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"Allison" 10 speed hard upshift gears 7-8

UWSkier

By UWSkier
in 6.6L Gas V8 (L8T)

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UWSkier Enthusiast

UWSkier

Posted
Posted (edited)

EDIT: I can for sure say the problem is upshifting from 7th to 8th gear now.  If I'm on the power at all for that shift, there's a clunk.  Every time.  If I'm coasting, no clunk.  Shifts below 8 and above 8 are fine.  Just going from 7 to 8 is no good.

 

__________original post below_____________

Truck has 10,500 on it and the transmission has been a non-issue until just recently.  Occasionally, when accelerating with part throttle, say coming up to 50 MPH or so gradually, I'll get a rough shift.  It doesn't seem to do it if I'm on the gas hard, and it doesn't happen every time.  Anyone else seen this on their 2024?  Do I just need to take it out and flog it a bit, or do I need to drive like I have a raw egg under the pedal?

 

I'm about to leave on a cross-country trip towing the camper so this is super great timing with the current cyber attack crippling the auto dealers (scheduler I talked to today said he wasn't sure when they could even begin a diagnosis).

Edited by UWSkier
lineman1234 Enthusiast

lineman1234

Posted
Posted

11,500ish miles on my 2024 3500 gasser and has always and still does shift perfect. 

Another JR Senior Enthusiast

Another JR

Posted
Posted

I’m just guessing, but if you are reducing your acceleration as you pass through 50 mph that 10 speed is probably skipping one or two gears on that shift, which may cause a harder shift due to the big rpm change. 

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BuckWallace Enthusiast

BuckWallace

Posted
Posted (edited)

I have a 2024 3500 gasser and also notice this every once in a while when it shifts in lower RPMs (around 1500-2000). What I really notice is the hard downshifts when in stop-and-go traffic on a slight decline, typically in line at a stop sign. Sometimes it will violently downshift making it feel like I got rear-ended.

 

Do you also get the "whine" noise in gears ~3-7 starting around 35mph under light acceleration? 

Edited by BuckWallace
KARNUT Senior Enthusiast

KARNUT

Posted
Posted

I’ve had even on 6 speed if you change your input on the throttle the up or downshift can get abrupt. One needed a fluid change to fix.

UWSkier Enthusiast

UWSkier

Posted
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17 hours ago, Another JR said:

I’m just guessing, but if you are reducing your acceleration as you pass through 50 mph that 10 speed is probably skipping one or two gears on that shift, which may cause a harder shift due to the big rpm change. 

Does the HD transmission skip shift?  It seems like I can watch my tach and count the steps up into higher and higher gear.

 

16 hours ago, BuckWallace said:

I have a 2024 3500 gasser and also notice this every once in a while when it shifts in lower RPMs (around 1500-2000). What I really notice is the hard downshifts when in stop-and-go traffic on a slight decline, typically in line at a stop sign. Sometimes it will violently downshift making it feel like I got rear-ended.

 

Do you also get the "whine" noise in gears ~3-7 starting around 35mph under light acceleration? 

I'll listen for the whine.  This truck makes so many funky noises, it's hard to keep track of them all.

UWSkier Enthusiast

UWSkier

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so I've noticed that if I manually cycle through the gears, this NEVER happens.  Every shift is butter smooth.  My truck has exclusively been a tow vehicle for my boat for the past 500 miles or so, and I like how it drives better with tow-haul disabled for my 3500 lb ski boat.  I'm thinking perhaps the transmission has learned that there's typically a load and has stiffened up that 8-9 shift point accordingly, which I feel unloaded.

 

About to head cross country with the RV and I will definitely be using tow-haul for that.  Will be pulling some 7% grades at 115F or hotter ambient.  Will see how hot the transmission gets on those stretches.

  • UWSkier changed the title to "Allison" 10 speed hard shift 7-8
UWSkier Enthusiast

UWSkier

Posted
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Posted

Did the hard hot climbs today and trans never got above 212F, which was good.  Shifting from 1-7 is buttery smooth.

 

What ISN'T at all good is the 7-8 upshift.  It's a clunk every time unless I'm coasting.  I've locked out 8th gear for the drive and only manually kick up to 8th when I'm on a flat or downhill section for a bit better fuel economy.

 

Truck has under 11k miles on it.  NOT happy...  How does this transmission hold up to a Duramax if a 6.6L gasser is too much for it?

  • UWSkier changed the title to "Allison" 10 speed hard upshift gears 7-8
UWSkier Enthusiast

UWSkier

Posted
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I've been told that I should try to reset the TCM by disconnecting the battery for a half hour.  Paging @newdude since he knows more about this stuff than just about anyone... would this work?  Or do I need a dealer handheld to force a fast learn... and is it even worth trying?  Still just that one gear change after 1200 miles towing.

newdude Senior Enthusiast

newdude

Posted
Posted
12 hours ago, UWSkier said:

I've been told that I should try to reset the TCM by disconnecting the battery for a half hour.  Paging @newdude since he knows more about this stuff than just about anyone... would this work?  Or do I need a dealer handheld to force a fast learn... and is it even worth trying?  Still just that one gear change after 1200 miles towing.

 

 

Battery disconnected won't accomplish a fast learn.

 

GM does know about the 7-8 shift.  Affects gas and diesel.  

 

#PIP5971: Harsh 7-8 Up Shift MGU MGM MKM - (Mar 22, 2024)

Subject: Harsh 7-8 Up Shift MGU MGM MKM

 

Firm/harsh 7-8 up shift.

Cause

Under investigation.

Correction

Do not replace any parts at this time.

"Service Fast Learn" and/or "Transmission Adaptive Pressure Reset" may correct the concern for a short time.

There is no durability concern, and the customer can continue to drive the vehicle during this investigation.

Engineering is investigating and this P/I will be updated or transferred to a TSB as needed.

  • Like 1
UWSkier Enthusiast

UWSkier

Posted
  • Author
Posted
37 minutes ago, newdude said:

 

 

Battery disconnected won't accomplish a fast learn.

 

GM does know about the 7-8 shift.  Affects gas and diesel.  

 

#PIP5971: Harsh 7-8 Up Shift MGU MGM MKM - (Mar 22, 2024)

Subject: Harsh 7-8 Up Shift MGU MGM MKM

 

Firm/harsh 7-8 up shift.

Cause

Under investigation.

Correction

Do not replace any parts at this time.

"Service Fast Learn" and/or "Transmission Adaptive Pressure Reset" may correct the concern for a short time.

There is no durability concern, and the customer can continue to drive the vehicle during this investigation.

Engineering is investigating and this P/I will be updated or transferred to a TSB as needed.

Thanks!  I may call a local dealer at my destination to see if they can at least get me reset so my wife doesn't think the truck is falling apart every time it shifts. LOL

UWSkier Enthusiast

UWSkier

Posted
  • Author
Posted (edited)

it's kindof crazy they've opened this in March and had no updates though.  It's pretty annoying when it happens towing.  It sounds like the truck is going to spit the whole driveline on the highway, and that's a gasser towing a travel trailer.  I have to imagine it sounds like the rapture itself if you're running a Duramax and a 25k lb toy hauler.

 

I've basically just had to drive it with anything above 7th gear locked out.  My truck shifts 7-8-7-8-7-8 a lot when towing my camper 70-75 MPH.  It'd be banging every 5 minutes or less.

Edited by UWSkier
  • 3 weeks later...
UWSkier Enthusiast

UWSkier

Posted
  • Author
Posted

Finally got into a dealership to have the shift relearn done and it's back to shifting smoothly.  It took 11k miles before it started acting up before.  Hope it's good for at least a few thousand miles of towing before it hits again.  Really sucks not having 8th gear on a cross country tow.

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