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So 3509 miles on Duramax Denali HD and love pretty much everything so far.
What doesn’t seem right, however, is the way the transmission seems to downshift so early that the rpms raise when simply coasting down hills. Today I was coasting downhill at about 35 mph and it downshifted so much it raised the rpms to 2000!
I would expect it should just be coasting at idle rpms going downhill.
Am I missing something? Does this use more fuel even though my foot is off the accelerator pedal?
 

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It will not coast as a regular old truck and automatic transmission because the torque converter locks up. I think the 2020 will lock up in 2nd gear. It the exhaust brake is engaged and you touch the brakes, even a little it will down shift. On the 17 to 19 model’s you have to engage the exhaust brake every key cycle, but I think on the 2020 it will stay on.
 

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I forgot to mention that no it is not in tow/haul mode and no the exhaust brake button is not selected (I’ve never noticed much of a difference when I have tried it anyway) and yes it turns off every time you restart the vehicle.
 

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Just an FYI - I don't think that transmission is an Allison. Whole new critter.
 

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Just an FYI - I don't think that transmission is an Allison. Whole new critter.
It is a allison, but it is a different Allison, as GM has bought Allison and is doing the pickup and med duty transmission design and build in-house. I imagine its going to be a learning experience, I read something about it being a sealed unit as well with out conventional dip stick and fills and drains? Can anyone that owns one confirm?
Over many miles of towing with Allison/Duramax I have discovered (most) of the time let the truck make the decisions and it will do well.
I would not worry about what seem to be a early downshift and no it will not effect fuel mileage because of RPM if your off the throttle.
Remember this is speculation from my limited brain function and a million + miles of towing.
 

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The new transmission is not a Allison as previously mentioned. However the previous transmission was “Allison designed” and had been in production for many years now with continual upgrades. The new 10 speed is produced by GM in a GM factory and supposedly approved by Allison for what ever that’s worth. Unfortunately, no more actual Allison designs for 1ton and under trucks.


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Honestly guys who cares if this is or isn’t a true Allison trans. Allison was involved in the design and just because it is not a true Allison does not make this a worse trans. I have a 19 ram HO with the Aisin and a 2020 at4 with the Allison and the GMC trans blows the Aisin away. Despite not being a true Allison I think this is hands down the best trans ever offered in a pickup, it’s buttery smooth and always seems to be in the right gear.
 

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So 3509 miles on Duramax Denali HD and love pretty much everything so far.
What doesn’t seem right, however, is the way the transmission seems to downshift so early that the rpms raise when simply coasting down hills. Today I was coasting downhill at about 35 mph and it downshifted so much it raised the rpms to 2000!
I would expect it should just be coasting at idle rpms going downhill.
Am I missing something? Does this use more fuel even though my foot is off the accelerator pedal?
Hello PGAIDUK how are you? Do you still own this vehicle and is the "auto grade brake" still operating as you describe is your post? If so can you please post a short video clip of the instrument cluster while this condition is present? I have a post here titled "Auto Grade Brake/High RPMs Downhill. I am trying to gather as much info on this condition as I can. My vehicle is a 2020 Silverado 3500 HC Duramax DRW. My truck use to do exactly what you describe but no longer does after a trip to the Chevy Service Dept. where it was Programmed for some SIBs. My issue is I absolutely LOVED that feature and now it's gone! The Service dept. looks at me like I'm crazy bc they say it is operating as designed, BS!! It no longer has an "Auto Grade Brake". They have absolutely nothing to compare so I have been posting on multiple forums and you are only the (3) person INCLUDING myself that has experienced this feature in all its glory!! Any feedback would be enormously appreciated.
 

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The new transmission is not a Allison as previously mentioned. However the previous transmission was “Allison designed” and had been in production for many years now with continual upgrades. The new 10 speed is produced by GM in a GM factory and supposedly approved by Allison for what ever that’s worth. Unfortunately, no more actual Allison designs for 1ton and under trucks.


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Yep you nailed it. Been same design for like forever with some improvements along the way. Was a original Allison design inside and out. But GM sold Allison years ago and now this new 10 speed is totally a GM design. Yes they are trying to use marketing to keep Allison somehow associated but it’s a totally and completely different animal. I’m sure at some point it will be fine though just need to get the kinks out.


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Nothing magic about Allison. It was just another division of GM when the 5/6 speed was designed.
GM sold it in about 2008, iirc.

6 speed was same gears/clutches packs as 5 speed, but with a different valve body that permitted a previously unused gear combo. Yes, there were enhancements to the unit to match the increases in engine torque in newer 6.6 generations.
 

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Nothing special? Really? Just happened to be the first reliable automatic transmission that GM had to handle diesel power output. Oh another little thing it was THE transmission that helped put them on the map with 3/4 and 1 ton trucks. Ford owned that market until Duramax and Allison were attached to the GM name plate. They were smart to partner with companies that could implement proven designs that could handle the application.


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I certainly wasn't trying to put down the talent gathered in the Allison division.
There is a remarkable history there.

Point was, purchased in 1929, Allison (originally an engine maker) was a GM division until they sold it in 2007.
So it was a division of GM that developed engines and transmissions that made Allison so respected. It was a division of GM the developed the 5/6 speed transmission that does such a remarkable job behind the Duramax.


Marketing folks prefer that customers to think otherwise, but it's not the 'name' that we are buying, it's the performance (and the price vs performance). Yes, naturally, branding has an impact -- so marketing and advertising types like to hang names on stuff. Sometimes a name on new feature. Sometimes they use are a division name or a manufacturing location name. Sometimes the a location name develops a life of it's own and becomes a 'brand name' or a 'division name'.
None of the 'names' are a guarantee of future performance. Lots of brands can and do end up with duds in the line-up.



Interesting list of Allison milestones here:
 
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Actually, Allison did have quite a bit to do with the development of the 10L1000 whether anyone wants to believe it or not.

Secondly, to the OP’s question, it sounds like the trans is doing it’s normal cruise/grade braking routine.
 

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Yep that’s right they’re called a Fordison.

Still can’t believe they did that but I guess times are changing. I wonder what will be next?

Maybe a CumMax Diesel engine?


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