2019 GMC transmission issues

[GT.....O?]

Hoping someone may have had a similar issue and can point me in the right direction.

Little history. Bought the car used in Feb 2023. Fast forward to later in July and noticed when the transmission shifts from 2nd to 3rd the rpm would “jump” 200-300 rpm when 2nd gear was released and before it grabbed third. This would create an obvious lag between gears as the power was cut for a slightly longer duration.

The jumping issue was fixed and a valve control body was replaced. However after a couple weeks it was noticed there was still a lag between the 2-3 gear shift. While under consistent acceleration the shift from 1st to 2nd gear results in a 500rpm drop, the shift from 2nd to 3rd results in a 900-1000rpm drop, and the shifts from 3rd to 4th and 4th to 5th return to a 500rpm drop. Dealership says this is within spec, but there is an obvious and consistent difference, the shift from 2nd to 3rd being the odd man out.

Video below for reference.

https://www.youtube.com/shorts/509j3yFEX14

Second issue that has developed is while in 3rd gear and again under consistent acceleration, the rpm will drop 200-300rpm while the speedometer continues to rise. There is no shutter, but you can hear the engine struggle through this sequence. Again, the dealership doesn’t have an answer and they say they are unable to replicate the issue unless I am driving with them. I have also only noticed this issue while in third gear, if it is happening in other gears the result is much more subtle and inconstant at best.

Second video for reference. Around the 6 second mark the rpm increases to 3100 rpm, drops back to around 2900rpm and then continues to accelerate. This is not a gear shift in my opinion, but something transmission or engine related.



Any thoughts? My transmission knowledge is limited at best, but my understanding of gearing doesn’t allow the sequence to make sense and the dealerships response of “within spec” doesn’t sit well with me. I understand they can’t replace parts at random, but I also don’t understand how they can deny there is an issue. Like the saying goes, “one of these things is not like the other”

Thanks in advance for any input!

[71/454]

What trans is it 6, 8, or 10 speed? How many km?
It sounds like you’re seeing the converter lockup engage after the shift. The 2-3 shift may be shifting and going lockup at the same time. I would assume they did a relearn after the first fix which sounds like a techm replacement.
Both the 6 and 8 are known for converter issues but the 8 speed has a specific fluid that needed to be changed out for the original fluid that the trucks shipped with new.

[Flexray]

I'm going to assume no trouble codes have been set. And seeing that you already had it at the dealer, I'm going to assume the transmission control module software is fully updated.

Using a scan tool, go into transmission data. There should be a data list of all of the last shift times. You are looking for the odd man out. If all of the shift times are about 150ms, but one is like 500ms, that is a good place to start. I would specifically be looking at the 3-5-R shift time. The transmission module calculates how fast or slow the transmission shifts. It will increase or decrease the pressure to that clutch to keep shifting uniform. There will be a data list of the adaptations for all of the clutches, again looking for the odd man out.

Next, drop the pan and access the valve body. Find the solenoid that controls the suspect hydraulic circuit. While activating the solenoid with the scan tool, graph its voltage and amperage. You can see how the solenoid is performing electrically and mechanically from this. Remove the solenoid out of the valve body. It will have a fine metal screen on it. Inspect it for metal and clutch material.

If everything so far has checked out. Remove the valve body. On the transmission case, find the feed hole for the suspected hydraulic circuit. Using compressed air, air check the circuit. When you apply a few psi to the circuit, you should hear the clutch engage with minimal leakage. If all hydraulic circuits pass air check. Remove the transmission from the vehicle and disassemble. Inspect the apply pistons and clutch assemblies. But, anything past the valve body or a solenoid is going to result in a new transmission.

[GT.....O?]

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What trans is it 6, 8, or 10 speed? How many km?
It sounds like you’re seeing the converter lockup engage after the shift. The 2-3 shift may be shifting and going lockup at the same time. I would assume they did a relearn after the first fix which sounds like a techm replacement.
Both the 6 and 8 are known for converter issues but the 8 speed has a specific fluid that needed to be changed out for the original fluid that the trucks shipped with new.

the tech was telling me he thought it was a 9speed, but i only count 6 shifts to get up to highway speed (+130km/h) (edit: guess it would be 5 shifts.... )

First issue of the jumping rpm arose around 76k kms maybe?, they did replace the control valve body under warranty along with trans fluid flush and fluid change. But since then have been unable to come up with any explanation or reason for what its doing. Currently sitting at 93k kms

[GT.....O?]

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I'm going to assume no trouble codes have been set. And seeing that you already had it at the dealer, I'm going to assume the transmission control module software is fully updated.

Using a scan tool, go into transmission data. There should be a data list of all of the last shift times. You are looking for the odd man out. If all of the shift times are about 150ms, but one is like 500ms, that is a good place to start. I would specifically be looking at the 3-5-R shift time. The transmission module calculates how fast or slow the transmission shifts. It will increase or decrease the pressure to that clutch to keep shifting uniform. There will be a data list of the adaptations for all of the clutches, again looking for the odd man out.

Next, drop the pan and access the valve body. Find the solenoid that controls the suspect hydraulic circuit. While activating the solenoid with the scan tool, graph its voltage and amperage. You can see how the solenoid is performing electrically and mechanically from this. Remove the solenoid out of the valve body. It will have a fine metal screen on it. Inspect it for metal and clutch material.

If everything so far has checked out. Remove the valve body. On the transmission case, find the feed hole for the suspected hydraulic circuit. Using compressed air, air check the circuit. When you apply a few psi to the circuit, you should hear the clutch engage with minimal leakage. If all hydraulic circuits pass air check. Remove the transmission from the vehicle and disassemble. Inspect the apply pistons and clutch assemblies. But, anything past the valve body or a solenoid is going to result in a new transmission.

@Flexray

Are you a transmission tech? do you work at a shop? I dont have access to a scan tool, and have never touched anything transmission related. I would be willing to pay for your time if you think you might be able to diagnose or give me anything to come back at the dealership with.

To answer your questions, they told me the transmission module wasnt reporting anything as off, "everything is within spec". They did update the transmission module, and reset the adaptive pressure control system or something. Its slightly annoying that they always seem to run to the excuse that they "were not able to replicate the issue". I took the tech out for a drive to show the shifting, and he wasnt paying close enough attention for the rpm slip mid-gear.

[Twin_Cam_Turbo]

Being a GM it’s either a 6, 8 or 10 speed. If the tech doesn’t know which it is he probably shouldn’t be working on GM transmissions. The 8 speed seems to have the most issues in my experience, I’ve had a few with similar symptoms, one got a new torque converter (6.2 and 8 speed) and one we just hawked it as is (2016 with 5.3 and 8 speed).

[HHURICANE1]

19 Acadia has a 6 spd. Either a 6T50 or a 6T70. These things like to eat control valve bodies. It has the TECM in it.

[Flexray]

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@Flexray

Are you a transmission tech? do you work at a shop? I dont have access to a scan tool, and have never touched anything transmission related. I would be willing to pay for your time if you think you might be able to diagnose or give me anything to come back at the dealership with.

To answer your questions, they told me the transmission module wasnt reporting anything as off, "everything is within spec". They did update the transmission module, and reset the adaptive pressure control system or something. Its slightly annoying that they always seem to run to the excuse that they "were not able to replicate the issue". I took the tech out for a drive to show the shifting, and he wasnt paying close enough attention for the rpm slip mid-gear.

This problem should still be under warranty. GM is footing the bill for this repair. This pulls money out of profit so they are going to do everything they can to keep costs down. Gm has a specific way they want problems diagnosed, and parts replaced. If it is not followed to the T, GM will not pay the dealership, and the dealer won't pay the technician.

A technician is going to be assigned your vehicle. He will be paid 0.00 to 1 hour to address your complaint. He has to get the keys, find the car, go for a drive, connect the scan tool, id the vehicle, look up tsb/service actions. At this point he is out if time and not being paid. He has to find another warranty operation code to bill to keep on getting paid. This is a minor complaint, that is going to be challenging to solve. The technician is going to state no fault found and move onto a job that he is actually being paid for. It's a shitty system that hurts customers and technicians. I would try going to another dealer.

I don't work out of my house, I don't have the proper tooling, resources, and insurance to do so.

[anomaly2]

Do you know if the relearned the transmission adaptive like as fast learn with the scan tool.
This is where the vehicle in drive and it learns with the brake pedal held and emergency brake engaged, and the transmission shifts all the dears and clunks till it learned and passes.

[corsvette]

Had a 2018 Acadia that started having late/flaring shift issues and a trans code (can’t remember what the code was). Turned out to be a faulty plastic disk looking thingy in the aforementioned TECM (bolts to the valve body). I ordered the new disks, seals, and tool kit needed from Amazon and did the repair myself. The worst part of the whole job was the mess from all the transmission fluid that you can’t fully drain. All in I think it cost me $150 with oil and preformed flawless after. Dealership wanted to replace the entire TECM for around $1800 iirc.

Here’s the link of the part numbers and explanation/video of what the repair is like and symptoms.
https://www.sonnax.com/parts/5414-pr...ch-rebuild-kit

[GT.....O?]

Thanks for the responses guys. The service guy at the dealership has actually been pretty good to deal with. I expressed my concerns with the issue on the last oil change around January, and had to pay the diagnostic fee. This is where they stated no issues first, he said bring it back when you get a more dialed diagnostic. We ended up having another kid at the end of the month, so didnt end up driving a bunch. Fast forward to March 28th, brought it in to the dealership, took the shop foreman/transmission specialist for a drive so i could show them the issues (wasnt accepting another "could not replicate" diagnostic). Warranty ended March 31st, so got the work order in for the 28th, before warranty expired.

This is the issue that facing me now, powertrain warranty is expired now however they will keep the Work order open for another week to see if anything changes or gets worse, but pretty much eluded to the fact that everything from the dataside was looking good.

My thoughts to go forward are:

Return and ask them to clear the transmission data, re-run the diagnostics but get a copy of the data and shift points, etc and start heading it up with GM canada to see if that gets anywhere. Im not sure if i can transfer the WorkOrder over to another dealership since the warranty is now expired. but at least with the data and the work order i can try push GM harder or maybe the dealership will cave and take a more extensive look.



Bringing it to a non-dealer shop or transmission shop and trying to get reimbursed would be the last step, but im doubtful that route would be successful.

[Flexray]

You have a few repair orders for this problem already. The fact that this problem started in the warranty period and has not been solved yet works in your favor.

I would bring the car back to the dealer, go for another drive with the foreman. Make sure you and him are on the same page. Give them your keys and tell them to keep it until it is fixed. In the shop, this will take pressure off of the job and gives the impression that no fault found is not an option. Transmission technicians basically don't exist, he going to be busy. It may take some time before he gets to your car. Keep in regular contact with them and don't be an a-hole.

GM won't cover a repair from an independent shop.

I think the torque converter clutch material is shedding, or one the apply clutches is failing. Clearing adaptations usually reveals problems. The transmission module will try to adapt around a failing part. Once you clear that data, it thinks it's controlling a brand new transmission. On the first drive after clearing, you will get a really hard or soft shift/apply on the failing gear.

[GT.....O?]

Heading back to the shop today to drive with the service manager.

Ive asked for the data on the shift times, whether it be a screenshot or the actual data but the dealer is unwilling to provide. Ive expressed my concerns that this issue will only continue to get worse and without the data to prove it was happening before the warranty expires, but my complaints have gotten nowhere.

Ive sent videos to the service manager who is adamant that the RPM bog is a "shift". Im at my wits end, i dont know how to explain it to them that the rpm bog/stall is not a shift and even recorded a video of the issue while moving through the gears manually.

Its beyond frustrating at this point.
@Flexray is there any sort of overdrive feature that could be causing this bog?? I agree with you, i think the torque converter is not applying the appropriate pressure and the bog is when it finally catches, thus the RPM loss.

[Flexray]

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Heading back to the shop today to drive with the service manager.

Ive asked for the data on the shift times, whether it be a screenshot or the actual data but the dealer is unwilling to provide. Ive expressed my concerns that this issue will only continue to get worse and without the data to prove it was happening before the warranty expires, but my complaints have gotten nowhere.

Ive sent videos to the service manager who is adamant that the RPM bog is a "shift". Im at my wits end, i dont know how to explain it to them that the rpm bog/stall is not a shift and even recorded a video of the issue while moving through the gears manually.

Its beyond frustrating at this point.
@Flexray is there any sort of overdrive feature that could be causing this bog?? I agree with you, i think the torque converter is not applying the appropriate pressure and the bog is when it finally catches, thus the RPM loss.

Painting with a broad brush, most service managers are not technicians. Back in the old days they were, now most are white collar number manipulators that don't understand modern day vehicle technology. The shop foreman will be GM master technician.

It is hard to tell from the videos what is actually happening. I would drive the vehicle while monitoring the transmission data. Compare what the transmission module is commanding to what the transmission is actually doing. But, this is a warranty repair. You cannot just make up your own diagnostic process. You have to follow a symptom chart or trouble code chart. If the problem doesn't fit into any symptom category, it is not going to get fixed. GM isn't going to pay for anything until a technician writes on a repair order "had this symptom, performed this flow chart, got these results".

During light to moderate acceleration and cruise the tcc will apply locking the transmission and engine together. When this happens it is normal to get a few hundred rpm drop. But it should be basically unnoticeable. The difference in between engine rpm and transmission input rpm is called "slip". The transmission module will request a certain amount of slip depending on driving conditions. A fault code will set when slip is above X rpm for X amount of time. You can have excessive slip that the driver will notice but doesn't meet the threshold to set a fault code. An overdrive gear means the transmission output speed is higher than its input speed.

Situations like this is why I have so much disdain for dealerships.