Transmission fluid hot. Idle engine

[franksbanks]

In 108 ambient temperature not hauling anything (other than two young kids, wife and I) I was coming up a mountain climb between el centro and descanso,CA and got a message about the transmission fluid being hot. I switched the driving mode from normal to towing to get the transmission temp and it was around the 270 if not more range.(hard to tell since I was trying to drive and merge)

I let it idle for 15mins and the message when away. Temp was then at 220. ~30mins later (cruising at 65) I had to accelerate to pass and caused it to spike from 220 back to 250 and kept climbing within a span of 1-2 minutes.

About to order the transmission bypass block but wanted to see if anyone has had the same issue? (Still under warranty)

2021 Yukon denali 6.2 l87 10speed without heavy duty tow package.

 

[JKeller]

How many miles on the vehicle and have you ever had the fluid and filter changed?

 

[franksbanks]

It’s got 71k dealer had done a fluid change prior my purchase according to Carfax

 

[jfoj]

The gas engine transmission thermostat is inside the valvebody and requires the valvebody to be removed in order to bypass the cooling loop thermostat. The external bypass block is for the 6 cylinder Diesels.

My 2024 with the basic towing package option never had trans temps above 165F in higher ambients. Based on the temp spike you NEED to change the trans fluid at least 2 times to get most of the old fluid out.

I vaguely remember a technical bulletin for a twisted flexible cooling line that would reduce the oil flow to the transmission cooler and cause high trans temps. I don't recall if that particular bulletin applied to the gasoline engine transmission, but you could search.

This is the bulletin and it appears to be for your year and model. Interesting the trans fluid was just changed, wonder if this problem has been happening for a while??

https://static.nhtsa.gov/odi/tsbs/2024/MC-11008962-0001.pdf

 

[franksbanks]

I’ll take a look and thanks for the additional info about the TBV

 

[tagexpcom]

I have a 2021 Yukon Denali 6.2... does not have tow package... but I tow a 5500lb cargo trailer (camper) a lot. Specifically tested towing up a 10mile, 6% grade at 107F in Aug as part of pre-purchase and trans only reached 200F. The only time I've seen it go higher is during stop-go (1mph) going up a mountain pass (Grapevine in Southern CA) or one time going up a multi-mile gravel road so rough I had to stay <10mph to a campsite... and this is due to zero air flow and towing. Even during the stop/go Grapevine experience, it slowly climbed to 226F after 30min of 1mph stop/go traffic jam and then cooled right back down to 180F once I was able to get speed.

I say the above to emphasize I think the rig is designed better than you're experiencing - so something must be wrong. You should not be getting such hi temps and you're not even towing. And sounds like you're doing highway speeds - so air-flow/cooling should be in affect and you should be in 170-180F range at most based on my experience.

 

[JKeller]

I agree with the above post about something not being right. I tow a 5,000 lbs trailer 75mph and have never seen my tranny temp go above 170. I don't think I've ever seen above 165 going 80 on the highway unloaded (i.e., not towing). That is both with my prior 2021 Tahoe HC with the 6.2 and my current 2026 Yukon Denali with the 6.2. I am located in Florida and we may not be at 108*, but we're not too far off at times.

 

[PG01]

Imo 1st thing is check level of fluid, 2nd thing is change it asap 3rd look through the grill and make sure there isn’t any thing blocking airflow… bugs, plastic bag etc.
If dealer really did do a flush and fill maybe they didn’t fill it completely.
Just some thoughts

 

[Joseph Garcia]

I agree with the folks above. Something is wrong, and you should take it back to the dealer to get diagnosed and corrected.

 

[jfoj]

Could also be an faulty temp sensor in the valve body. Get an IR thermometer and measure the bottom of pan and compare to onboard reading. 270F is crazy high and the trans fluid would be immediately cooked and broken down. Given most of the vehicles rarely even have trans temps that even get high enough open the trans thermostat, I think this may be a faulty reading/bad sensor/bad connection to the temp sensor.

The only way I see the temps climbing that high would be a problem with the torque converter not locking up or a bad on way clutch for the stator inside the torque converter. The bulk of heat in automatic transmissions is generated within the torque converter when it is slipping and not locked up