Autotrac transfer case vs regular

[newdude]

On 1/6/2025 at 6:09 PM, Skiff Lowrey said:

This thread is a few years old, but seems to be on topic for me.  I am looking at 2024/2025 Silvarado 1500 4x4. The 2spd transfer case option is a $200 upgrade. With this selectorI can chose 2 hi, 4hi, and 4lo. Where as the Auto has 4 hi setting and Auto that kicks in as it determines. More like an adaptive AWD, than a true 4wd. If someone could clear this up for me I would appreciate it. 

 

 

First off, this thread is in the HD section.  The HD transfer cases are a bit different.  Both are 2 speed cases, one has Auto, the other doesn't.

 

In your case on the 1500, the standard case is the single speed with AutoTrac.  The optional case is the 2 speed AutoTrac.  

 

Single speed is standard on all trucks with regular 4x4.  The 2 speed case is optional on WT, standard on all trucks optioned with Z71, Trail Boss and High Country.

 

Auto is in theory a "full time" function of these transfer cases.  In Auto mode, the clutch pack in the transfer case will vary its power output to the front wheels as needed.  When there is no traction loss and to keep the front end from binding, it only applies 5-10% power to the front wheels.  When slip is detected, it can deliver up to 50/50 power distribution front/rear.    4HI overrides this and just locks everything 50/50 power distribution.  

[newdude]

24 minutes ago, Another JR said:

To me it makes no sense not to get the two speed transfer case. It’s probably one of the only features that is worth 5 to 10 times what you paid for it at resale because many people will not even consider the truck if it doesn’t have 4 Lo.). It is essential for off roading and for gardening (stump pulling), and really nice to have for steep boat ramps.  Most people not off roading or using steep boat ramps rarely use it, but when you need it, you will be very glad you spent that $200. 

 

 

To be fair, the single speed case in the 1500s does have Terrain Mode which is intended to be used in those boat ramp scenarios or off roading.  Only issue with it is you don't get the mechanical torque multiplication provided by the true low range.  I've used it a couple times, works fairly well.  

[Another JR]

10 hours ago, newdude said:

 

…

4HI overrides this and just locks everything 50/50 power distribution.  

The first part of this is correct but isn’t the second part incorrect?  4hi just locks the front and rear systems together with no center differential or clutch. Torque distribution in that configuration is purely determined by traction, and can be anywhere from 0/100 to 100/0. 

[asilverblazer]

4 hours ago, Another JR said:

The first part of this is correct but isn’t the second part incorrect?  4hi just locks the front and rear systems together with no center differential or clutch. Torque distribution in that configuration is purely determined by traction, and can be anywhere from 0/100 to 100/0. 

I might not be interpreting your comment correctly but in 4HI you will not get a difference (differential) in front to rear power distribution. It is locked 50:50 regardless of traction. Ignoring any traction devices or traction controls, at a minimum, one tire in the front AND one tire in the rear will get power (50%) and in poor traction spin. Versus what you describe as one front tire OR one rear tire getting power (100%) and spinning.

 

EDIT:

If I re-read your comment another way... In 4 Auto, it cannot vary the power beyond locking the front and rear together. The front will never get more power than the rear. 

Something like...

F/R

10/90

20/80

30/70

40/60

50/50

 

Never

F/R

60/40

70/30

80/20

90/10

100/0

Edited January 8, 2025 by asilverblazer

[Another JR]

31 minutes ago, asilverblazer said:

I might not be interpreting your comment correctly but in 4HI you will not get a difference (differential) in front to rear power distribution. It is locked 50:50 regardless of traction. Ignoring any traction devices or traction controls, at a minimum, one tire in the front AND one tire in the rear will get power (50%) and in poor traction spin. Versus what you describe as one front tire OR one rear tire getting power (100%) and spinning.

 

EDIT:

If I re-read your comment another way... In 4 Auto, it cannot vary the power beyond locking the front and rear together. The front will never get more power than the rear. 

Something like...

F/R

10/90

20/80

30/70

40/60

50/50

 

Never

F/R

60/40

70/30

80/20

90/10

100/0

I was not talking about 4 auto. Others above have described its clutch function, and I don’t know if it will deliver 100% of the torque to the front axle via the clutch. It’s probably limited, and I don’t know if the limit has any relationship to the actual torque being delivered to the rear axle. 
 

I was trying to explain how 4 hi and 4 lo work when the 4 auto clutch is not involved. It may be more complicated than this in the auto transfer cases, but I think that 4 hi and 4 lo in the auto transfer cases work just like a plain transfer case. A plain old-school transfer case with no clutch and no differential will deliver the same rotational speed to the front and rear, but the torque applied is determined by where the traction is. To simplify, imagine there is no front differential and no rear differential. If you put either both front or both rear tires on a wet ice surface, the other axle will be the only axle applying torque. The common example of this is the slippery boat ramp at low tide. The rear tires are on slime and have no grip (hence all those cringeworthy Florida boat ramp videos showing 2wd trucks slipping into the water), but if you have it in 4 hi or 4 lo the front wheels will pull you up the ramp until the rears get grip and start helping. 
 

I could be wrong, but that’s how I understand our two speed auto transfer cases to work. 

[asilverblazer]

5 minutes ago, Another JR said:

I was trying to explain how 4 hi and 4 lo work when the 4 auto clutch is not involved. It may be more complicated than this in the auto transfer cases, but I think that 4 hi and 4 lo in the auto transfer cases work just like a plain transfer case. A plain old-school transfer case with no clutch and no differential will deliver the same rotational speed to the front and rear, but the torque applied is determined by where the traction is. To simplify, imagine there is no front differential and no rear differential. If you put either both front or both rear tires on a wet ice surface, the other axle will be the only axle applying torque. The common example of this is the slippery boat ramp at low tide. The rear tires are on slime and have no grip (hence all those cringeworthy Florida boat ramp videos showing 2wd trucks slipping into the water), but if you have it in 4 hi or 4 lo the front wheels will pull you up the ramp until the rears get grip and start helping. 
 

I could be wrong, but that’s how I understand our two speed auto transfer cases to work. 

I think we are saying/understanding the same thing. 

[carkhz316]

4HI (with an autotrac T-case) is simply front and rear receive full output with no bias (because the clutch is fully applied/ locked). The only limiting factor would be if the clutch pack were to slip from failure. Without Autotrac, everything is a mechanical gears/ chain connection. If something is to give, its the tires or metal parts breaking.

[redwngr]

......

Edited January 12, 2025 by redwngr

[truckguy82]

On 1/8/2025 at 2:46 PM, Another JR said:

I was not talking about 4 auto. Others above have described its clutch function, and I don’t know if it will deliver 100% of the torque to the front axle via the clutch. It’s probably limited, and I don’t know if the limit has any relationship to the actual torque being delivered to the rear axle. 
 

I was trying to explain how 4 hi and 4 lo work when the 4 auto clutch is not involved. It may be more complicated than this in the auto transfer cases, but I think that 4 hi and 4 lo in the auto transfer cases work just like a plain transfer case. A plain old-school transfer case with no clutch and no differential will deliver the same rotational speed to the front and rear, but the torque applied is determined by where the traction is. To simplify, imagine there is no front differential and no rear differential. If you put either both front or both rear tires on a wet ice surface, the other axle will be the only axle applying torque. The common example of this is the slippery boat ramp at low tide. The rear tires are on slime and have no grip (hence all those cringeworthy Florida boat ramp videos showing 2wd trucks slipping into the water), but if you have it in 4 hi or 4 lo the front wheels will pull you up the ramp until the rears get grip and start helping. 
 

I could be wrong, but that’s how I understand our two speed auto transfer cases to work. 

I went from my sierra to a jeep rubicon. Both have 4auto.

 

While I was on jeep forums, someone posted up scan data of the transfer case in action while in the different modes. I learned that the transfer case isn’t actually locked in 4hi.  In 4hi the jeep applies 100% clutch pressure (effectively locked) but then it starts to reduce the clutch pressure as speeds increase. It is actually only locked until about 25mph. At highway speeds it is pretty much back to 2wd (yes this is in 4hi setting not 4auto)

 

I was very surprised.

 

4lo it was locked all the time.

 

I don’t know the GM transfer case operates in a similar manner but I wouldn’t be surprised.

 

Also interesting, in 4auto in the jeep, it doesn’t just adjust the clutch pressure based on wheel slip, it also does it according to throttle position. Full throttle is 100% lock, 50% throttle was like 60% clutch pressure if I recall. Which explains why 4auto seems to react telepathically in both vehicles when you give it the business.

[Another JR]

A friend of mine has a 2016 (I think) Tahoe he bought used a few years ago. He was getting ready to put new tires on it and wanted me to look at the tire wear pattern (all tires, but especially the fronts, were worn a lot more on the outside 2 inches of the tread). I told him he needs to slow down around corners and make sure he maintains his tire pressure, but checking the alignment is probably a good idea.
 

Then he told me he was wondering if it might be due to his having driven the thing in 4 auto since he bought it - maybe 15,000 miles. The tire place told him he shouldn’t be doing that. 
 

I told him he should change is transfer case and differential oil and see how the fluid looks. I told him i have no idea if driving that much in 4 auto would wear out the clutch. I suggested that next time he’s in the snow he put it in 4 auto, then find a place to put his rear wheels on ice and his front wheels on pavement and try to briskly accelerate. 
 

Anybody have knowledge about whether  his driving in 4 auto so long would wear out the transfer case clutch or do other damage, like to the front differential?  Thanks. 

Edited February 10, 2025 by Another JR