2 or Auto

Mark Diedrich · January 15, 2017

But that's not how it works. Zero literature on the subject agrees with the above statement. If fully locked the same binding/crow hopping issues that occur in 4Hi would occur and the system would be of little value on paved roads.

From the theory of operation from GM in a bulletin when it was upgraded:

You had mentioned NP and that information applies to the NVG 246. Of course we no longer use that as it has been replaced by the MP 3023/24 which do have internal differences but the theory of operation is much the same:

-----Transmission Digest, March 2013

----Gears May/June 2011

Mark Diedrich · January 15, 2017

So I read your articles, and maybe I don't understand? But it looks to me like if your under 25mph AND low torque it puts less pressure on the clutches to avoid crow hoping..... However, if you had higher torque while turning (mashed the accelerator) you could get crow hoping, like someone posted earlier because the clutches would have more pressure, correct,?

Once you are over 25 mph and under a higher load the clutches are loaded with more pressure, (since you wouldn't be turning at intersections)

At this point when it detects wheel slippage it is trying to make the 2 shafts spin at the same speed?

If they are spinning at the same speed, then they become 50/50 split right?

If that is right, then the only time it is really variable is at low speed?

j3st3r · January 15, 2017

someone mentioned their DIC displays AWD while in 4 auto, that must be a change on newer models, mine says 4wd system shift in progress when i move my dial to 4auto.

While it's shifting it says 4wd system shift in progress. If you bring the off road section up on the DIC, that is where it shows AWD while in 4 auto mode or 4WD while its in 4hi and 4lo.

Knotsure · January 15, 2017

While it's shifting it says 4wd system shift in progress. If you bring the off road section up on the DIC, that is where it shows AWD while in 4 auto mode or 4WD while its in 4hi and 4lo.

I love my 2014 like no other, but it doesn't have the off-road screen in there. I have looked into what it would take/if I could add that but sadly I don't have it. Wish I did because I geek out on little things like that.

2014 Chevy silverado Z71 DCSB w/Bilstein 5100 level

Jon A · January 15, 2017

But, it's seems hard to change anyone's mind on here so I guess most people on here are also hard headed.

But there's a difference. Above I quoted facts from GM & Industry technical documents--not my opinion. You changing your mind or not won't affect what they say or how the system operates. Mechanical systems don't change their operation based upon the opinions of the operator, informed or not, of how they should operate.

Jon could of saved us all a lot of time effort and pain had he posted several days ago...lol

Heh, sorry, I can't be everywhere. Don't worry, there's a "Auto mode in the rain" thread here once a week so maybe I'll hit the next one earlier.

All the info i was able to find, including the video i posted from GM

That video made me want to cry. Sometimes they dumb things down to the point they do more harm than good.

I have noticed a slight amount of wheel hop on one occasion while engaged in 4auto while attempting to park on an ice glazed parking lot; so either my truck has a slip clutch going out in the transfer case, it was an anomaly, or it was those rare instances where the traction needed happened while my system was in the process of transferring power to the front axle or the transfer case or didn't reduce power in the amount of time the incident happened.

When turning sharply there's a lot of stuff going on besides the 4WD system that you can notice from time to time if you are paying attention. I have sometimes noticed what sounds/feels like a mini-crow hop on a couple particular corners on wet pavement if I shut off the radio and lower the window and really feel for it--even when in 2WD.

Without pure Ackerman steering, one of your front tires is forced to scrub sideways a bit since each wheel is not turned the perfect amount to reflect the different radii they're following. Under just the right traction conditions this can sometimes cause a tire to "skip" sideways instead of slowly and progressively scrubbing.

If the ground is uneven, you'll get side scrub due to that alone. This is made worse by leveling kits since they raise the instant center of the suspension.

Also you may have had one rear tire spinning and felt the G80 "clunk in."

In short, don't get too excited if you feel something a bit odd when turning sharply every once in a while.

I still wouldnt refer to my system as AWD

I wouldn't either. While yes, it can be argued it operates in a similar manor to some AWD systems, I personally wouldn't call it that. The prior gen Denali, however, did have a legit AWD system (planetary gear system in the transfer case allowing differential speeds unrelated to torque split) which definitely causes some confusion.

Jon A · January 15, 2017

But it looks to me like if your under 25mph AND low torque it puts less pressure on the clutches to avoid crow hoping.....

At low speeds, high steering angles can be expected. Excessive preload, even if it didn't cause crow hopping would cause more wear and tear, steering effort, etc, on high traction surfaces. At higher speeds you won't have nearly as sharp turning angles so these issues are diminished so they can get away with more.

Having some torque go to the front wheels all the time adds safety by actually preventing rear wheelspin from happening in the first place under some conditions. When the rear tires do spin the back of the vehicle often wants to slide out--the torque from the front tires provides a righting moment helping to straighten the vehicle as this occurs, without delay waiting for the clutches to react. In some situations this will be enough to prevent a crash. Those miliseconds matter more at higher speeds.

However, if you had higher torque while turning (mashed the accelerator) you could get crow hoping, like someone posted earlier because the clutches would have more pressure, correct,?

No. I do it all the time and you don't get crow-hopping like you do in 4Hi. That's the beauty of auto mode, the axles aren't locked together even when significant torque is sent to the front. Don't get me wrong, if you put your foot to the floor on a right angle corner from a stop all sorts of weird and violent things can occur. Usually as the torque ramps up you can feel a steering effort increase and there can certainly be some feedback through the wheel depending upon what sort of traction the front wheels are finding, but that's a whole different thing. [Edit to add: I didn't mean to make it sound like crow hopping is impossible in Auto mode--if you try hard enough you might be able to make it happen. But in regular use, even quite abusive use, it's not an issue. In 4Hi, however, you won't make it around the block on wet pavement without feeling like you may be doing real damage to the vehicle.]

Since you probably won't break anything on wet pavement, if you want to really feel the difference that'll illustrate it for you. On a right angle corner, from a stop, with the traction control off, mash the pedal as much as you dare in 2 Hi, Auto and 4 Hi. They'll all give you quite different results and you'll see Auto is easily the best tool for the job when on pavement.

At this point when it detects wheel slippage it is trying to make the 2 shafts spin at the same speed?

If they are spinning at the same speed, then they become 50/50 split right?

If that is right, then the only time it is really variable is at low speed?

That's the part I think many are mis-understanding. Making the shafts spin the same speed does not mean they are locked together or that the torque split is 50/50. Think about what it means--on dry pavement cruising down the road, the shafts will be spinning at the same speed even with 0% going to the front. If you floor it to the point your back tires can break loose, that takes a lot of torque. Adding in only a small amount of torque to the front will be enough to propel the vehicle forward and stop the rear wheelspin. In other words, the shaft speeds will likely equalize long before the torque to the front has ramped all the way up to 50%.

On lower traction surfaces in more normal use, think of it like this--if you can accelerate at 1/2 throttle on a surface without spinning the rear tires, if you do it at 2/3 throttle in auto mode it doesn't take much torque on the front axle to stop the rear tires from spinning.

Equal shaft speeds rarely will require equal torque split. In deep snow, etc, sure, but in those situations you aren't worried about bind anyway (and should probably just shift to 4 HI).

Silverado Steve · January 15, 2017

The 2014s dont have the off road screen. It only states on the DIC when you turn the knob to any of the 4wd settings, "4wd shift in progress".

While it's shifting it says 4wd system shift in progress. If you bring the off road section up on the DIC, that is where it shows AWD while in 4 auto mode or 4WD while its in 4hi and 4lo.

Mark Diedrich · January 15, 2017

At low speeds, high steering angles can be expected. Excessive preload, even if it didn't cause crow hopping would cause more wear and tear, steering effort, etc, on high traction surfaces. At higher speeds you won't have nearly as sharp turning angles so these issues are diminished so they can get away with more.

Having some torque go to the front wheels all the time adds safety by actually preventing rear wheelspin from happening in the first place under some conditions. When the rear tires do spin the back of the vehicle often wants to slide out--the torque from the front tires provides a righting moment helping to straighten the vehicle as this occurs, without delay waiting for the clutches to react. In some situations this will be enough to prevent a crash. Those miliseconds matter more at higher speeds.

No. I do it all the time and you don't get crow-hopping like you do in 4Hi. That's the beauty of auto mode, the axles aren't locked together even when significant torque is sent to the front. Don't get me wrong, if you put your foot to the floor on a right angle corner from a stop all sorts of weird and violent things can occur. Usually as the torque ramps up you can feel a steering effort increase and there can certainly be some feedback through the wheel depending upon what sort of traction the front wheels are finding, but that's a whole different thing. [Edit to add: I didn't mean to make it sound like crow hopping is impossible in Auto mode--if you try hard enough you might be able to make it happen. But in regular use, even quite abusive use, it's not an issue. In 4Hi, however, you won't make it around the block on wet pavement without feeling like you may be doing real damage to the vehicle.]

Since you probably won't break anything on wet pavement, if you want to really feel the difference that'll illustrate it for you. On a right angle corner, from a stop, with the traction control off, mash the pedal as much as you dare in 2 Hi, Auto and 4 Hi. They'll all give you quite different results and you'll see Auto is easily the best tool for the job when on pavement.

That's the part I think many are mis-understanding. Making the shafts spin the same speed does not mean they are locked together or that the torque split is 50/50. Think about what it means--on dry pavement cruising down the road, the shafts will be spinning at the same speed even with 0% going to the front. If you floor it to the point your back tires can break loose, that takes a lot of torque. Adding in only a small amount of torque to the front will be enough to propel the vehicle forward and stop the rear wheelspin. In other words, the shaft speeds will likely equalize long before the torque to the front has ramped all the way up to 50%.

On lower traction surfaces in more normal use, think of it like this--if you can accelerate at 1/2 throttle on a surface without spinning the rear tires, if you do it at 2/3 throttle in auto mode it doesn't take much torque on the front axle to stop the rear tires from spinning.

Equal shaft speeds rarely will require equal torque split. In deep snow, etc, sure, but in those situations you aren't worried about bind anyway (and should probably just shift to 4 HI).

Mark Diedrich · January 15, 2017

Thanks!

MUDCVRD · January 16, 2017

I suppose it's safe to say that the term "All-Wheel-Drive" (AWD) is open to interpretation considering the large number of varying systems within the AWD family. If you take the term literal, then any mode where power is being sent to every wheel is in fact All-Wheel-Drive whether it be 4WD HI, 4WD LOW, Auto4. If the manufacturers had designated the term ATAWD (All-Time-All-Wheel-Drive) to a system which is simply always delivering power to every wheel and cannot be switched out of that mode, this topic would certainly have less confusion and debate. But as most people see it now, a system that is designated AWD is a system in which power remains in affect to all wheels and at all times and therefore is All-Time-AWD. 4Auto is not even close to being AWD as long as we understand an AWD system as being one that cannot be disengaged. It's all just word play.

Thomcat · January 16, 2017

You can drive as fast as you want in auto.

It should be labeled "AWD" instead of "auto 4wd"

x2

The graphic display of the truck's drive wheels in the DIC labels it as AWD showing all wheels driven for auto and 4WD showing all wheels driven for 4Hi/Lo matching Auto and 4Hi/Lo settings respectively on the transfer case selector knob.

truckguy82 · January 16, 2017

Man I am very familiar with GM systems, I have worked on them for years along with working on them with my dad who retired from GM. GM does not use any sort of vectoring or torque split in their truck 4wd system, PLAIN AND SIMPLE...!!!

The only reason that while in 4Auto that the transfer case loads the clutches 10% is because it takes the transfer case actuator 10-15 seconds under a normal move from 2Hi to 4Hi. While you can shift from 2Hi to 4Hi while on the fly, if you tried to do that after reacting to a slippage event you would be 20-30 down the road before the 4Hi locked into place and you were in 4wd mode with 50/50 power split. When you go into 4Auto the actuator moves over into 4Hi with the 10% clutch hold, that way you don't have to wait on the actuator to move the hard parts over from 2Hi to 4Hi. Instead now all you wait on is the clutch pack loading to go from 10% to 100%. There is no variableness to the transfer case at all, it is either 0%, 10% or 100% engaged.

So please continue to show how ignorant you are to people who know how this system works and tell us that it works like a AWD system and we will continue to tell you that you are wrong, so please quit posting inaccurate info to confuse people who will then trust you and get into a situation that would of have normally been in had they listened to someone who knows how the actual system operates.

I have owned a 1996 Yukon 4x4, 2001 Silverado 4x4, 2010 Sierra 4x4, 2014 Sierra 4x4 and my 2016 Sierra 4x4. So to say I know how the GM 4wd system operates would be saying it lightly.

Donstar · January 16, 2017

Shouting and name calling are not productive. It may be soothing to vent but it doesn't help to prove your point, imo. There was some good explanations on this thread and it is not difficult to discern fact from opinion. Many of the "opinions" may not be technically accurate to others but they are all valid contributions.

truckguy82 · January 16, 2017

Shouting and name calling are not productive. It may be soothing to vent but it doesn't help to prove your point, imo. There was some good explanations on this thread and it is not difficult to discern fact from opinion. Many of the "opinions" may not be technically accurate to others but they are all valid contributions.

I hope you know I did not type that

truckguy82 · January 16, 2017

I would just like to note, that I think we can officially refer to 4auto as "just like awd"

While the term itself is rather irrelevant to myself, and probably most of you. I think it will be easier for people to comprehend that instead of going into detail about clutches and what-not. I mean literally every single time someone asks about 4auto we go into this lengthy description of the mechanicals and how it should and shouldn't be used. It just easier to say, "it's just like awd"

Even if you disagree via mechanical design (not sure how you would at this point), from the driver's and owners perspective, it is infact just like an AWD setting.