2019 at4 5.3 rear end or trans slip.

[ATCinc]

Trying to find away to duplicate 

But getting periodic slippage. Anyone having issue.

[HondaHawkGT]

Are you sure you aren't just feeling the torque converter clutch as it begins to lock up?

[Steve Martin]

I get something weird like when repositioning in a spot backing in on occasion.  Maybe has happened about 10 times now..  I'll put in drive again and there is a horrible hesitation and then bang drops into gear.  Dealer hasn't been able to reproduce.  I think its a pressure thing or something.  In any case, shifting from reverse and forward vice versa, will sometimes do it for me.  Frustrating because I bought this truck for a transmission issue on my old one.

[leafspring]

Mines at the dealership rate now for this very issue, mechanic stated that he’s had a couple others in for this. It’s almost as if it starts in second gear and then bangs down into first. I have had mine do it quite a few times, usually when backing out of my driveway and going to take off in drive.

[Rednex]

Following / 19 TB here doing the same, Bang.  

 

I'm getting it sometimes when downshifting to pass as well, but more so from reverse to forward, or shift 1 / 2

[ATCinc]

6 hours ago, leafspring said:

Mines at the dealership rate now for this very issue, mechanic stated that he’s had a couple others in for this. It’s almost as if it starts in second gear and then bangs down into first. I have had mine do it quite a few times, usually when backing out of my driveway and going to take off in drive.

Thank you for the replies and better describing it. So yes this seems like the same issue. It was greatly amplified when I had my travel trailer hooked up. Even the wife asked if we are going to make it home lol. 

[Cpl_Punishment]

Which of you guys with the banging are running 8 speeds and which have 10 speeds? 

[Thomcat]

Take this for what it is worth - an educated guess.  I'm no tranny genius, only from reading operation of start stop as it applied my 9 speed, but logic dictates it may also apply to all start stops. If so, these new gen 8/10 speeds or even 6 speeds if it has start stop must share a the design function like the 2nd gen Traverses 9 speeds. I've no idea if this will occur only if start stop is active on every initial movement after a gear change or if the function operates only on release from an auto stop, but should likely also apply to all initial forward movement situations or transitions from "R" to "D" when servo pressures to different drums are being applied or braked for transitioning gears.

 

By design when the engine is stopped the hydraulic pressure pump inside the tranny is not turning so no hydraulic pressure, in particular on the servo applying pressure to the drum holding the planetary gearset for first gear which is necessary for immediate engagement and movement in "D" or "L".  A loss in pressure will result in a delayed engagement and slamming into first as the pressure is restored or could result in starting movement of a different gear whose operation requires release of its drum. Maybe this is why a delayed first engagement or why tranny starts in a different gear when the first gear drum is in released position and then slams into first when the pressure is applied????

 

But why only on start stop? As a result of sporadic pressure cycling of the main hydraulic pump in the tranny ALL the start stop trannys use an "accumulator" system that maintains pressure on the servo holding 1st gear with the engine off in an autostop.  Not sure if this will also occur on a normal stop like when shifting from "R" to "D" with the engine is still running, but it is likely since the accumulator fed by the pump and not the pump itself is designed for full time pressure application to all tranny servos. I would guess just like the vacuum reservoir in a power brake system.  Bu "accumulator", the best description of what is used in start stop systems is listed in the second document below as: "the auxiliary transmission fluid pump motor or auxiliary transmission fluid accumulator solenoid valve that maintains proper transmission fluid pressures while the engine is off".

 

Whether this is the case or not the failures described would fit nicely in a situation where the the tranny's hydraulic pump is functioning, but there is a problem causing bleed down of hydraulic pressure in the accumulator or alternatively an electric auxiliary transmission fluid pump if used.

 

http://gmauthority.com/blog/gm/gm-transmissions/9txx/

 

https://gm-techlink.com/?p=8495

 

 

 

 

[leafspring]

I always turn auto stop - start off, so I know that mine isn’t shutting down. I have had mine do it when starting and taking off without reversing as well.

[ATCinc]

I can't see it being a start stop issue as it happens when it wouldn't be in a start stop instance. 

Mine is an 8 speed. 

[Sin City Trail Boss]

My six speed does it too, but only happened a few times. Usually when I am coming to a stop, I'm almost completely stopped then give it gas again and it feels as if someone taps me in the rear. Hear it's normal in these six speeds though. My 16 did it, and my friends 18 does it too.

[ATCinc]

On 10/14/2019 at 10:33 PM, leafspring said:

Mines at the dealership rate now for this very issue, mechanic stated that he’s had a couple others in for this. It’s almost as if it starts in second gear and then bangs down into first. I have had mine do it quite a few times, usually when backing out of my driveway and going to take off in drive.

They come to any conclusions?

[leafspring]

On 10/18/2019 at 11:20 PM, ATCinc said:

They come to any conclusions?

They said that it was doing a 2-1 shift, they undated the computer and said that there's really nothing else they can do until until GM comes out with a specific update for this issue. So basically live with it lol.

[flat3500]

Mine with an 8 sp seems to be clunky first thing in the morn. pulling out of the drive and sometimes if you do tight left and right turns.First time it did it I looked in the mirror to see what hit me.

[Rgpie75]

I have this issue with my 2019 AT4 5.3 w/8 spd as well. I'll be in for the first service in a couple weeks and will see what the dealer has to say.

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