How does 'auto' mode work?

jvbronco · November 19, 2011

I've done a bit of searching without luck, so I figured I'd post a quick question...

I have a 2011 Silverado, 4x4, z-71, 5.3l, ext cab

Snow is here in northern Idaho (as it is with many other parts of the country) and I'm trying to figure out the best driving configuration for making my way through town with poor road conditions (whether it's better to use 'auto' or keep it in 4 high).

I have a question about how the 'auto' mode works on my truck. From what I understand, when 'auto' is activated, some internal hardware is engaged in order to be ready to flip to 4WD when needed. Given this, what it the typical lag time between when the computer determines that 4WD is needed (e.g., by hitting a patch of ice) and when 4WD is actually engaged (i.e., more than the two back wheels are being powered)?

I ask because I would like to understand how the system works. If there is a 2-3 second delay from detection to engagement, then 'auto' mode might not be useful for a patch of ice (for example), and 4 high would be more effective.

Thanks in advance!

Rolling · November 19, 2011

Auto 4x4 engages the front differential actuator and applies about 10% to the clutch pack in the transfer case. When the wheel speed sensors detect slip, the transfer case encoder motor applys the clutches to fully engage four wheel drive. Once the computer does not detect anymore wheel spin it releases the transfer case clutches so that the system does not have the typical four wheel drive charactersitics, like crow hop.

My 07 and my 11 Silverado both had the auto transfer case and driving in winter is actually very similar to an AWD vehicle. 4x4 engagement is very smooth and very quick. You might feel about a half second delay if you romp on it on ice. Most the time I feel a little torque steer, thats about it.

As far as what works, for example I live in Anchorage Alaska, snow is common. I primarily drive my Silverado in Auto and if it gets DEEP I will switch it to 4x4. Since the clutches are allowed to slip, I change the transfercase fluid quite frequently ( about every other oil change) to keep the fluid from getting too full of debris.

jvbronco · November 19, 2011

Great info! Exactly what I was looking for. Thanks for the heads up on the transfer case fluid too!

I was hoping that it would be similar (in response time) to AWD vehicles but I wasn't sure if there were any truck-specific characteristics that would prevent a quick engagement.

have a great day!

LURCH HD · November 19, 2011

You need to remember that it will not hold you back in auto mode like it would if you were in 4 high.

music · November 20, 2011

I'll second the "like AWD" opinion. If I'm in auto and punch it on a slick surface, it locks in *very* quick, with no noticeable spin. You'll be impressed.

Ryan00TJ · November 20, 2011

I agree! It's a great option. I also just leave it in Auto for uncertain winter conditions and go to 4HI when it gets deep.

jvbronco · November 20, 2011

It's been snowing here (Moscow, ID - Northern Idaho) at night, then melting during the day, then freezing at night. It has been great to be able to leave it in auto and let it do it's thing when I hit a slippery surface, rather than using 4h around time. To me, the only downside of getting a truck (as compared to a capable AWD vehicle) was the performance in the snow. 4wd is great, but not always the most practical (seems like it would be excessive wear on the truck for driving around town).

--LURCH, what do you mean by it "holding me back" in 4 high (and not holding me back in auto)?

--Another general question, should I feel any difference between 2 high and auto while I'm driving? I'm not sure if it's just my own paranoia or if I can feel a difference in the pedals while i'm in auto mode. Any thought on this?

jandersinc · November 20, 2011

I can feel a very minor, but noticeable bind when turning sharp in auto. There is also some additional road noise (actually mechanical noise I think) while driving. These are hardly noticeable most of the time, but every now and then I can still detect it.

uchi · November 20, 2011

my 06 had a slight lag in engagement, it would get about 1.4 turn out of the back tires when she broke traction in the wet intentionally, my 09 however is instant. we were trying out my tune in the rain the other night and she would happily blast into second gear rear tires roasting, auto mode from a dead stop i hammer it and she just grabs and goes, no issues, no hesitation, no crap, starts off like she was in 4wd

jvbronco · November 21, 2011

I can feel a very minor, but noticeable bind when turning sharp in auto. There is also some additional road noise (actually mechanical noise I think) while driving. These are hardly noticeable most of the time, but every now and then I can still detect it.

That's exactly what I am experiencing. Just wanted to make sure that it's normal before driving around all winter like that.

uchi: nice to see that they're improving the design to make it a quick engagement - having that lag almost makes it pointless if you hit a small patch of ice on the road.

KMGZ400 · November 21, 2011

suprisingly there is no lag at all in auto but it binds if i make a pretty sharp turn

LURCH HD · November 21, 2011

It's been snowing here (Moscow, ID - Northern Idaho) at night, then melting during the day, then freezing at night. It has been great to be able to leave it in auto and let it do it's thing when I hit a slippery surface, rather than using 4h around time. To me, the only downside of getting a truck (as compared to a capable AWD vehicle) was the performance in the snow. 4wd is great, but not always the most practical (seems like it would be excessive wear on the truck for driving around town).

--LURCH, what do you mean by it "holding me back" in 4 high (and not holding me back in auto)?

--Another general question, should I feel any difference between 2 high and auto while I'm driving? I'm not sure if it's just my own paranoia or if I can feel a difference in the pedals while i'm in auto mode. Any thought on this?

If you are going down a slick hill in 4 high and say your in 1 st gear letting the engine hold you back to keep off of the brakes both front and rear diff's are slowing you down ( holding back ). If you are in auto the rear or 1 tire is all that's holding back. In auto the truck has to detect wheel spin before it engages the front diff. I'm not good at putting it in words, hope you understand what I mean.

In 2 WD nothing in front turns when in auto the axles start turning ( like the old full time 4 wheel drive ) they just don't engage until rear wheel slippage is detected.

jvbronco · November 21, 2011

I'm not good at putting it in words, hope you understand what I mean.

I completely understand - great to know, too here in Moscow, ID because our roads are built on top of the rolling hill landscape of the Palouse region (lots of wheat fields - kinda looks like the hill screenshot from Windows XP). Thanks for giving me the heads up, so many things that you don't think about until you actually need them. It's great to have this resource!

kdixer · November 21, 2011

It's been snowing here (Moscow, ID - Northern Idaho) at night, then melting during the day, then freezing at night. It has been great to be able to leave it in auto and let it do it's thing when I hit a slippery surface, rather than using 4h around time. To me, the only downside of getting a truck (as compared to a capable AWD vehicle) was the performance in the snow. 4wd is great, but not always the most practical (seems like it would be excessive wear on the truck for driving around town).

--LURCH, what do you mean by it "holding me back" in 4 high (and not holding me back in auto)?

--Another general question, should I feel any difference between 2 high and auto while I'm driving? I'm not sure if it's just my own paranoia or if I can feel a difference in the pedals while i'm in auto mode. Any thought on this?

If you are going down a slick hill in 4 high and say your in 1 st gear letting the engine hold you back to keep off of the brakes both front and rear diff's are slowing you down ( holding back ). If you are in auto the rear or 1 tire is all that's holding back. In auto the truck has to detect wheel spin before it engages the front diff. I'm not good at putting it in words, hope you understand what I mean.

In 2 WD nothing in front turns when in auto the axles start turning ( like the old full time 4 wheel drive ) they just don't engage until rear wheel slippage is detected.

Actually everything in the front turns all the time. But when in auto or 4wd the diff actuator couples the passenger side axle to the carrier. At no time do the hubs disengage from the axles. In 2wd the actuator un-couples the passenger side axle from the carrier in an effort to free up driveline resistance. This allows the right and left axles to rotate at different speeds without involving the carriers spider gears. The resistance in the un-coupled carrier often causes the front driveshaft to spin even when driving around in 2wd.

In auto mode the front diff is engaged and ready for action. And when wheelslip is detected the clutch in the transfer case directs power to the front driveshaft.

I usually use Auto in intermitantly slick driving consitions. But if I find the auto keeps engaging I just go ahead and put it in 4wd to save the wear and tear on the clutch pack. I am not in the habit of letting 4wd hold me back going down hill. I would rather have full control of my steering. I usually let the rear tires hold by putting it in a low gear and feather the brakes' letting up if I detect a slide. ABS usually prevents full lockup as to where I have had 4wd cause an uncontrollable slide where the truck just goes where it wants pointing any direction it wants. The only way to recover from a 4wd slide is to apply throttle and often there is just not enough room for that. At least in 2wd I can let up on the brakes and regain some steering control.

Just remember that if you get stuck you should turn off stabilitrak.