2018 Denali - 8spd Trans - Bad idea?

[Tkaldridge]

Hey everyone! New to the group and excited to join.

 

I've found a 2018 Sierra 1500 Denali w/ 6.2L & 8spd A/T.  It's got the right features at a decent starting price.

 

My concern and/or lack of clarity is around the 8spd transmission reliability. I've read for many days and hours now on the issues with this transmission for what appears to be the entirety of this generation. Most people come to the internet to vent and share their problems so it can be hard to gauge how many good experiences there are with this trans (I know about the lawsuit and the 'infamous' past). What I cannot discern is whether this is purely a hardware reliability issue or can be mostly mitigated with custom/improved tuning? Are regulations driving the issues with this 8spd design or is it truly a multi year engineering disaster? I read a lot of concerns and dislike for the 'feel' of the drive but no total catastrophic failures to mention.

 

I'm wanting to go eyes wide open into this purchase since it is a large one for me. I'm a gear head and don't mind knowing the troubles now versus later with any new vehicle, especially if this are some good fixes already found. How do we know the issues are completely resolved with the new 10spd? Anyone have an inside source on the significant changes from 8spd to 10spd (hardware vs. tuning)? 

 

Looking for any kind of advice and appreciate the support from this group! 

[mookdoc6]

7 hours ago, Tkaldridge said:

Hey everyone! New to the group and excited to join.

 

I've found a 2018 Sierra 1500 Denali w/ 6.2L & 8spd A/T.  It's got the right features at a decent starting price.

 

My concern and/or lack of clarity is around the 8spd transmission reliability. I've read for many days and hours now on the issues with this transmission for what appears to be the entirety of this generation. Most people come to the internet to vent and share their problems so it can be hard to gauge how many good experiences there are with this trans (I know about the lawsuit and the 'infamous' past). What I cannot discern is whether this is purely a hardware reliability issue or can be mostly mitigated with custom/improved tuning? Are regulations driving the issues with this 8spd design or is it truly a multi year engineering disaster? I read a lot of concerns and dislike for the 'feel' of the drive but no total catastrophic failures to mention.

 

I'm wanting to go eyes wide open into this purchase since it is a large one for me. I'm a gear head and don't mind knowing the troubles now versus later with any new vehicle, especially if this are some good fixes already found. How do we know the issues are completely resolved with the new 10spd? Anyone have an inside source on the significant changes from 8spd to 10spd (hardware vs. tuning)? 

 

Looking for any kind of advice and appreciate the support from this group! 

First..it's robust as it gets and can handle the input and reliability is without question far superior than the 6speed imo.  Regulations yes probably partly to blame in order to generate those MPG'S it does act and shift differently.  Drive the truck your looking at on Dead/Cold start and see how it acts?  It's never going to change from that perspective.  Make sure the truck has the updated spec for fluid etc.  The 8L90 is quirky until it's hot...they want the fluid warm/hot and quick as possible for efficiency...which is why the first shifts when cold are FIRM.

 

When I compare my K-2 to the build quality of the T-1's I am not quite sure I am sold as the SOLID TANK FEEL on K2's still impress me with the steel structure...THE 800's had that solid feel.  The 900's I was never impressed and Finally the K2's gave me the inside and outside SOLID Durable construction that can take years of abuse.  

 

For what it's worth the T1's have somewhat different 8speed because of DOD and I have driven one with 5.3L and it drove way more responsive and car like than mine.  

[16LT4]

Another thing to consider is if you like having a transmission dipstick and easy AF fluid changes, because if so, the 6 speed is your huckleberry. The 8 speed isn’t horrible (based on my experience with Audi, VW and BMW transmissions with similar procedures), but is something to consider if you need to farm out a fluid change. I like dipsticks, so I like the 6. I have the 6L45 in another vehicle, sans dipstick, and checking it’s level is far from straightforward. 

[truckguy82]

On 4/26/2020 at 9:29 AM, Tkaldridge said:

Hey everyone! New to the group and excited to join.

 

I've found a 2018 Sierra 1500 Denali w/ 6.2L & 8spd A/T.  It's got the right features at a decent starting price.

 

 

 

My concern and/or lack of clarity is around the 8spd transmission reliability. I've read for many days and hours now on the issues with this transmission for what appears to be the entirety of this generation. Most people come to the internet to vent and share their problems so it can be hard to gauge how many good experiences there are with this trans (I know about the lawsuit and the 'infamous' past). What I cannot discern is whether this is purely a hardware reliability issue or can be mostly mitigated with custom/improved tuning? Are regulations driving the issues with this 8spd design or is it truly a multi year engineering disaster? I read a lot of concerns and dislike for the 'feel' of the drive but no total catastrophic failures to mention.

 

I'm wanting to go eyes wide open into this purchase since it is a large one for me. I'm a gear head and don't mind knowing the troubles now versus later with any new vehicle, especially if this are some good fixes already found. How do we know the issues are completely resolved with the new 10spd? Anyone have an inside source on the significant changes from 8spd to 10spd (hardware vs. tuning)? 

 

Looking for any kind of advice and appreciate the support from this group! 

I think it’s a crap shoot, and I think it’s hardware related

it’s an amazing trans if you get a good one. Like stout enough for 1000whp.

 

I have yet to drive a bad one, but I did put a 1000 miles on a 2014 cts 3.6 with an 8 speed of some sort. Jesus christ all mighty was that trans bad. It literally ruined the entire car. It was fine if your just cruising, but get it confused, and you will think the car just nuked the drivetrain. Not the same trans, but it is from the same architecture and it is a gm 8 speed.

 

if it’s got decent mileage on it, you simply need to test drive it and try your best to confuse it. Like slowing down from highway speeds to 25 mph and then floor it. Alternate from casual driving to aggressive driving. If you feel no problems, I’d say you got a good one.

[lucas287]

I think most of my complaints are tuning-related to the 8L90. The first thing you'll notice is that low speed shifts feel like all momentum stops. The ignition timing goes negative during shifts. Seeking fuel efficiency, GM tries to lock the converter pretty much all the time. So imagine this in your mind, first gear, accelerate, cut timing, shift to second, accelerate - lock TCC - oh wait time to shift - unlock TCC - pull timing - shift to third, so on and so forth. 

 

The old fluid and being cold, as mentioned, only accentuates the funkiness. I've had the new Mobil1 fluid for about 300 miles and it does, generally, feel better but there are some things I notice now but not before. 

 

I look forward to purchasing HPTuners and working with a couple GEN V gurus over on that forum. They claim that with a few simple changes, like keeping the converter unlocked, slightly positive timing on shifts, and raising part throttle shift points a bit, that it transforms the truck. 

[lucas287]

Just realized you're talking about a 2018! The same tuning principles apply - BUT - I'm pretty sure you'll need to have the TCM removed, sent off, and unlocked to do it.