Transmission Deceleration on throttle release

[ericandrews]

I regularly drive up to Northern Arizona, the Pinetop - Show Low area and drove the 2015.5 Denali there for the first time since I purchased it in February. It is quite a elevation change compared to the Phoenix area (1117 ft) to Show Low (6,400 ft).

I felt that the transmission acted strangely while in a downhill coast. The felling was similar to multiple downshifts on a manual transmission vehicle. The Yukon would reach high RPM 5-6K while coasting down large hills. I felt that I had to accelerate to keep the Yukon a decent speed. I have driven a 2005 Silverado 4x4 with the 5.3L back and forth for the last 10 years and never had this same experience. I would expect when you don't apply throttle the vehicle would coast without downshifts.

I just hit 4200 miles on the Yukon. Has anyone else had this issue? or is this common with the 8-speed?

I have also noticed a clunk/late catch issue when accelerating from a stop. But the issue is random and does not occur all the time.

Thanks.....

 

[Tahoe14]

They are replacing some of the 8speeds, are you part of that group?

 

[ericandrews]

They are replacing some of the 8speeds, are you part of that group?

I have not mentioned this issue to my service department yet. The build date of my Denali is late October 2014. I will bring it to their attention. Does anyone have a service bulletin on the transmission issues?

 

[gravitypad]

Could that be "automatic grade braking"? I just had that feature show up on my driver info display today as I was coasting down a hill. I didn't feel much difference with it on or off, so I'm not sure exactly how it would feel.

 

[ericandrews]

Could that be "automatic grade braking"? I just had that feature show up on my driver info display today as I was coasting down a hill. I didn't feel much difference with it on or off, so I'm not sure exactly how it would feel.

Interesting that could be what is going on. I just found this from the 2015 Yukon Model Overview Doc.

"AUTO GRADE BRAKING This feature automatically
senses when you want to slow your Yukon and
engages a Grade Braking shift pattern within the
transmission to help maintain your desired speed."

I wonder if it can be disabled? It seemed excessive considering I was going down mile long hills and the speed limit was 65.

 

[Tahoe14]

Look at this post which describes the clunk noise you are experiencing and maybe take a copy when you go to the dealer "Clunk noise at acceleration until warms up". Good luck.

 

[johnnybogey]

Same problem here. Coasting downhill, the auto grade braking kicks it, RPM's jump but at the same time, I got these two violent kicks. Literally sounded like the tranny fell out.

 

[ericandrews]

I am taking it in for service tomorrow. They are replacing some of the dash padding and aluminum trim pieces that have been destroyed from disassembling my dash so many times to fix the onstar/nav/screen flicker issues.

I already spoke to them about the transmission downshifting but will have the service department investigate tomorrow. I will keep you posted.

 

[ericandrews]

I have confirmed with the service department this is Grade Braking.

I also found the following excerpt from the manual how to disable Grade Braking:

Normal Mode Grade Braking
If equipped with a gasoline engine and an automatic transmission, Normal Mode Grade Braking is enabled when the vehicle is started, but is not enabled in Range Selection Mode. It assists in maintaining desired vehicle speeds when driving on downhill grades by using the engine and transmission to slow the vehicle. The first time the system engages for each ignition key cycle, a DIC message will be displayed.

To disable or enable Normal Mode Grade Braking within the current ignition key cycle, press and hold the Tow/Haul button for five seconds. When the button is released, the requested mode change is made. A DIC message displays.

 

[cjweb11]

sounds like a dumb feature. one that should be auto-off and you can turn on if u want it.