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Discussion Starter · #1 ·
Hi,

Due to the recent projected events of 10-15" snow in specific parts of NYS by Monday. I've always been a little curious which to put my truck in.
I'm still new to owning my truck (roughly 3 weeks now) and have been curious. The stealership mentions to put the truck in "Auto" when the conditions
on the road are wintery.

What is the real difference between Auto or 4HI? Should I put the truck into auto mode as intended or 4HI? Do either have a speed limiter to
how fast I can go in either mode? Since we are projecting to have a ton of snow on Monday, I'm just seeking advice to which one to put the truck into (4x4)

I appreciate the answers in advanced, I'm not well versed in this as my previous cars have had FWD and AWD.
 

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2015 High Country Duramax
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I never used the auto mode in either of my trucks that had it. Was either 2wd or 4wd.

But it is designed to be used with mixed road conditions. So for example, the roads have been plowed for the most part, but there are still some slick and snow covered areas.

Some people love it, others don't use it. My father uses it quite often. Heck his 2009 he kept in auto all the time for basically 120k miles. I definitely wouldn't do that though. It is un needed wear on the t case and extra drag on the truck due to the front axle always being engaged (granted not a whole lot extra).
 

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2022 3500HD
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Auto is nice when conditions are off/on slippery, as you will not have the front diff binding on dry pavement when turning corners. If you are in constantly snow or slippery conditions i would leave it in 4wd and keep it within the recommended speed, which is usually around 50mph.

The transfer case in My 2004 standard duty 2500 went bad, which i believe was brought on by the previous owner constantly using auto 4wd when the front wheels would "slam" into gear by hitting the throttle hard, spinning the rear and engaging the front.

On the new truck, I typically leave it in auto when the wife is driving, and I will either use 2wd or 4wd as conditions dictate. I also run dedicated snow/ice tires which make a huge difference.

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We just returned home from skiing at Purgatory. Used Auto on the highway that was mixed with snow/slush/wet but when the roads were covered 4Hi was the choice.
Snow Slope Tree Freezing Motor vehicle
Photograph Snow Nature Slope Automotive tire
 

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The responses above basically mirror my thoughts. I don't use 4auto very often, but it is great for mixed road conditions. I like to use 4HI only when conditions are bad, but if I find myself shifting back and forth to 2Hi too often, it is time to just put it in 4auto.
 

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2015 Sierra SLE 5.3 4x4
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Auto is going to leave it in basically 2wd mode until it needs 4wd then it will instantly switch to it, then when not needed it will turn back off. It says it uses a little more gas using auto over 2wd if that matters. It's handing the control over to the truck on when it needs 4wd instead of the driver.

Best place to look is your owners manual.
In my 2015 manual it says for automatic transfer case the following

(Automatic Four-Wheel
Drive): Use when road surface
traction conditions are variable.
When driving in AUTO, the front
axle is engaged, and the vehicle's
power is sent to the front and rear
wheels automatically based on
driving conditions. This setting
provides slightly lower fuel economy
than 2wd.
Do not use AUTO mode to park on
a steep grade with poor traction
such as ice, snow, mud, or gravel.
In AUTO mode only the rear wheels
will hold the vehicle from sliding
when parked. If parking on a steep
grade, use 4 m to keep all four
wheels engaged.

(Four-Wheel Drive High): Use
this position when extra traction is
needed, such as when driving on
snowy or icy roads, when
off-roading, or when plowing snow.

(Four-Wheel Drive Low): This
setting engages the front axle and
delivers extra torque. Choose 4 Low
when driving off-road in deep sand,
deep mud, or deep snow, and while
climbing or descending steep hills.
Shifting into 4 Low will turn Traction
Control and StabiliTrak off. See
Traction Control/Electronic Stability
Control on page 9-50.


Shifting Into 4 High or AUTO
Turn the knob to the 4 High or AUTO
position at any speed, except from
Neutral. The indicator light will flash
while shifting and will remain on
when the shift is completed.
The only limit on speed I see is where it says, When 4 Low is engaged, keep vehicle
speed below 72 km/h (45 mph).

I just looked at the 2020 Sierra manual here 20_GMC_Sierra_OM_en_US_U_84186890A_2019APR11.pdf
and it says the same thing.
I did see one weird thing in the 2020 manual which doesn't make sense to me
When a shift to 2 wheel drive is completed successfully while in P (Park), the parking brake will engage. To resume driving, shift the transmission to the desired gear and manually release the parking brake or press the accelerator pedal to begin driving.
 
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Possibly because the operator may not have their foot on the brake when the vehicle is in park.

If so, when it shifts thru neutral the vehicle could move -- so it auto applies the parking brake to prevent this.
Seems like it ignores what the parking brake position was prior to the shift request.
 

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2015 Sierra SLE 5.3 4x4
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But you have to have your foot on the brake to even shift out of park don't you? If so then why apply the parking brake as well?
What do you mean shift through neutral? It says while the transmission is in park if you switch the transfer case to 2wd. You are not moving the gear shift, it's in park already. The transfer case isn't going into neutral either.
 

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If the roads are pure slippery like today I use 4x4. No reason not to. Auto would have been sufficient but why wouldn't I use the feature I paid for and have?
 

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But you have to have your foot on the brake to even shift out of park don't you? If so then why apply the parking brake as well?
What do you mean shift through neutral? It says while the transmission is in park if you switch the transfer case to 2wd. You are not moving the gear shift, it's in park already. The transfer case isn't going into neutral either.
You don't need to have foot on brake to shift transfer case.
Transfer case (the part that shifts from 2wd-N-4wd) is behind the transmission.
When transfer case is in neutral, the transmission being in park cannot prevent the truck from rolling.
 

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2015 Sierra SLE 5.3 4x4
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That is correct except when you switch from 4wd to 2wd the transfer case is not going into neutral. You have to do a whole other operation to get the transfer case into neutral.
I don't see why the parking brake goes on when you switch into 2wd while transmission is in park. If what you say is true that the transfer case goes into neutral inbetween the shift wouldn't they want it to apply the parking brake as well when switching into 4wd when the transmission is in park as well? Why only 2wd?
 

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i am still overwhelmed with joy at my floor shift on the fly magna transfer case in my 2500hd, compared to getting out of my 72 to lock the front hubs and then shifting the transmission every which way until the np205 stops making loud grinding noises and finally goes into gear ... this auto 4x4 sounds magical! sorcery, i say! don’t trust it!
 

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2022 3500HD
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i am still overwhelmed with joy at my floor shift on the fly magna transfer case in my 2500hd, compared to getting out of my 72 to lock the front hubs and then shifting the transmission every which way until the np205 stops making loud grinding noises and finally goes into gear ... this auto 4x4 sounds magical! sorcery, i say! don’t trust it!
Sounds like a guy who drives old Pontiacs lol.

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2015 Sierra SLE 5.3 4x4
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I usually don't trust auto this or auto that but things have really improved from years ago.
I didn't trust the auto reheat on our Panasonic microwave but gave it a go and holy shit it reheats things perfectly. We then got a new dryer and it had auto dry. I tried it skeptically and low and behold it dries perfectly. My wife was even amazed because she thought it was dumb until she tried it.
We don't get much bad weather around here and I'm glad but I do admit I kinda wish it would snow just so I could test out the auto 4wd since I really have no other need for it otherwise.
 

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Both my 02 and 2011 Sierra's had the auto feature which I never used at first in the 02. After using it in the snow, I rarely use the 4hi anymore. As said, it does not bind up the front end on corners and is best in variable conditions. We just got through a long bout of snow and ice and "auto" never left me feeling like I was only in 2 wheel drive. It always kicked in immediately when needed.

The funniest part about my 2011 was when I wanted to do some donuts in 2wd so I turned the wheel and floored it.....HA.....It did NOTHING!! Realized it has traction control....disabled that and then had some fun!
 

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Discussion Starter · #16 ·
Update: We had about 13" of snow fall from Sunday night into Monday. Due to it being a federal holiday, and barely any snow plowing being done, 4HI is what I had running pretty much all day. And WOW is it a difference, and it's noticeable.

I took a trip into the city to train, and they never clean the city roads (unless important ones), so I was able to navigate comfortably. Definitely yesterday made me feel like the truck investment was worth every penny. Ironically I notice about a 1MPG decrease, (2.7T) but that was impressive.

This mornings commute, mostly the roads were clean, some icy/slushy spots I kept the truck in Auto which handled it nicely. I will say the difference from owning a Subaru (AWD) to this, is dramatically an improvement.
 

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2015 Sierra SLE 5.3 4x4
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Still no snow here. They are again saying a chance for the weekend but I doubt it. I may get to use 4wd mode in April hopefully if we go to the Outer Banks.
 
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