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Discussion Starter · #1 · (Edited)
9 Speed Automatic Transmission

The Hydra-Matic 9T45 transmission is a fully automatic, 9-speed, transverse-mounted, electronically-controlled transmission. It consists primarily of a 4-element torque converter, a compound planetary gear set, friction and mechanical clutch assemblies, and a hydraulic pressurization and control system. The multiple disc clutches combine with the selectable one-way clutch to deliver ten different gear ratios — nine forward and one reverse.



The selectable one-way clutch has two modes of operation. In the released mode, it will hold in one direction and free wheel in the other direction. In the applied mode, it holds in both directions.

The Transmission Control Module (TCM) is a standalone unit mounted to the front of the transmission. The 9T45 9-speed transmission uses a line pressure control system during upshifts to compensate for new transmission build variation and the normal wear of transmission components. The TCM adjusts the pressure commands to the various control solenoids to maintain the originally calibrated shift timing.


Gear Ratios
1st - 4.69
2nd - 3.31
3rd - 3.01
4th - 2.45
5th - 1.92
6th - 1.45
7th - 1.00
8th - 0.75
9th - 0.62
Reverse 2.96
Final Drive Ratio 3.17:1


CVT Transmission

The VT40 is a fully automatic single mode continuously variable, front-wheel drive transmission. The transmission features:
  • a four-element torque converter
  • one planetary gear set
  • an electronic hydraulic pressurization and control system
  • a variable drive and driven pulley assembly
  • variable drive chain
  • two friction clutches
  • a differential assembly


The four-element torque converter contains a pump, a turbine, a pressure plate splined to the turbine, and a stator assembly.

The transmission fluid pump is a binary vane type, which effectively allows for two pumps in the packaging size of one. The design of the pump provides for ideal oil routing to the controls system, with the pump located in the valve body itself.

Variator Ratio Spread 7.0:1
Underdrive 2.621
Overdrive 0.374
Overall Ratio Spread 7.0:1
Reverse 0.67:1
Final Drive Ratio 5.45:1
 

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I really wonder how these CVTs are going to compare to previous generations. I don't know if they've made major mechanical advancements to where we won't see the failure rates like we did with Nissan.
 

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They've been using them in the 2019+ Malibu and apparently have been very reliable. They've been able to tune them and hold over 200 lb. ft. of torque.
 

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It's chevy I'm sure they will have plenty of poor calibration issues. Mechanically they've probably fixed most of any issues. Only time and a few winters will tell.
 

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Mosaic Black 2021 Trailblazer RS AWD
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Anyone ever get a low pitched humming noise when the car is in a higher gear at low rpm? I know it stays in higher gears to cut fuel consumption, but when I took it on a road trip with the wife and dogs I noticed it on the back roads going around 45-50 mph. As soon as you give it gas and it shifts to a lower gear it goes away. Is this common?
 

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Discussion Starter · #6 ·
Anyone ever get a low pitched humming noise when the car is in a higher gear at low rpm? I know it stays in higher gears to cut fuel consumption, but when I took it on a road trip with the wife and dogs I noticed it on the back roads going around 45-50 mph. As soon as you give it gas and it shifts to a lower gear it goes away. Is this common?
I can't say I've noticed that on my RS, but I almost always have the stereo turned up. 3cyl engines are very unbalanced which is noticeable at lower rpms. I can sometimes feel this unbalance while stopped at a light and idling. I'll have to listen more carefully when the engine is at a lower rpm and higher gear range to see if I can hear it, but it isn't anything I'd be concerned about assuming it's the engine and not something in the drivetrain.
 

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I can't say I've noticed that on my RS, but I almost always have the stereo turned up. 3cyl engines are very unbalanced which is noticeable at lower rpms. I can sometimes feel this unbalance while stopped at a light and idling. I'll have to listen more carefully when the engine is at a lower rpm and higher gear range to see if I can hear it, but it isn't anything I'd be concerned about assuming it's the engine and not something in the drivetrain.
Ah, thank you. I only noticed because the stereo was turned down lol. I don’t think it’s something to be concerned about, but I was wondering if anyone else noticed this.
 

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Anyone ever get a low pitched humming noise when the car is in a higher gear at low rpm? I know it stays in higher gears to cut fuel consumption, but when I took it on a road trip with the wife and dogs I noticed it on the back roads going around 45-50 mph. As soon as you give it gas and it shifts to a lower gear it goes away. Is this common?
Yep, i have found that going around 45-50 sometimes it seems stuck in a higher gear and turning sub 2000 rpm (i think its around 1.8 if i remember correctly). Which in my previous experience with a manual transmission that this is normal if you are in to high of a gear). But if i give the TB some gas and force it to down shift then the hum goes away.
 

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Yep, i have found that going around 45-50 sometimes it seems stuck in a higher gear and turning sub 2000 rpm (i think its around 1.8 if i remember correctly). Which in my previous experience with a manual transmission that this is normal if you are in to high of a gear). But if i give the TB some gas and force it to down shift then the hum goes away.
Yes, that’s exactly what I’m experiencing! Weird that the computer wouldn’t down shift in a lower gear to avoid doing that. Only complaint I have really. Thanks for reassuring me on that one, Shogunmark!
 

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Yes, that’s exactly what I’m experiencing! Weird that the computer wouldn’t down shift in a lower gear to avoid doing that. Only complaint I have really. Thanks for reassuring me on that one, Shogunmark!
My other complaint is in sport mode it will hang out in the 4.5-5 range too long when decelerating. Mine will sit in that range so long that i sometimes turn off sport so it will shift. I understand that it's normal for sport mode to stay in a lower gear, but mine just seems a little excessive.
 

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My other complaint is in sport mode it will hang out in the 4.5-5 range too long when decelerating. Mine will sit in that range so long that i sometimes turn off sport so it will shift. I understand that it's normal for sport mode to stay in a lower gear, but mine just seems a little excessive.
I wonder if there’s going to be a software update later down the road for that. It’s almost like it replicates down shifting (in a manual transmission) and uses the drivetrain to slow the vehicle down. Hope they come out with something to fix that!
 

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Well it hangs out in the lower gear for acceleration purposes. But at a point it's excessive. I'm my case I'm going downhill for at least a quarter of a mile and it just hangs there, running at nearly 5k on the tach. And it makes me uncomfortable.
But i think the shifting should be improved in either mode. And if not then i have my work sounds.
 

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Discussion Starter · #13 ·
Well it hangs out in the lower gear for acceleration purposes. But at a point it's excessive. I'm my case I'm going downhill for at least a quarter of a mile and it just hangs there, running at nearly 5k on the tach. And it makes me uncomfortable.
But i think the shifting should be improved in either mode. And if not then i have my work sounds.
Our RS does the same and it's definitely a programming issue. My 18 Colorado also does it occasionally as if I wanted engine braking, lol.
 

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Our RS does the same and it's definitely a programming issue. My 18 Colorado also does it occasionally as if I wanted engine braking, lol.
I actually notice it more if the engine is not fully warmed up. So this is usually when i am leaving my house as its straight downhill out of the subdivision. but after the cars run for a bit it doesnt do it anymore..
Makes me wonder, could it possibly be hanging in the lower gear to improve engine temp? could it be that smart?
 
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