“General Motors’ in-house towing team expert provided RV Clinic with this statement: The maximum allowable automatic transmission fluid temperature is dictated by the transmission oil itself. The oil begins to degrade significantly above 270 degrees Fahrenheit, so we design  http://www.silveradosierra.com/transmission/maximum-tranny-temp-t1877.html#">vehicles  so that in all but the most extreme conditions, the fluid temperature in the transmission sump stays below 270 degrees F.

We allow for up to 285 degrees F in extreme conditions (i.e. towing a trailer with combination loaded at GCWR in Death Valley). But for customer usage anywhere else in the country, even at GCWR,  http://www.silveradosierra.com/transmission/maximum-tranny-temp-t1877.html#">transmission  sump temperature should stay well below 270 degrees F. Above this point, certain internal components, such as seals, begin to disintegrate rather quickly. Although newer synthetic fluids can withstand higher temperatures we still recommend this (270F) as a maximum temperature. "

Recommended service interval for tranny fluid change for my truck is 50K, I had mine done at 65K just because I had an extra $150 and nothing to spend it on, ha ha.

 

I could tell when the computer turned the fans on because the engine temp would slowly creep to 235, then quickly drop to 210 (where it always stays in normal driving), then slowly creep up again.  The tranny temps were less variable but the higher readings often corresponded to higher engine temps, too.

 

I also found that info supposedly from GM, I did a search while we were on the road, so I didn't worry too much.  The fluid is still mostly pink and no burned smell.  However, I'm considering getting this external cooler:  http://www.etrailer.com/Transmission-Coolers/Derale/D13960.html - http://www.etrailer.com/Transmission-Coolers/Derale/D13960.html   Its thermostat triggers its own fan at 180, so that tells you where Derale thinks your tranny temps should be.

 

 

My towing temperatures with both the Taco and Tundra (different engines and transmissions) have been similar at 185-195 regular towing rising to 215-235 climbing or w/lotsa stop and go. These are conditions we see a lot of here in WA State. I have found that towing with the Tundra, I can minimize the number of higher temp episodes by towing in T/H, locking out 6th and manually shifting to 4th around town, climbing, or in rollers. W/out the Scan Gauge, I would be clueless about trans temp control, as, even though the Tundra has a trans temp gauge, it is little more than a fancy idiot light, not showing a temp rise until after about 270 degrees. Both the Tacoma and Tundra come with WS transmission fluid, which is pretty new age and is supposed to easily stand the temps I've been seeing.

 

Also, Tidalwave, Scan Gauge occasionally adds new codes to their web site. The one that is successful for the Tundra is supposed to be good for lots of Toyotas.

 

Normal driving (not towing), the transmission fluid temp (TFT) is in 170-180 range.  While towing its in the 205-220 range.  The maximum temperature its reached - very hot day, long uphill pull - is 229, which is my maximum comfort level; any higher and I would have pulled over to let it cool down.

I drain and replace the tranny fluid once a year (generally in the fall after I've winterized the trailer) and by then the fluid is quite dark and obviously in need of replacement.