2018 Sierra, 6-speed or 8-speed trans? And other gripes.

Z45 · July 10, 2018

I have a 2017 Denali Ultimate with th 8sp, its been perfect since day 1. My bro has a 2017 SLT 6.2L with max trailering and all options. He has almost 30,000 miles and has not had one problem with the 8sp.

I had a 2015 Denali with the 8sp and there were problem to start but after the new TC and fluid flush it was good for 50,000 miles til I traded if for the 2017.

M5U

Transmission, 8-speed automatic, electronically controlled with overdrive and tow/haul mode. Includes Cruise Grade Braking and Powertrain Grade Braking
1 - Included and only available on Double Cab models with (L86) 6.2L EcoTec3 V8 engine. Included and only available on Crew Cab models with (GAT) All Terrain Package, (PDT) SLT Crew Cab Premium Plus Package, (NHT) Max Trailering Package or (L86) 6.2L EcoTec3 V8 engine.

mookdoc6 · July 10, 2018

1 hour ago, newdude said:

The only really big deal seems to be the fishbite/chugging resulting in a triple fluid flush, and if it comes back, replacement of the torque converter. Goes away after that. 8L45 and 8L90 both have the same deal. We've done maybe 6-8 torque converters here at work so far. No valve bodies, etc. Have yet to see one torn apart.

That is something more inline probably than....They suck, falling out of vehicles, slipping, grinding....loosing gears etc. The programing is not appropriate and can be tweaked in aftermarket products thankfully...but the robustness of the 8L90 is no doubt extremely robust! I like the ratios and honestly I don't think I want more and will NIX the 10speed and just get a dolled up 8speed in 2020 timeframe..................

Z45 · July 10, 2018

One thing is I caught the dealer about to install the wrong ATF in my truck. They (service dept) were unaware that the Dexon VI was not the correct ATF for the 8-sp. When I questioned them about having the correct fluid they had to reschedule the appointment (truck was going on the lift) because neither their parts dept or the 3 other dealers they called had Dexron HP. in stock Even the SM was shocked to find out their parts dept had dropped the ball on carrying Dexron HP. They apologized for the screw up, gave me coupons for several car washes, filled my tank and even delivered the vehicle back to me (100 miles) to compensate me for having to come back.

Mine was the first one they were doing the flush on, early 2015 model.

mookdoc6 · July 10, 2018

3 minutes ago, elcamino said:

One thing is I caught the dealer about to install the wrong ATF in my truck. They (service dept) were unaware that the Dexon VI was not the correct ATF for the 8-sp. When I questioned them about having the correct fluid they had to reschedule the appointment (truck was going on the lift) because neither their parts dept or the 3 other dealers they called had Dexron HP. in stock Even the SM was shocked to find out their parts dept had dropped the ball on carrying Dexron HP. They apologized for the screw up, gave me coupons for several car washes, filled my tank and even delivered the vehicle back to me (100 miles) to compensate me for having to come back.

Mine was the first one they were doing the flush on, early 2015 model.

Yeah, I had similar experience they just hustled from other dealers to round it up and make it happen for me. Fast forward 2 years later....Dealer says, AC Delco does not make it anymore and the replacement is Mobil1 HP? Not sure about all that but they put in my Truck from the Dealer?

SouthpawHD · July 10, 2018

3 hours ago, newdude said:

Not trying to come off as a smart arse, but did you look the truck over before purchasing to notice it didn't have features you wanted in it? I wonder if the truck was either built during a possible constraint, or the dealer pulled a fast one on you and obviously stated it had features it didn't really have. You wouldn't happen to have the VIN handy? I wonder what the build really shows for options on it.

The truck was in transit, so no physical truck to inspect prior to signing. Yes, I knew the risks. The dealer made a mistake in comparing my truck (that was in transit) to another truck that was in stock.

They printed a partial window sticker that only indicated it was the Premium package.

3 hours ago, JamesAT18 said:

Check the window sticker, it should state what all options came with your truck. I'd try to work it out with the dealer to get the options you're lacking.

But there is SLT and then SLT Premium packages.... You probably ended up with just a SLT. Premium packages come with all things you stated you're lacking

Check out the different names of each here:

https://www.gmc.com/trucks/sierra-1500-pickup-truck/build-and-price/packages

Best of luck,

James

I have the SLT Premium. And the truck the dealer compare this to was the Premium Plus, as I have learned now.

3 hours ago, mookdoc6 said:

I believe it's one of those GM package deals.....Premium Plus...gets you the LED's, sprayed in bed liner, running boards, rear slider window, parking assist front,back.

The actual M5U is added option or 8speed. I would not get all worked up over the 8speed/6peed. If your pulling things all the time on hills the 8speed will be where it's at...if your driving in town the 6speed is where it's at.......MPG's nadda difference you or I would be able to tell

Yes, I have the LEDs and HIDs (which are AWESOME), bedliner, running boards.

3 hours ago, mjm-1957 said:

Your much better off without the 8 speed, nothing but a headache.I am the transmission guy at a GMC Buick dealer and the M5U leaves alot to be desired.....

Good to know about the trans, thanks!

Z45 · July 10, 2018

PDT

SLT Crew Cab Premium Plus Package, Requires Crew Cab model. Includes (NZP) 20" chrome clad wheels, (QSS) P275/55R20 all-season, blackwall tires, (UD5) Front and Rear Park Assist, (UVD) heated steering wheel, (A48) power sliding rear window, (BVQ) 6" rectangular chrome tubular assist steps, (UQA) Bose premium audio system, (IO6) GMC Infotainment 8" Diagonal Color Touch audio system with Navigation, (JL1) trailer brake controller, (Y86) Enhanced Driver Alert Package, (AN3) front bucket seats, (KQV) heated and ventilated seating, (CGN) spray-on bed liner and (T4L) full LED headlamps. Available to substitute (NZP) 20" chrome clad wheels with (RD5) 20" x 9" polished aluminum wheels.
1 - Not available with (PCM) SLT Preferred Package, (PDG) All Terrain SLT Premium Package, (PDB) SLT Premium Package, (PDC) Texas SLT Premium Package, (GAT) All Terrain Package or any LPO wheels. Note: No changes or substitutions to the included Premium Plus Package equipment will be allowed. If not specifically mentioned as not available as above, all other normally available 4SA options will be allowed.

PK8 · July 11, 2018

I’ve been driving a Sierra 6.2 with the 8-speed for just over a year now. I don’t “hate” it, but I do think it’s too many gears (I’m sure due to GM’s programming, in the name of fuel economy).

When I stop at a light, then make a right turn (for example), it seems like it’s in 3rd or 4th gear by the time I’ve completed the turn/straightened out, which then prevents “spirited” acceleration (or at least makes it no fun because it doesn’t like to downshift...).

You also have to really stomp on the gas to get it to downshift with anything approaching urgency.

I also have the “buck” at low speeds- especially when cold.

I drove a 5.3 6-speed for a week+, before getting my 6.2, and I didn’t notice those issues.

It’s unfortunate because, IMO, the 8-speed holds back the 6.2 from being as great as it could be. When this is really bugging me, I just put the trans in either “manual” mode or tow/haul mode for a bit. Then I feel better.

Sent from my iPhone using Tapatalk

Kubs · July 11, 2018

I have had my 2018 8 speed Silverado since March. I now have almost 5K miles on it, 2 trips towing the race car, and absolutely ZERO trans issues. When it was only a couple weeks old we still had some cold weather here in Ohio and I would notice a harder 1-2 upshift on occasion for the first couple shifts, but then it was fine. It could be the warmer weather or the learning process, but I have not had a hard shift in more than 2K miles. I love the 8 speed, but im sure there are some bad apples in the bunch which give them the bad rap.

Loco-diablo · July 11, 2018

I have a 2017 LT+ with the 5.3 6 speed. I'm ok with the trans. It shifts good. The only time i've had it act up it when coming to an abrupt stop from 40+ MPH, it will sometimes bang as it downshifts. It's only happened a couple of times.

I'm surprised an SLT does not have the options you listed, but I guess there are different options at different levels of trim and different levels within the trim you select.

I was looking for an LTZ when I came upon my LT+. It has more options than some LTZ's that I looked at including rear slider, spray in bed liner, homelink, leather, Bose, in vehicle nav (io6), but it doesn't have some base LTZ features like LED lighting front and rear. I wish I had front park assist, but I only have the rear assist. The BIG incentives were on the LT at the time, not the LTZ, so I saved good money vs buying an LTZ at the time.

rrmccabe · July 13, 2018

If you feel the dealer mislead you, then hold the GM survey over their head. Do NOT fill that thing out until you are happy. If you feel it was your mistake,. then live in learn. But if they are in error then I would not give in.

That survey is literally money to them.

BCMoto · July 25, 2018

I just got to 1k miles with the truck and its been good but now i will be hauling bikes down to southern california next week and going to be the first long trip with a load so I hope the trans does good.

jameshb · June 3, 2019

On 7/10/2018 at 1:37 PM, mjm-1957 said:

That transmission has been a problem since it was released in 2014. The first ones were built with too much clearance between the steel clutch plates and the case. The clunking and noises they made were unbelievable. Unfixable also, we were checking the build dates and the Julian dates on the trans themselves and relaying that off to GM and they would then send a complete replacement.Then the shifting problems started,delayed engagement into reverse after sitting overnight, delayed engagement into drive anytime, harsh 3-2 or 2-1 downshift that felt like you got rearended.We were updating the updates, performing fast learn adapts etc etc etc.Then the TCC shudder started showing up,are you kidding me? First we were told to drop the pan ,replace the filter and blow out the cooler lines and run it for ten minutes on the lift, drop the pan again, drive it on the lift for another ten minutes and another drop of the pan and maybe it will get better after it is driven 200 miles.All of this is done while burning your arm on the cat because they couldn't even put a ****ing dipstick on that piece of shit.I almost forgot the countless valve body replacements to try and correct some shift quality concerns. The six speeds have been virtually trouble free compared to the eight speeds.

I am in the market for a 2016-2017 Sierra. I want the All-Terrain package which requires the 8-speed transmission. I used to have a 2015 Yukon denali with 8 speed and it had issues shifting specially up hills. It would have noticeable shudders. Dealer told me suspension was leaking and I attributed the shuddering to that but maybe it was the transmission. I do not want to end up with the same bad transmission but I really want the All-terrain package. Anyone have an 8-speed and can share experience between 40-80k miles?

jameshb · June 5, 2019

Went to GMC dealer today to talk to a mechanic there. I told him I was in the market for a sierra and asked which transmission I should get. Without hesitation he told me to get the 6-speed. I then asked if he had seen a lot more problems with the 8-speed and he said he had. Told me all I needed to know.