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

[dorkweed]

Brand new 2016 Silverado here.................just over 600 miles driven so far. Signature tells what's on my truck. Tried the "grade braking off" thing today.

 

I've driven almost exclusively in M5 since getting this truck. Want to disable V4 shortly.

 

I put my truck into M5................pushing in the tow/haul button for 5+ seconds.....................and even in M5, the "grade braking off" comes up on my dash display. I didn't notice a difference at all while driving in M5 with it off. The tranny still keeps downshifting as I slow. Put it into D, and it was the same........................still had the tranny downshifting as I slowed. I can see this on the tachometer. Did not really notice a difference in coasting in D vs. M5; nor did I notice any difference in shifting in the tranny.

 

First tank of fuel on this truck netted me 19mpg average hand calculated with the display reading 19.4mpg average. As I type this, the display is reading 21.6mpg....................Driving almost exclusively in M5 also.

[HondaHawkGT]

I am among many who have seen an increase in fuel mileage with Grade Braking Disabled.

 

So GM spends hundreds of millions of dollars in R&D to find ways to save grams of fuel, but programmed the truck to negate all of that investment by turning on grade braking by default? That's absurd. So GM turned on GB by default, for the sole purpose of annoying customers, all at the expense of decreased fuel economy. That's not sound logic.

 

Consider the two different scenarios happening when grade braking is on and off. With GB active, when you lift your foot off the gas and begin to coast, fuel cutoff activates so you're no longer burning fuel and the engine continues to turn as your roll forward. Instead of having to apply the brakes to convert that forward energy into heat, the truck is conserving fuel and brake wear by absorbing that energy to drive the engine and all accessories (alternator, AC compressor, brake booster vac pump,etc). It's very, very similar to the concept of regenerative braking on a hybrid/electric vehicle - recapture the energy that's otherwise wasted on decel.

 

Now in the same scenario with grade braking deactivated, as you lift your foot off, the truck essentially goes into neutral. The engine must continue burning fuel to keep the engine and accessories running.

 

You could claim it's easier to drive your truck (like a hypermiler in a prius, barely touching the pedal and coasting as much as possible) when GB is off, but the truck isn't operating more efficiently. It's anecdotal at best. Kind of like the guys that install a cold air intake or throttle body spacer, and report on the forums that they gained 2 mpg. If GM thought they could gain .2 mpg with a throttle body spacer or grade braking off by default, they would do it even if it cost them millions of dollars to do it.

[No Tyme]

 

So GM spends hundreds of millions of dollars in R&D to find ways to save grams of fuel, but programmed the truck to negate all of that investment by turning on grade braking by default? That's absurd. So GM turned on GB by default, for the sole purpose of annoying customers, all at the expense of decreased fuel economy. That's not sound logic.

Sorry, your not going to draw me into a pissin match about Grade Braking. I have over 20,000 miles on my truck and see a noticeable difference in transmission shifting along with increase fuel mileage. I have been disabling for 2 months now with the same results. I have also spoke with people who have obtained the same results and that is fact. Maybe that is why this posting has 19 pages currently.

Maybe it's the way we drive our trucks, the region were in or just pure luck. I can't answer that. All I know is my trucks drive-ability is more pleasing to me with GB off, and I do like the slight increase in fuel mileage. I am running 89 octane fuel because of the compression in my 5.3 and I personally dislike that feature being turned on by default.

That's my opinion and I'm stickin to it.

[pl8man]

GMC tech told me disabling grade braking increases transmission line pressure resulting in cleaner, firmer shifting. I have to agree, it does! I also keep it in M5

[CounterMeasure]

 

So GM spends hundreds of millions of dollars in R&D to find ways to save grams of fuel, but programmed the truck to negate all of that investment by turning on grade braking by default? That's absurd. So GM turned on GB by default, for the sole purpose of annoying customers, all at the expense of decreased fuel economy. That's not sound logic.

 

Consider the two different scenarios happening when grade braking is on and off. With GB active, when you lift your foot off the gas and begin to coast, fuel cutoff activates so you're no longer burning fuel and the engine continues to turn as your roll forward. Instead of having to apply the brakes to convert that forward energy into heat, the truck is conserving fuel and brake wear by absorbing that energy to drive the engine and all accessories (alternator, AC compressor, brake booster vac pump,etc). It's very, very similar to the concept of regenerative braking on a hybrid/electric vehicle - recapture the energy that's otherwise wasted on decel.

 

Now in the same scenario with grade braking deactivated, as you lift your foot off, the truck essentially goes into neutral. The engine must continue burning fuel to keep the engine and accessories running.

 

You could claim it's easier to drive your truck (like a hypermiler in a prius, barely touching the pedal and coasting as much as possible) when GB is off, but the truck isn't operating more efficiently. It's anecdotal at best. Kind of like the guys that install a cold air intake or throttle body spacer, and report on the forums that they gained 2 mpg. If GM thought they could gain .2 mpg with a throttle body spacer or grade braking off by default, they would do it even if it cost them millions of dollars to do it.

 

 

Its entirely possible that since this is all software, there's a software bug at play here that has yet to be isolated, and not because of some specific design decision by GM. I say that for two reasons.

 

First, I've seen such situations first hand as a senior software lead with 25+ years developing software and hardware. I've been in situations where myself and others have good faith investigated an issue and sworn to management there is no bug only to not think of some specific case or variable. Then we find it and (hopefully) learn from it.

 

Second, while running an errand this morning, I got to experience flooring my truck, a bang noise from the floorboard, and NO downshift nor RPM increase. The RPMs held steady at ~1200. Then about 4-5 seconds later while I was thinking "WTF?", all the while the pedal was to the floor, the truck finally woke up, downshifted hard, applied the gas, and off I went. That is 100% a software bug, either in ECM or TCM, the exhibits itself in specific conditions that hasn't been caught and/or fixed yet. And if that one can happen, one can happen related to grade braking.

[wurgs]

I was going to wait a little longer to reply with my mileage report but since it seems to be a current discussion I will post my results.

 

I started to turn off grade braking in the 3rd week of May and my mileage has definitely increased. I have kept track of all mpg from each fill up since I bought the truck. Over the same time period from last year to this year my mileage went from an average of 18.9 to 20.1. This is with taking the same path to work and 3-4 weekends each month to our seasonal camper so the driving type is very similar but is 263 miles less this year compared to last.

 

There is of course the variable of different weather and winds so can't say the increase is all from turning it off but it's safe to say it does make some difference.

[kp52]

 

 

Its entirely possible that since this is all software, there's a software bug at play here that has yet to be isolated, and not because of some specific design decision by GM. I say that for two reasons.

 

First, I've seen such situations first hand as a senior software lead with 25+ years developing software and hardware. I've been in situations where myself and others have good faith investigated an issue and sworn to management there is no bug only to not think of some specific case or variable. Then we find it and (hopefully) learn from it.

 

Second, while running an errand this morning, I got to experience flooring my truck, a bang noise from the floorboard, and NO downshift nor RPM increase. The RPMs held steady at ~1200. Then about 4-5 seconds later while I was thinking "WTF?", all the while the pedal was to the floor, the truck finally woke up, downshifted hard, applied the gas, and off I went. That is 100% a software bug, either in ECM or TCM, the exhibits itself in specific conditions that hasn't been caught and/or fixed yet. And if that one can happen, one can happen related to grade braking.

 

This is what I hate the most, its happened to me a few times when slowing down then needing to speed up to merge or change lanes. Its not 4-5 seconds, more like 1-3 seconds (although it seems like it when a semi is about to run you down) but I hate it. Turning off grade braking does seem to help but to be fair I have had the same thing happen in both my previous 6 and 8 speed Ram pickups.

 

Its most likely a combination of trans programming, ETC mapping and traction control causing some delay. Someone has to drive these things before they go into production, just cant imagine no one had it happen to them, or any of the clunky downshifts and other auto trans weirdness these have.

[JacobC1983]

Has anybody gotten any software updates to their transmission or TCM recently that has helped?

[Sean Neilan]

I tried this on my drive to work today and didn't notice a difference. But I wasn't having any issues with my trans to begin with. 2015 Sierra SLT CC 5.3 3.42 6 spd

[[email protected]]

o the GM Rep,

I've owned a 85 Z28 (for 15 years) 91 Surburban, 90 and 91 K5 Blazer and a 2009 Yukon (I still have the k5, with 6.2 diesel). I have always GM guy. Since the initial product launch in 2013 of the new GM trucks, I couldn't have been more excited for several reasons. On the day of the launch, I had knee surgery so I set up my laptop on the couch, fully charged ready to watch the unveiling. I was that excited. Well, I purchased a GMC SLT CC, 5.3 4x4. All the bells, driver alert package, heated cooled seats....After a few weeks, at slow speeds the tranny began clunking, missing shifts, acting confused, etc, always at the same location on the way to work, every day. Then it began to occur more frequently. The local dealership said they all do that. Long story short, I had no idea what caused the problem. No other truck ever did this. I assumed something was wrong and didn't know if it would get worse! Never heard of grade braking! Well, I wasn't going to pay well over 40k, for a vehicle that couldn't even shift normally, so I traded it in for a new Subaru for my wife after 9 months. We lost about $5k total. This was my military retirement present to myself so this was very upsettng.

 

After the release of the 8 speed tranny, I began looking into another GM truck. I was about to order a Double cab, with 8 spd tranny and NHT max tow package. Well, I guess the issue isn't fixed, so F150 or Titan here I come. Maybe even the manual tranny Ram 2500 (Also, A GM truck can't be optioned this way)

 

Sorry for the long post but I just read this and thought I'd share my 2 cents.

 

You need to fix this problem. Most of us care about our vehicles and having these issues are not an option. Good luck to all of you who have these trucks.

[truckguy82]

Has anybody gotten any software updates to their transmission or TCM recently that has helped?

Yes, from blackbear lol, it's great

 

On another note, I rented a cadillac cts about a year ago, had an 8l45. It's my only experience with a GM 8speed.

 

It was hands down the worst transmission I have ever experienced. I feel for you guys if your 8l90 is anything like that thing. It was good at WOT, it was good just cruising around. Any sort of transition from creeping to aggressive driving that required a significant downshift it was just aweful. Always getting confused. Slammed into gears sometimes. I could not live with it, and I'm not a complainer. If the 8l45 is anything like the 8l90, I would get an aftermarket tune or get rid of the truck.

[JacobC1983]

So nobody has been to the dealer lately to get their 6 speed looked at with any results?

[No Tyme]

So nobody has been to the dealer lately to get their 6 speed looked at with any results?

I had my trans (6 speed) rebuilt by my dealer. I was having shift flares between 2nd and 3rd and they replaced the 3 5 R clutch drum. Took care of the trans shift flares.

I went the BB route also... fixed my shifting for the most part. Didn't care for what I called "indecision" on the trans part when shifting. Turning off grade braking helps with the downshifting clunking and driving in M5 keeps the engine in V8 mode and also seems to help with shifting around town.

I still have a touch of a lag between 1st and 2nd. This trans will never be like the 4L60.

[JacobC1983]

So the black bear tune didn't help with the downshift clunking...?

[No Tyme]

So the black bear tune didn't help with the downshift clunking...?

If your question is to me, I did not have a lot of downshift clunking, but occasional clunking. I found that turning off grade braking did help, but most all my issues were resolved with the BB tune.

My biggest reason for the BB tune was because the trans sometimes shifted from 2nd to 4th or acted as if it wasn't sure when it wanted to shift.

My only issue now is a slight pause between 1st and 2nd during the shift. It is hard to explain but it is as the engine shuts off for one or 2 cylinder fires during the shift. It is NOT and instant bump and only for a very short moment. I did NOT eliminate torque management, so at this point I do not know if that would help, but I am happy with the way the trans performs now. It is much better than it was originally. Quicker shifts certainly is nice but it is as if BB gave the trans an education. Smarter now since the tune.

I also had BB tune my truck so it shifted in accordance to pedal pressure: Meaning a slower take off the trans shifted at lower rpm's: harder pedal the higher rps's before the shifts. Just my personal preference.

If the guys from BB were ever in my area I would most likely sign up for a hands on tune to see if they could address this issue, but it is a small issue and something I can live with; for a while at least.