Auto 4x4

[4weight]

Robert,

 

I am not an expert but what I meant is in Auto, the front axle is engaged but it is not in 4WD high until necessary.  It's an auto function.  When I took mine from Auto to 2WD high, you could hear it disengage.

 

Sorry I did not make myself clear.  Just going by what the manual said.  Bottom line, better gas mileage for normal driving in 2WD. 

 

Merc

[ftwhite]

So, it seems:

4High, the center differential/transfer case is locked, so front and rear spin at the same speed. 50/50 split in torque. 

4Auto transfer case doubles as a limited slip dif., allowing the front wheels to spin at a different speed than the rears. Split from 5% -50% it is rumored. 

4Low, the gearing changes in the transfer case, so each turn of the wheels are accomplished with higher revs of the engine (ie: more optimal power/torque band, without forcing the wheels to spin)

Both 2x and 4x, the rear has a limited slip diff that allows the axel to lock up when one wheel looses traction, to prevent all the power (or 50% of it) going to a free spinning wheel.

What allows the front left and right to spin at different speeds? Does this change in 2x/4x/auto?  

 

I still wonder just how much gas mileage is different between auto and 4x4. Thinking about it, even if the engine power is not being used to drive the front wheels, what parts stop moving and/or causing friction? The axels don't, they are connected to the wheels. The transfer case is still doing work, and, with both front wheels off the ground, the front drive shaft still spins in 2WD. so the only friction I can think of that is reduced is the friction between the clutch plates in the transfer case. Seems like that should be fairly small....

      

[Bikemobile]

I just recently borrowed a co-worker’s company provided Tahoe for a few days. When I started driving it was in “auto.” The roads were clear and so I switched it to 2wd. When I saw him again I asked him why he had it in auto and he said he’s had it in auto since he got the vehicle, 60k miles ago..... 

 

I bet this scenario is more common than you think. 

 

I use auto to get out of my driveway and neighborhood most of the winter. Out on the main roads the pavement is clear and dry so I switch it back to 2hi. Perfect system that alot of new half ton trucks lack these days. 

 

 

[BB68]

On 11/30/2018 at 11:25 AM, aseibel said:

Well that is news to me that it is a variable system. I thought it was all or nothing. I just found another post on an older thread where somebody stated that in 4auto, the clutches are loaded to 10% when no slippage occurs (who know how much power is transferred to the front at 10% of clutch grab, you said 5%, that could be true). That's fine, but again, this is why I won't use it on clear pavement, even if wet. I don't want to be in 10% 4WD all the time. If the roads are so bad that I need 4 powered wheels, I put it in 4HI. Wet pavement doesn't create enough slippage for me to drive in 4WD.

You were correct to start with. There is only one clutch engaging the front axle in the transfer case, rear is connected with a steel collar. Try slipping that clutch 10% for a few hundred miles. When these "auto" systems first came out a shopping trip to Des Moines and back (180 miles) was enough to take them out. Its no different than riding/slipping a clutch on a manual trans. 

 

One of the reasons I buy new vehicles....

[NWI Denali]

7 hours ago, Bikemobile said:

I just recently borrowed a co-worker’s company provided Tahoe for a few days. When I started driving it was in “auto.” The roads were clear and so I switched it to 2wd. When I saw him again I asked him why he had it in auto and he said he’s had it in auto since he got the vehicle, 60k miles ago..... 

 

I bet this scenario is more common than you think. 

Man I would love to open up that transfer case and compare the clutch against a new one.

[truckguy82]

7 hours ago, BB68 said:

You were correct to start with. There is only one clutch engaging the front axle in the transfer case, rear is connected with a steel collar. Try slipping that clutch 10% for a few hundred miles. When these "auto" systems first came out a shopping trip to Des Moines and back (180 miles) was enough to take them out. Its no different than riding/slipping a clutch on a manual trans. 

 

One of the reasons I buy new vehicles....

Is that why tons of people leave them engaged for 100k+ miles and don’t have any issues?

 

Also it’s 5%, not 10%

[j-ten-ner]

7 hours ago, BB68 said:

You were correct to start with. There is only one clutch engaging the front axle in the transfer case, rear is connected with a steel collar. Try slipping that clutch 10% for a few hundred miles. When these "auto" systems first came out a shopping trip to Des Moines and back (180 miles) was enough to take them out. Its no different than riding/slipping a clutch on a manual trans. 

 

One of the reasons I buy new vehicles....

 

43 minutes ago, NWI Denali said:

Man I would love to open up that transfer case and compare the clutch against a new one.

The clutch will look fine. There 's no slippage as long as the front and rear axle are spinning with the same rpm.

 

so long

j-ten-ner

Edited December 4, 2018 by j-ten-ner

['17 Sierra]

I use AWD only when I see random ice or snow on the roads. When you hit dry or even slightly wet pavement and you're in 4WD, you can get driveline wind up once you turn even slightly. You could even have a tire that is worn down more than the other three and that will also tighten up the driveline.

When it's said that the front axles engage in AWD, that is actually incorrect. The axles are always engaged; always turning with the wheels. We don't have locking/unlocking hubs anymore. It's the front drive shaft that starts turning when AWD (and 4WD) is selected. That synchronizes the front drive system in the transfer case to the rear drive but is still disengaged until A) clutch pack engages due to tire speed differentiation or B) 4WD is manually selected.

Edited December 5, 2018 by '17 Sierra

[BB68]

On 12/4/2018 at 6:51 AM, j-ten-ner said:

 

The clutch will look fine. There 's no slippage as long as the front and rear axle are spinning with the same rpm.

 

so long

j-ten-ner

I guess I left out that the roads that day were 50/50 ice covered. I dont know the wheel speed differential, but it was constantly shifting the front end in and out took out the t-case clutch. So either programming allowed to much speed differential, or didnt engage it fast enough, or didnt leave it engaged long enough -  I really dont care I just dont use auto when the roads are bad.

 

I know another guy that had me replace his at 80k as the clutch was gone. HIs t-case oil was black as coal. I spoke with my local dealer and they kept clutch kits on hand.

 

Im sure on just wet roads a person would never have problem. Their will always be certain situations where clutches will be burnt out of the norm.

[BlackZ71SLT]

Heres a good video that shows how the older gm AWD/4WD transfer case worked. I assume our's are much of the same.

 

 

[diyer2]

Great video.

Thanks.

['17 Sierra]

Nice Video. I also believe that when engaging AWD mode, the front differential makes a shift to engage the front driveshaft because in 2WD, the front driveshaft does not turn.

[SteveC]

When I check my display it seems that when in Auto mode, I'm always in 4x4. Whenever I check to see, even on a dry road, it's in 4x4 mode. I just run in 2WD unless we get snow or it's icy.

['17 Sierra]

2 hours ago, SteveC said:

When I check my display it seems that when in Auto mode, I'm always in 4x4. Whenever I check to see, even on a dry road, it's in 4x4 mode. I just run in 2WD unless we get snow or it's icy.

When you say display you mean "Off Road" mode on your DIC? It won't change when AWD becomes active (kicks in) but when AWD has been selected, it should say AWD on the OffRoad display.

[SteveC]

Right