Hi all, looking for any working knowledge of the ATF Thermal Degradation Estimate number that you can read using OBDII or Techstream. Have searched a good bit, and have found only limited information on various Toyota forums, but not too much that is helpful. The number maxes out at 65535, and mine has been there as long as I can remember. As in, I think it was already at max when I first got my OBD fusion app to be able to read it (maybe 55-60k miles or so).

All that said, I question the accuracy or meaningfulness of this number. Currently at 74k miles. I have watched my transmission temps pretty closely during all my normal driving and typical temps rarely spike above 170degF, and I don't know I've ever seen a temp above 200degF. Never tow, and have only been to the mountains once in this vehicle. So pretty easy on the transmission overall I think. I can't find anything in the 4Runner manual, but other Toyota manuals do talk about replacing fluid at thermal degradation > 50000. Being that the manual has the first transmission fluid replacement at 60k only if towing or other heavy loading conditions, I'm surprised that my fluid would be indicating thermally degraded at this point. Since I can't find anything about it in the service manual, I'm thinking it's quite possible the 4Runner simply doesn't use this ATF thermal degradation number, and the OBDII just reads it at max, and it's not valid.

I do intend to start doing some periodic drain and refills on the transmission, but wondering if there is any benefit to resetting this thermal degradation number, or any downside to not resetting it. I do not have Techstream so would be some effort to get it reset.

Also wondering if I have any reason to be concerned about already being at the max reading of 65535 (and having been there for some time).

Any input appreciated.

@renau001

I was able to get answers for questions about ATF and thermal breakdown for my 4th Gen by talking to the Master Mechanic and Parts Manager at my local Dealership.
If you don't have any luck with Toyota, You might try reaching out to the Folks at Scanguage.

Boort

Any chance it's influenced by engine cycles? Numerous short trips on a cold engine are harder on equipment than one long drive cycle. Woikd this apply to the transmission?

I just recently flushed my tranny and tried to reset the thermal degradation number via techstream. I encountered an error and reset didnt happen. My techstream may have an issue that I am posting about in another thread.

I have come to the conclusion that this 4Runner doesn’t use the ATF thermal degradation and it just reads at max on OBD readers since it’s not used.

I’m basing this on the fact that the manual does not address reset in the 4Runner manual but it does in the Tacoma manual. And I haven’t seen anybody report anything other than the max value.

Maybe somebody that knows better can correct me. Or maybe somebody with a new 4Runner and an OBD reader can check their number.

Transmission fluid is exposed to transient temperatures significantly higher than 200 F even if you don't tow or drive in the mountains.

Every time an automatic clutch pack engages, transmission fluid between the plates heats up. Fluid that undergoes frictional heating during engagement is rapidly cooled through bulk mixing with a much larger amount of cooler fluid in the transmission. The amount of fluid that heats up due to engagement is very small and yet the accumulative effects of a transient thermal spike with each transmission shift eventually degrades the fluid.

I saw a paper that presented measured ATF transient temperature spikes at the clutch pack in a test vehicle several months ago, but I can't find it now. This paper, https://www.diva-portal.org/smash/ge...FULLTEXT01.pdf presents analysis if you are interested in the details of a thermal model that matches a 250 F transient fluid temperature peak measured during a long clutch engagement time in a test rig. A short engagement time results in a 360 F peak transient temperature. High torque loads increase peak transient temperature, under various conditions such as driving a loaded vehicle, rapid vehicle acceleration, frequent acceleration to high speeds, towing or climbing a grade, etc. You'll never observe these transient temperature spikes by reading transmission temperature with a scan gauge. The transient spike is very short and rapidly quenched through bulk mixing.

Google "transient fluid temperature during wet clutch engagement" for a list of technical papers on this effect.

Toyota recommends replacing transmission fluid at 60,000 miles under special operating conditions. I occasionally drive to a 7000 feet elevation, so I have elected to replace transmission fluid at 90,000 miles.

Why not just flush the whole trans? Its not hard.

I'd wholly recommend this after 60k as well. Doesn't have to be OEM fluid either. Just about anything will do.

I certainly understand ATF degradation, was just curious about the ATF thermal degradation estimate value on my OBDII reader. On our new to us GX460 with about 17k miles, the number is ~900, with the manual saying to replace fluid at 50000. My 4Runner has been maxed out at 65535 for as long as I can remember, that's what I was trying to find out about. I've come to the conclusion that the 4Runner doesn't have the sensor that the Lexus and the new Tacomas do.

I don’t own a vehicle with an oil life estimator. Looking at the numbers you quoted, it appears you have consumed 1.8% life in the first 17,000 miles. I’m curious about the algorithm used to compute oil life.

I’m assuming the ECU continuously monitors operating conditions such as speed, load, ATF temperature, trip time, etc., and uses an algorithm to increment the oil life number. If oil degradation is linear and if you do not change your driving habits, then ATF oil life will be exhausted at 945,000 miles. That doesn’t make sense for typical daily commuting with a mix of city and highway driving, mostly short and some long trips, therefore I'm wondering if ATF life degradation accelerates as mileage accumulates.