Transmission Temp - did I make a mistake?

[Jhwhite05]

My reman transmission with less than 20k miles with an updated torque converter typically runs below 185 on warm days and 230 when going uphill on mountain pass at 45-55mph in tow mode. I have an external Derele transmission cooler.
Recently while towing in tow-mode on a very steep grade for 25 minutes at 15-40mph, the transmission temp on the dash got to 261. The car seemed to perform normally.
Was it unwise to take the transmission to this temperature and/or it is OK to do so occasionally?

 

[Doubeleive]

if it was a one time thing I am sure it's fine, but I wouldn't make a habit of it
The optimal temperature range for transmission fluid is 175 to 220 degrees. Above that, for every 20 degrees bad things happen, starting with formation of varnish at 240 degrees, followed by seals hardening, plates slipping, seals and clutches burn out, carbon is formed, and, ultimately, failure.

 

[NickTransmissions]

My reman transmission with less than 20k miles with an updated torque converter typically runs below 185 on warm days and 230 when going uphill on mountain pass at 45-55mph in tow mode. I have an external Derele transmission cooler.
Recently while towing in tow-mode on a very steep grade for 25 minutes at 15-40mph, the transmission temp on the dash got to 261. The car seemed to perform normally.
Was it unwise to take the transmission to this temperature and/or it is OK to do so occasionally?

At 250 degress, fluid starts to break down and lose its lubricative properties and fails to do its job.

Exchange the fluid with fresh dex6 (id try to do a full exchange vs just a partial)

ETC: removed references to the cooler bypass valve - for some dumb reason i thought they were installed onto the 07-14 6Ls...

 

[Doubeleive]

At 250 degress, fluid starts to break down and lose its lubricative properties and fails to do its job.

Do the following immediately:

1) exchange the fluid with fresh dex6
2) install GMs updated thermal bypass valve or flip the pill on yours
3) pray your 4-5-6 clutch pack didnt burn

the gmt900's do not have the thermal valve

 

[swathdiver]

My reman transmission with less than 20k miles with an updated torque converter typically runs below 185 on warm days and 230 when going uphill on mountain pass at 45-55mph in tow mode. I have an external Derele transmission cooler.
Recently while towing in tow-mode on a very steep grade for 25 minutes at 15-40mph, the transmission temp on the dash got to 261. The car seemed to perform normally.
Was it unwise to take the transmission to this temperature and/or it is OK to do so occasionally?

See post #290 in this thread:

Useless Information

On Sunday my youngest ran the battery dead and had to get jumped. This somehow caused my engine hour meter to reset to zero! Didn't discover that that had happened until today while refueling. It'll be a pain to reprogram and cost $55 at Tis2Web to boot. Last tank just finished up, 13.2 mpg...

www.tahoeyukonforum.com

Is your Derale larger than the OE cooler and did you mount it in the same place as the old or move it down and out of the way?

if your truck were mine and it got that hot, I would change the fluid when I got home. As it is, we change it every 36K miles nowadays unless we tow often then we do it sooner.

Some came with 6L80s. 2007-2014. 6L80 went into production in 2006 and was installed into numerous gmt900 platforms including submodels of the tahoe and yukon.

The OP Nick will have a 6L80 for 2009. The GMT900s did not get the thermal bypass valve. That started with the K2s when they moved the cooler to the condenser.

 

[NickTransmissions]

See post #290 in this thread:

Useless Information

On Sunday my youngest ran the battery dead and had to get jumped. This somehow caused my engine hour meter to reset to zero! Didn't discover that that had happened until today while refueling. It'll be a pain to reprogram and cost $55 at Tis2Web to boot. Last tank just finished up, 13.2 mpg...

www.tahoeyukonforum.com

Is your Derale larger than the OE cooler and did you mount it in the same place as the old or move it down and out of the way?

if your truck were mine and it got that hot, I would change the fluid when I got home. As it is, we change it every 36K miles nowadays unless we tow often then we do it sooner.


The OP Nick will have a 6L80 for 2009. The GMT900s did not get the thermal bypass valve. That started with the K2s when they moved the cooler to the condenser.

Gotcha, thanks @swathdiver , .

@Doubeleive - my bad, disregard.

 

[Doubeleive]

Some came with 6L80s. 2007-2014. 6L80 went into production in 2006 and was installed into numerous gmt900 platforms including submodels of the tahoe and yukon.

ETA: he mentions "updated torque converter" in his first post - no need to specify that as there are/were no "updates" required for TMBX converters as they didnt have the failure patterns like the JMBX converters that came with the 6L80s. That's what lead me to believe his is a 6L and not a 4L.

If he has the 4l60e, swap "4-5-6 clutch" with "3-4 clutch" and change the fluid. No TBV to worry about. Either way, trans overheated; hope there was no damage.

the 07-14 gmt900's did not have a thermal bypass valve, that started in 2015 and changed (mounting) design shortly after and then was recently updated in the last couple years with the lower temp valve

 

[Marky Dissod]

After searching several ATF transmission charts (which were admittedly written before DexRon6), oversimplified rules of thumb appear to be as follows:

176°F ... 100,000 Miles ... Ideal sustained operating temperature
194°F ... 50,000 Miles ... Maximum recommended sustained temp
212°F ... 25,000 Miles ... Pressure drops

230°F ... 12,500 Miles ... Varnishes Form
248°F ... 6,250 Miles ... Seals Harden
275°F ... 3,125 Miles ... Clutches Slip
293°F ... 1,562 Miles ... Oil forms carbon, seals and clutches burn

Over 293°F, measure in minutes instead of miles - or just rebuild the damn thing already ...

 

[swathdiver]

After searching several ATF transmission charts (which were admittedly written before DexRon6), oversimplified rules of thumb appear to be as follows:

176°F ... 100,000 Miles ... Ideal sustained operating temperature
194°F ... 50,000 Miles ... Maximum recommended sustained temp
212°F ... 25,000 Miles ... Pressure drops

230°F ... 12,500 Miles ... Varnishes Form
248°F ... 6,250 Miles ... Seals Harden
275°F ... 3,125 Miles ... Clutches Slip
293°F ... 1,562 Miles ... Oil forms carbon, seals and clutches burn

Over 293°F, measure in minutes instead of miles - or just rebuild the damn thing already ...

We've seen that the K2s generally give up the ghost by about 100K miles and they run around 195-200 all the time.

 

[Marky Dissod]

We've seen that the K2s generally give up the ghost by about 100K miles and they run around 195-200 all the time.

GMT K2xx used 6L80E & 8L90E. Does that apply to both, or one moreso than the other?