 that I used to drain & fill (DEX VI) every 40,000 to 45,000. It always shifted nicely, and the fluid never really showed signs of being used up or sustained thermal stress.

 

[chubbs1]

I would not mess with it before 50k, these more modern fluids can handle more heat and stress than we give them credit for. Unless you are racing or hauling through mountains (which I doubt you are doing) the fluid should hold up fine for a minimum of 50k.

 

[JHZR2]

Originally Posted By: NMBurb02
Originally Posted By: JHZR2
Most reaction rates align with Arrhenius, which means that the rates relate to the exponential e^(-E/RT) type of characteristic.

Usually that means that rates can double roughly each 10 degrees celsius. But remember that the rate may be slow, the reaction may be impeded or a lot of AO additive may be in there that depletes at some rate that may or may not behave in such a manner, etc. Lots to it.

In general, ATFs have been designed with this king of thing in mind, and while as a rule, cooler is better, the design attributes for the fluid and service intervals suggested account far beyond any elementary handwaving that we can do, and instead uses FTIR and other advanced techniques to identify side reactions occurring in time.


Wow...that went waaay over my head. The only part that I understood was the first 2/3 of the last sentance and going off of that, I'll stick with 50k, if I even get that far before it gets totalled or sold as it has 140k miles and only 1k on the current fluid (full line off exchange and filter change).


Short version - oxidation and other degrading chemical reactions all occur faster at higher temperatures. But the additives have already been designed to resist this at an interval at least of what is claimed to be severe service.