Help understanding Auto drive mode

[GN2018]

Is there really any difference in driving in 2WD versus Auto as long as I’m driving on dry pavement?  I’ve always driven in Auto mode, but to be honest, I don’t really understand what it does.  I understand the other modes.  Reason I’m asking about Auto, beyond just understanding it, is that I’m wondering if I’ll get better gas mileage in 2WD versus Auto - again, on dry pavement.  Thanks.

[ChriZ71]

Quick reply from someone who has a very limited understanding:

Auto runs RWD but activates front if senses slip. There is always "some" engagement with the front which will potentially lead to premature wear (and less gas mileage?). 

In general, no need for Auto in the conditions you describe. Use 2wd for dry pavement, Auto for Rain/Snow type conditions on pavement. You said know about 4wd hi/lo. 

Edited September 19, 2021 by ChriZ71

[redwngr]

In GM's wording: 

 

AUTO (Automatic Four-Wheel Drive)
 

Use when road surface 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 2 m. 

 

 

 

[yofxr]

Auto has the same affect as driving with the hubs locked .It readies the truck to be placed in 4wd instantly.

Drag created uses more fuel and wear/tear on front driveline. Some lift kits recommend not using auto mode with their kits installed.

 

[stevejones]

Auto mode is the woman button.  When it's sunny & 85 degrees outside, put your truck in 2WD.

[Gangly]

From GM's website:

If road conditions frequently alternate between high- and low-traction areas, consider using the “AUTO” setting found on select GMC electronic transfer cases. This setting allows your GMC to automatically distribute torque to the front axle by anticipating the need for additional traction.  Shifting into “auto” engages the front axle, but the transfer case sends power primarily to the rear wheels in normal conditions and the clutches modulate torque forward to provide stability and enhance traction to the vehicle.  Although not always optimal for efficiency and wear of your vehicle 4wd driveline, AUTOMATIC 4 HI can be used on any road condition without risk of damaging your vehicle.


 

While 4WD Auto is engaged, the front differential is engaged, albeit in small amounts when grip is adequate, and therefore additional wear is occurring to the system.  In layman's terms, 4WD Auto is 95% RWD and 5% FWD until the clutches in the transfer case sense slippage, at which time they tighten down and transfer more power to the front wheels.  If there is no indication of adverse weather or dangerous roadway conditions, there is no reason to be in 4WD Auto as you are only adding wear and tear to your vehicle's 4WD system.

 

Edited September 20, 2021 by Gangly

[GN2018]

I put it in 2WD today and it did feel a bit lighter for steering.  Hard to explain, but it does seem different.  I’ll leave it there until winter.

[Gangly]

17 minutes ago, GN2018 said:

I put it in 2WD today and it did feel a bit lighter for steering.  Hard to explain, but it does seem different.  I’ll leave it there until winter.

Most anytime you switch from 2wd to 4wd, or vice verse, on concrete you can usually tell a difference in the steering.  That felt difference is the force generated from a drive axle trying to steer vs a neutral/rolling axle trying to steer.

[JimCost2014]

1 hour ago, GN2018 said:

I put it in 2WD today and it did feel a bit lighter for steering.  Hard to explain, but it does seem different.  I’ll leave it there until winter.

Take a look at this, it does not use the verbiage AUTO 4WD, but think you will understand how it works from the video.

 

[Gangly]

Great video JimCost2014

[Ioniq MPG]

In auto there is a slight whirl sound when starting from a stop vs. 2wd.

[newdude]

Zero need to run AUTO on dry roads.  Heavy rain you could if you really wanted.  AUTO is right at home in the winter so like trace to 3-4 inches of snow on the roads or slushy muck.  If the snow is deep, unplowed 6 inches+ 4HI.  

 

GM's AUTO position:

 

- Front axle engages.  The truck activates the front axle actuator to engage the front axle so it can put power to the ground.

 

- Transfer case.  Inside the AutoTrac transfer cases, there is a clutch pack.  This pack when placed in AUTO regulates the power flow to the front axle.  Power flow is the same as in 4HI.  When there is zero slip detected, the truck sends 95% of its power to the rear wheels, and 5% to the front.  This 5% allows the front axle to turn with no binding/crow hop.  

 

- When slip is detected in AUTO, the clutch pack in the transfer case will start to engage and send more power to the front axle, up to 50% of available power.  The transfer case actuator varies its position to increase or decrease the torque through the clutches.

 

GM's description and operation:

 

During the Auto 4WD mode, the power flow is the same as it is in the 4HI mode. Except, during the Auto 4WD mode, the 2/4 wheel drive actuator assembly (13) rotates the control actuator shaft to the correct torque level positions. Rotating the control actuator shaft to the various positions changes the clutch torque level. When a difference of front wheel speed to rear wheel speed is recognized, the transfer case control module commands for more, or less clutch torque.

 

Front Axle Operation:

 

When the driver engages the 4WD system, the Transfer Case Control Module sends a signal to the electric motor actuator to energize and extend the plunger inside. The extended plunger moves the clutch fork and drive axle clutch sleeve across the intermediate axle shaft and the wheel drive shaft and locks the two shafts together. The locking of the two shafts allows the axle to operate in the same manner as a semi-floating rear axle. A propeller shaft connects the transfer case to the front axle.

 

 

So 4HI everything is engaged in the case.  Actuator shifts to 4HI and applies the clutch to full on and the front axle engages.  In AUTO, the front axle locks, transfer case goes to a form of 4HI which allows for control of power flow in the case to the front axle.  

Edited October 11, 2021 by newdude

[TxTruckMan]

38 minutes ago, newdude said:

Zero need to run AUTO on dry roads.  Heavy rain you could if you really wanted.  AUTO is right at home in the winter so like trace to 3-4 inches of snow on the roads or slushy muck.  If the snow is deep, unplowed 6 inches+ 4HI.  

I agree with your first sentence, but disagree with your second one.  AUTO is great any time the roads are slick - light rain, heavy rain, snow, ice, gravel, etc.  Usually the roads are slicker with light rain as all of the slime and oils have not yet washed away.  And people usually try to drive "normal" when it's only a light rain, and more traction is needed when driving quicker on wet roads.  In heavy rains people usually slow down due to poor visibility and ponding issues, so I find AUTO less needed (but I still use it).

 

I usually use the pulling into traffic example when trying to determine if you need AUTO.  If you need to turn right onto a busy street and then quickly accelerate, do you think you'll lose traction when driving normally?  If so, then use AUTO.