GMC Sierra AT4X 1500 - 4WD FRONT/REAR LOCKERS Question

Jeremy Stevens · January 31

I'm Wondering How The 4WD AUTO Mode Works In Comparison To Having The FRONT/REAR LOCKERS Engaged...

Assuming It Acts Like An Open Differential When The LOCKERS Are NOT Engaged...

How Does The 4WD System Work Let's Say On An Icy Highway With The Lockers NOT Engaged?(it says to NOT operate At High Speeds When Engaged)..

I think Having 1 Wheel Spinning Driving On Icy Highway Conditions Could Be Dangerous And SPIN YOU OUT Does That Make Sense?

The 4WD Dial Or Button Area Is In A Different Place Than The FRONT/REAR LOCKER Buttons...

So I Wonder How The 4WD AUTO Mode Works For Driving On A Icy Highway?...

The GMC Sierra AT4X Looks Right For Me but Wonder About That And Then You Can't Even Get FRONT LOCKERS On A Standard Bed HD ... So Assuming The Front Diff On The HD Is A Limited Slip ... But No FRONT Locker

Only The 1500 SHORT BED CREW CAB Configuration Has The FRONT LOCKERS...

So I'm Probably Going To Go With The Short Bed 1500 For The Front And Rear Lockers.......

Just Wonder How it Will drive On A Icy Highway Tho With 4WD AUTO And if That Operates Separately Than the Lockers

Like if Possible Still Being In 4WD AUTO With The LOCKERS NOT ON?...

newdude · January 31

Let's whiddle down the emojis and words a bit here.

"I'm wondering how the 4WD AUTO mode works in comparison to having the front and rear lockers engaged. Can I assume that the differentials act as an open diff?

On an icy road for instance, how does the 4WD work with the lockers unlocked?"

The AT4X 1500. The electronic rear locker can be locked in all modes of the transfer case, so 2HI, 4HI, 4LO and AUTO under certain conditions.

It can be locked during as follows:

-Normal mode engagement allowed if vehicle speed is less than 30 km/h (19 mph)

-Off road mode engagement allowed if vehicle speed is less than 30 km/h (19 mph)

It can unlock itself under the following circumstances -

-A driver request to disengage

-Vehicle speed is greater than 40 km/h (25 mph) in Normal Mode

-Vehicle speed is greater than 160 km/h (100 mph) in Off-Road Mode

-An ABS or ESC event in OFF Road Mode and vehicle speed is greater than 40 km/h (25 mph)

The front axle however, can only be locked when the rear axle is locked AND it must be in 4LO. The front won't lock in 2HI, 4HI or AUTO.

In order for the front axle differential lock actuator to energize, the vehicle must be operating under the following conditions:

Transfer case 4 LO range is engaged
Rear axle differential lock is engaged
Hill Descent Control is not active
Normal mode engagement allowed if vehicle speed is less than 30 km/h (19 mph).
Off road mode engagement allowed if vehicle speed is less than 30 km/h (19 mph).

The front axle will remain in the locked position until one of the following events occur:

A driver request to disengage
Rear axle differential lock is disengaged
Vehicle speed is greater than 40 km/h (25 mph) in Normal Mode
Vehicle speed is greater than 56 km/h (35 mph) in Off-Road Mode
Transfer case is shifted into 4 HI or NEUTRAL
An ABS or ESC event in OFF Road Mode and vehicle speed is greater than 40 km/h (25 mph)

HD truck only has the rear locker, no front.

So to use the rear locker over 25mph, you must have the drive mode selector set to Off Road mode, which also remaps throttle input and shift times, so that "might" actually be a hinderance on icy roads.

As for AUTO. AUTO mode, the front axle engages the 4wd actuator, but power flow to the front wheels is controlled via the transfer case. The clutch pack inside the case can vary its apply based on wheel slip. If no slip is detected, only 5% of the power flow is sent to the front axle. Once slip is detected, the transfer case can then send the full apply power to the front, thus splitting to pretty much a 50/50 power distribution to the wheels, but then is no slip is detected, go back to 5% power flow.

AUTO is great for things like slushy roads, light snow like a couple inches at most, and roads that have scattered dry and snow conditions.

TLDR, front locker operation is limited to 4LO and 25mph in normal or 35mph in off road mode, with the rear locked first, so useless in day to day operations but great when going rock climbing or off road park operations.

Edited January 31 by newdude

GETGONE · February 2

On 1/31/2024 at 3:10 PM, Jeremy Stevens said:

I think Having 1 Wheel Spinning Driving On Icy Highway Conditions Could Be Dangerous And SPIN YOU OUT

You must be super young. NO cars when they were first mass produced had a rear locker and had the "one wheel peel". Only in the 60's were locking differentials offered from the factory. On ice you just get the wheel with the least traction spinning while the other just sits there.

@newdude doesn't the G80 unlock at a certain speed even though it's mechanical? I know there has to be ~100rpm difference between the wheels before the locker engages.

newdude · February 2

12 hours ago, GETGONE said:

@newdude doesn't the G80 unlock at a certain speed even though it's mechanical? I know there has to be ~100rpm difference between the wheels before the locker engages.

Correct, over 25mph.

Edited February 2 by newdude

AJMBLAZER · February 2

18 hours ago, GETGONE said:

You must be super young. NO cars when they were first mass produced had a rear locker and had the "one wheel peel". Only in the 60's were locking differentials offered from the factory. On ice you just get the wheel with the least traction spinning while the other just sits there.

@newdude doesn't the G80 unlock at a certain speed even though it's mechanical? I know there has to be ~100rpm difference between the wheels before the locker engages.

Even today most vehicles have open differentials.