Interesting... progress with odd trans behavior? (2015 6-speed)

scottl1346 · April 28, 2016

went out to get gas i tried this and the shifting seems better i thought i herd a clunk but may have been the trailer hitch in the bed will post after a few days of driving.

Jeff5347 · April 28, 2016

Just curious. I held the tow/haul button and never saw a display come on the DIC like if you hold traction control down for 5 secs it will say stability has been disabled. I held it for 10 secs and figured oh well and once I let go it said grade braking disabled. Does this enable itself again like the stability after 35? I only drove a mile so probably not nearly enough to see anything happen.

What do you guys notice different with it off? Now my regimen for driving is shift all the way to manual and engage only 5 SPD and no hold tow mode to disable grade braking. Waiting to see what other items I have to disable for a better ride[emoji19]

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JacobC1983 · April 28, 2016

Will this help anybody with the 8 speed?

Deanjo · April 28, 2016

Just curious. I held the tow/haul button and never saw a display come on the DIC like if you hold traction control down for 5 secs it will say stability has been disabled. I held it for 10 secs and figured oh well and once I let go it said grade braking disabled. Does this enable itself again like the stability after 35? I only drove a mile so probably not nearly enough to see anything happen.

What do you guys notice different with it off? Now my regimen for driving is shift all the way to manual and engage only 5 SPD and no hold tow mode to disable grade braking. Waiting to see what other items I have to disable for a better ride[emoji19]

Sent from my Nexus 6P using Tapatalk

No it does not reenable itself after 35. I just took mine out on the highway after for a long test drive and it remains off until the ignition is turned off. It is a night a day difference around town. The truck actually feels like it has 355 HP under the hood now. If you press on the gas while in v4 mode at city speeds it also doesn't try to do the v4/v8 switch and the long gear change at lower RPMs anymore. It doesn't feel like it is searching for gears anymore. Throttle response is better as well, travelling around clover leafs and merging no longer feels like you have to step on the pedal harder to get back up to speed. On the highway which I have travelled hundreds of times before I noticed my v4/v8 switches were nearly imperceptible if it wasn't for slight tone in exhaust sound. With it on I could easily tell the switch back and forth. I also noticed on the highway when it came to the hills with the cruise on, my v4 mode was holding even going up hills against the wind@ 65 mph. With it on, it will rarely shift into v4 at that speed in my truck. The only time it kicked in before was when I was when I was on a gradual downslope. If the DIC is to believed, over the last 50 km I am getting the best gas mileage that I have ever had.

If you haven't tried this out yet, grab your keys and run to your truck and do some driving tonight. It really is like a different truck.

I hope GM sends out a flash to fix this or at the very least make it so the setting sticks between startups.

Deanjo · April 28, 2016

Will this help anybody with the 8 speed?

Maybe, there is only one way to find out.

JacobC1983 · April 28, 2016

I tried it this morning with the 8 speed and still had some clunks when shifting... will keep trying it to make more of a judgement.

So I will need to re-disable this every time I start the truck, correct?

The Mad Veteran · April 28, 2016

I tried it this morning with the 8 speed and still had some clunks when shifting... will keep trying it to make more of a judgement.

So I will need to re-disable this every time I start the truck, correct?

Keep us posted because the truck I ordered has the 8 speed and I'm curious.

THEREED · April 28, 2016

I just tried this (6 speed) and I can confirm this makes a NOTICEABLE difference! The truck is more responsive and way smoother after coasting along and then pressing the gas. Before this was clunky and high revving and now smooth as it feels like engine torque is pulling you ahead not just a scramble of the transmission to overcompensate to try to find a new gear. Annoying as it is to disable manually each time u start I will. E doing this EVERY TIME I DRIVE!

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JacobC1983 · April 28, 2016

So while certainly not as vocal... I do notice occasional odd behavior with the trans and shifting in my 2015 Sierra 1500 SLT.

I was chatting with a friend of mine I know through a local car club, and it turns out he's actually one of the senior techs at a GMC dealer a few towns over. So I've been trying to pay closer attention when the trans does something funny (harsh 2-3 upshift, delayed downshifts, occasional shudders, feels like it gets "stuck" in a lower gear then pops, etc.) and relaying it back to him.

He hit me up the other day and told me to disable grade braking, and let him know how the truck feels.

To disable grade braking, you hold the Tow/Haul button down for a few seconds, then release it. (This is NOT "hill descent control"). You get a popup on the DIC that says it's been disabled.

It definitely makes a difference. I was concerned it was just placebo effect... but he confirmed my observations are what they expect to see. Much smoother shifting. No more "confused" feeling by the trans. No more odd clunks.

He told me "they" think they're narrowing in on the problem. I assume he's talking about GM/factory. Says one of his co-workers (also a tech) is a big truck nerd and has been trying to sort this stuff out with his own truck.

Give it a shot. Let me know if you feel the same thing I do (for my own curiosity's sake).

Can you ask your buddy if he knows if this helps the 8 speed in our trucks.... or if he has seen any other ways to fix the 8 speed? I have a field service engineer from GM coming into town at some point over the next week or two, and would love any and all info to get my truck fixed.

Rkaroll953 · April 28, 2016

Drove 30 miles with grade braking off yesterday and i noticed no difference at all. still very lazy... V4 is disabled also

davidio840 · April 28, 2016

I did this on my way home from work yesterday and on my way to work this morning. I definitely notice a different in the shifting. It is smoother for me.

I also noticed my miles per gallon has increased by 3-4 miles. I know the DIC may not be completely accurate, and this is an extremely small sample size, but I looked at the last 25 miles and I went from around 16.5 to almost 20 mpg in that time frame. Mind you this is driving in M5, in the city... so there was no V4/V8 switch.

Deanjo · April 28, 2016

I did this on my way home from work yesterday and on my way to work this morning. I definitely notice a different in the shifting. It is smoother for me.

I also noticed my miles per gallon has increased by 3-4 miles. I know the DIC may not be completely accurate, and this is an extremely small sample size, but I looked at the last 25 miles and I went from around 16.5 to almost 20 mpg in that time frame. Mind you this is driving in M5, in the city... so there was no V4/V8 switch.

Mine is actually getting close to the average EPA rated 9 l/100 km now according to the DIC on the highway. Lowest is has ever been prior is 12.5 l / 100km avg. (19 Miles/US Gallon vs 26 Miles/ US gallon).

But here is the thing with you. If you are driving in M5, grade braking is automatically disabled.

ACTIVATION

Normal mode powertrain grade braking defaults to "ON" after each ignition key cycle.

To disable or enable all grade braking within the current ignition key cycle, press and hold the Tow/Haul button for three seconds.

No grade braking is available in Range Selection Mode (Manual Shift Mode).

For grade braking to activate it requires the driver to apply steady brake pedal pressure to maintain desired speed while driving on a downhill grade. The vehicle's modules monitor the amount of brake pedal apply, vehicle deceleration rates, and other factors to determine if normal mode powertrain grade braking is necessary. When the feature activates for the first time that ignition cycle, the DIC will display "Grade Braking On" or "Grade Braking Active" and you will notice the transmission down shifts and engine RPM increases. At this point, if the brake pedal is released the transmission will hold the current gear. If additional engine/transmission braking is needed to help maintain desired speed, reapply steady brake pedal pressure and additional down shifts may occur.

Normal mode powertrain grade braking will have a less aggressive transmission down shift schedule than Tow/Haul mode powertrain grade braking.

Grade-braking can be exited by depressing the accelerator pedal.

DAY11 · April 28, 2016

So I have a 2016 with the 8 speed. I disabled it this morning to take my son to daycare, about 10 -12 miles round trip.

My old symptoms were mostly during the first few miles and seemed to go away after. It would coast then clunk on let off. Hard shift -slip then clunk around town.

With the system disabled it did not coast -slip on let off when backing out of the driveway and then pulling onto the street. It also seemed more responsive under throttle. It certainly coasted better after let off as would be expected.

I did see at least one hard shift but way better.

I tried Turning it off two ways...

-At start up I held the tow haul before even moving truck or putting it in Gear. Held it for 10 seconds

-while in gear and in motion I held the button and it also shut it off.

davidio840 · April 28, 2016

Mine is actually getting close to the average EPA rated 9 l/100 km now according to the DIC on the highway. Lowest is has ever been prior is 12.5 l / 100km avg. (19 Miles/US Gallon vs 26 Miles/ US gallon).

But here is the thing with you. If you are driving in M5, grade braking is automatically disabled.

Thanks for that info.. What is even more weird is that when I disabled it (at least the DIC said "Grade Braking Disabled" I noticed immediately when I left of the gas the truck wants to roll a lot easier. Before it would almost seem like the truck would slow down on its own. Now it just rolls forward without any restriction.. as in I have to press the brake harder..

It may just be me thinking this and my mind is playing tricks on me, but it does seem different to me. Placebo effect maybe?

Dunc · April 28, 2016

Will this help anybody with the 8 speed?

I tried it on my '15 8-speed and while it's only been about 50 miles so far I am definitely leaning toward noticing a difference. I want to drive more to confirm, but so far I'm interested in the results. It's not like it's 100% better, but a noticeable difference.

It didn't seem to change the clunk/jolt I get when shifting from park to drive or to drive after reversing. Once moving though it did seem less confused what the transmission should be doing. Going around neighborhood turns before was miserable. It would slow a lot make it seem like I've never driven a car before and was the most unsmooth operator of all time. Now it does seem more smooth going through turns. It does seem to sometimes still shift up then down and then back up in rapid succession when trying to accelerate while already in motion, but overall I'd say so far it seems to be more decisive with what to do.

So I will need to re-disable this every time I start the truck, correct?

Seems to be the case for me.

Can you ask your buddy if he knows if this helps the 8 speed in our trucks.... or if he has seen any other ways to fix the 8 speed? I have a field service engineer from GM coming into town at some point over the next week or two, and would love any and all info to get my truck fixed.

See above...