Clarity on Auto/AWD system

[NewfieEnvoy]

Looking for some clarity on how the Auto/AWD system works in our rigs. I know it has a bad rap so I'm looking for more details why. As I under stand it while in Auto/AWD there's very little (maybe 5%) power sent to the front axle. Upon sensing slip there's a set of clutches in the transfer case that will gradually engage transferring more power to the front axle, than releasing as traction is regained.
Questions:
1) So these clutches "wear out"? is there any way to test how worn they are? what happens if they totally wear out?
2) are these same clutches used while the 4HI is engaged?
3) how does this Auto/AWD differ from something like the AWD in the Acadia?
4) do we have any records of members who have damaged their transfer case by using Auto/AWD a lot?

I've heard all the usual comments, AWD is just for the wimps afraid to put it 4HI but I do find a use for it. Example; we have currently have a very thin dusting of greasy snow on the roads. Not really enough to justify 4HI and not sure it would provide enough slip around tight corners. Taking a left at the lights in 2wd the traction control kicks in a leaves me feeling helpless with out power. Turn TC off and I'm doing donuts. The Auto/AWD gives me that extra traction just when I need it but I don't have to worry about clicking in and out of 4HI.

Can someone shed some light on the subject for me.

 

[CaptainXL]

Are you talking about Auto/4WD not AWD? AWD is on all the time.

I don't think there are any major issue with Auto/4WD so long as it is engaged without flooring the gas for instance. And make sure the fluid level in the TC is full and in good condition.

 

[NewfieEnvoy]

Are you talking about Auto/4WD not AWD? AWD is on all the time.

I don't think there are any major issue with Auto/4WD so long as it is engaged without flooring the gas for instance. And make sure the fluid level in the TC is full and in good condition.

Yes the Auto/4WD. However, isn't that a sort of AWD? Does the AWD system in the Acadia (just using this one as an example) always send power to the wheels equally? How is it different then our Auto setting?

 

[CaptainXL]

Yes the Auto/4WD. However, isn't that a sort of AWD? Does the AWD system in the Acadia (just using this one as an example) always send power to the wheels equally? How is it different then our Auto setting?

AWD transfer cases include an additional differential near the transfer case to allow front and rear independant axle speeds. This allows it to be used as a daily driver sort of how our rear differential allows you to turn without binding.

The Auto/4WD setting utilizes the rear wheel speed sensor to know if the vehicle is not moving and to engage 4WD. However when in 4WD our front and rear drive shafts are connected together and at least two wheels (one front and one rear) must turn at the same rpm.

So there you have it.In the AWD system you basically have all 4 wheels which can each have independant rpm's and 4wd where only two can unless the rear is locked up using the G80 for instance. In that case your won't be able to turn very easy on dry pavement for instance.

 

[blazinlow89]

I think he is referring more towards the A4WD mode on the selector switch. Trying to compare that mode to a true AWD.

IIRC, the A4WD basically keeps the front hubs in a ready condition, when the vehicle senses slippage it engages the hubs. I do not understand the system like the offroad guys or Roadie, but I think I have a basic understanding of it.

I think the reason alot of guys do not like the A4WD is the amount of stress put on components when the front hubs engage at higher speeds. As compared with a true AWD system that has power going to the front and rear simultaneously.

If any of this is incorrect please edit or let me know.

 

[RayVoy]

Newfie, I'll toss my 2 cents into the mix. The way you are using the auto selection is the correct way it should be used. Used correctly, it is a great tool.

The problem that the t-case has, is that when the truck is moving very slow (parking lots, backing into driveways, etc) and the rear wheels slip on ice, the t-case locks the clutches and sends power forward. The front wheels are almost stopped, the rear(s) are spinning, there is a noticeable thump as the clutches engage the front shaft, the spinning rears are now tied to the almost stopped fronts, all of the engine torque is obsorbed by the t-case.
In a number of the early t-cases, the torque actually broke a bearing (or the post holding the bearing), this area was upgraded in the '05 and newer t-cases.

IMO, continuing to use the Auto4WD, as you use it, will not damage it, just be mindful of ice when almost stopped.

Some of the full time AWD vehicles (including the Trailblazer SS trucks) use a posi-traction type of transfer case. Like the old posi-traction rear diffs, a very little torque is sent to the wheels that do not normally provide drive function; however, when drive wheel slippage is detected, the posi-traction t-case applies additional torque to the non-driving wheels. The posi clutches apply torque progressively so the traction is corrected progressively, without the noticeable thump.

 

[The_Roadie]

The TCCM is supposed to sense clutch wear and automatically adapt as time goes on. Eventually, they wear enough and the A4WD function quits working.

Otherwise Ray said everything I would have. And accurately. For more reading on the SS system, Google for Torsen Differential. They are not cheap.

 

[NewfieEnvoy]

So Roadie if the A4WD "wears out" am I correct to say that the transfer case will still work perfectly fine in 2WD/4HI/4LO?

So by the sounds of it the clutches used for the Auto system engagement don't provide a gradual slippage and transfer of power like most some AWD systems, they do more of a quick hard lock up. Which especially in the older TC caused shock loading damage. So what I'm sensing is the Auto system is great in theory but GM's implementation has had some weak points.

 

[The_Roadie]

If the clutches wear thin, they may not work for 4HI and 4LO. But unless the fragments contaminate the fluid and damage bearings, it should be fine for 2WD.