2021 Yukon Denali - How best and when to use 4wd low / how does it work

[tagexpcom]

Hey Folks

I've had my 2021 Yukon Denali 2+ years now and have tried 4wd low twice - both with unexpected/not-useful results. This also generated a few comments but it's clear I don't understand how it works.

I'm coming from Cadillac SRX 2004 with AWD - it just works all the time. I'm comfortable driving in foot of snow or county roads or even bouncy off-road moderate 'trails' but not rock climbing. 4wd 'hi' or awd been perfectly adequate in all these scenarios.

I've been driving for 56years in all types of conditions, have changed a engine and rebuilt several transmissions on my 66 Mustang back in the day. So not an expert but not totally devoid of basic mechanics.



Back to GMC Yukon 4wd low....
First time I tried it was backing a trailer into a camp spot in the snow. The 2wd was slipping, so tried 4wd low, but... I got horrible sounds (kind of tight steering wheel turns) and just went 4wd Hi.

Second time was recently - here's my post in another thread....

For what it's worth, I've had 3 4WD Tahoes and 1 4wd F150 - all of them acted this way in 4 low. 4 low is meant for low traction situations where you need max torque. Be careful using it in situations with high traction as you can damage the system.

Yep, that's what I'm finding out. I pull a 5500lb trailer over my front lawn out to the street. I have a 'grass driveway' with 9" stone and those plastic grids with 1-2" dirt/grass on top. Slight uphill to the street.

Normally I just 2WD over the grass to the street but due to tons of rain, the left rear tire slipped in the grass->mud and I wasn't moving. Put in 4WD-Low and slipping (left rear) tire spun a bit -> 'clunky engage' -> more clunks -> general spinning / could not move forward and tore up the grass a bit more.

Put in 4WD-Hi and pulled right across the lawn to the street.
--------------------------------------------------

To me (the layman) it seems like 4wd low would be perfect for maximum traction at 2mph on muddy lawn but 4wd hi was the better choice.

I'm not posting to complain, just interested in....
1) Why all the noise on tight turns such as 3 point turn-around in snow - it sounds horrible! and someone posted that I could damage the vehicle.
2) Why is 4wd hi better for moderate mud that 4wd low? - why was 4wd low so clunky and actually made things worse?
3) What am I missing in my thinking....

Appreciate getting educated / helpful comments. Search of 4wd low youtubes were not helpful. We're doing more and more boondock camping and want to know how to best use the rig's features if I get in a tight spot

 

[DoubleDingo]

Did you put the transmission in neutral before trying 4Lo? If not, try that. Also, sometimes, after engaging 4Hi or 4Lo, it's best to try reverse first, and then forward. Shouldn't have to, but if gear teeth aren't meshing, that should get them meshed. The clunking is most likely the teeth trying to mesh. Tight turns in 4WD is always ha rd on the drivetrain, don't do tight turns.

 

[DoubleDingo]

When I got the 2005 Tahoe, I couldn't try the 4x4 system until weeks later. 4Lo wouldn't engage unless the transmission was in neutral, and I think it states that in the owner's manual, but it shifted into 4Lo just fine with the transmission in neutral, and moved the vehicle just fine, albeit I wasn't towing anything. 4Hi I shifted into no problem at a slow roll. Disengaging 4Lo or 4Hi, I always shift the transmission into neutral, select 2Hi, then shift into reverse and go maybe 3 to 5 feet, then go forward. That releases the tension in the drivetrain and makes sure everything is freed up on the front axle and transfer case.

 

[Marky Dissod]

You may never need 4Lo. I've literally no experience with 4Lo, so I'll defer to those with experience.

All-Wheel Drive (most usually do not actually drive all the wheels,with few exceptions - Porsche? Subaru?) is a euphemism for 'don't think about it'.
If you help keep AWD out of serious trouble it'll have your back when needed.

4-Wheel Drive (Hi or Lo), so far as I know, always drives all four wheels. Most 4WD systems are not meant for use on well-paved roads with good grip.
In theory it's ok in a straight line, because there's no difference between left and right, but as the difference between left and right grows
(unless it's raining / snowing / icy / dirty / muddy / somehow a bit slippy)
the tighter the turn, the quicker the front differential's guts are wearing out.
It's more capable than AWD, but it can also get schmuckdiots into more trouble than it can get them out of.

How I think of it:
if road conditions permit, I'll enable part-time 4WD
if road conditions DEMAND I'll enable full-time 4WD (4Hi)

Tight turns in 4WD is always
ha rd on
the drivetrain, don't do tight turns.

So, anything that might make Beavis and/or Butt-Head laugh? Anything?

 

[Doubeleive]

Hey Folks

I've had my 2021 Yukon Denali 2+ years now and have tried 4wd low twice - both with unexpected/not-useful results. This also generated a few comments but it's clear I don't understand how it works.

I'm coming from Cadillac SRX 2004 with AWD - it just works all the time. I'm comfortable driving in foot of snow or county roads or even bouncy off-road moderate 'trails' but not rock climbing. 4wd 'hi' or awd been perfectly adequate in all these scenarios.

I've been driving for 56years in all types of conditions, have changed a engine and rebuilt several transmissions on my 66 Mustang back in the day. So not an expert but not totally devoid of basic mechanics.



Back to GMC Yukon 4wd low....
First time I tried it was backing a trailer into a camp spot in the snow. The 2wd was slipping, so tried 4wd low, but... I got horrible sounds (kind of tight steering wheel turns) and just went 4wd Hi.

Second time was recently - here's my post in another thread....

Yep, that's what I'm finding out. I pull a 5500lb trailer over my front lawn out to the street. I have a 'grass driveway' with 9" stone and those plastic grids with 1-2" dirt/grass on top. Slight uphill to the street.
View attachment 473172


Normally I just 2WD over the grass to the street but due to tons of rain, the left rear tire slipped in the grass->mud and I wasn't moving. Put in 4WD-Low and slipping (left rear) tire spun a bit -> 'clunky engage' -> more clunks -> general spinning / could not move forward and tore up the grass a bit more.

Put in 4WD-Hi and pulled right across the lawn to the street.
--------------------------------------------------

To me (the layman) it seems like 4wd low would be perfect for maximum traction at 2mph on muddy lawn but 4wd hi was the better choice.

I'm not posting to complain, just interested in....
1) Why all the noise on tight turns such as 3 point turn-around in snow - it sounds horrible! and someone posted that I could damage the vehicle.
2) Why is 4wd hi better for moderate mud that 4wd low? - why was 4wd low so clunky and actually made things worse?
3) What am I missing in my thinking....

Appreciate getting educated / helpful comments. Search of 4wd low youtubes were not helpful. We're doing more and more boondock camping and want to know how to best use the rig's features if I get in a tight spot

Based on your picture I would say the issue is simply traction, not the 4wd system
if you just need to get moving off the grass or from a slippery spot then a couple sets of traction mat's or recovery boards are easy to just throw down, get out of the spot, throw them back in the truck and keep on moving. a lot easier and a lot less work than cables or chains.

 

[thefrey]

I would assume that the clunk you hear is some form of the rear differential locking. It seems to be a little different with the independent rear suspension vs. solid axle, but I believe that it works based on wheel slip. If the computer catches one side slipping, it'll lock the differential to get traction to the opposite wheel.

For most people, 4lo is going to be if they're doing more off-roading type applications, or if you're stuck. Mud, deeper snow, etc... Very steep hills and rocks are good for 4lo as well. It just basically gives you as much torque as possible. I would equate it to the lowest gear on your bike. Gets you the most torque but you don't get anywhere very quick. Very little throttle will be your friend in 4lo from my experience.

 

[Bigburb3500]

Auto 4wd: Does not lock the center differential and I think uses a clutch system in our trucks. This allows for the “set it and forget it.” This will allow the truck to be driven on any surface and likely the best one to use in this situation.
4wd Hi: True 50/50 power split. This gives you a true 2wd vehicle. Back left and front right. Turning tight will create drivetrain binding on a hard/high traction surfaces casing a “cluck” or “hop.”
4wd Low: I believe most of these trucks are a 4-8x multiplier of 4wd Hi. It will bind and hop faster putting more stress on the system. It’s really for steep grade and high torque required situations.

Lots of ppl chimed in on your last post because 4wd Hi was probably the “better” choice. I use 4wd low SOMETIMES on the beach but primarily when I’m at an offroad park going up a 30% grade, off-camber, and need extra control.

Hope this helps!

Edit: Trucks with the G80 or a locker is a 3x4 where the back two wheels will receive equal power along with 1 front tire. Only a triple-locked truck is “technically” 4x4 where all 4 wheels receive equal power.

 

[tagexpcom]

Based on your picture I would say the issue is simply traction, not the 4wd system

As far as driving forward - yes.

if you just need to get moving off the grass or from a slippery spot then a couple sets of traction mat's or recovery boards are easy to just throw down, get out of the spot, throw them back in the truck and keep on moving. a lot easier and a lot less work than cables or chains.

My question is not so much traction but how the 4wd lo works. I was surprised that 4wd lo failed to help me move forward whereas the 4wd hi worked smoothly.

I was stuck due to left rear spinning in 2wd. Backed up a bit but when it hit the same spot, the left rear spun and I was stalled again. Choose 4wd lo, and gave it light gas. The slipping wheel (left rear) spun/then clunked in a 'rough manner' and then one or more other wheel(s) spun (did not get traction) and I was still stuck but it tore up more yard. The mechanical operation was rough - not smooth.

And as others describe, 4wd lo is max torque but this was not a case of needing extra torque - and maybe the extra torque actually caused my failure to move as the power to the wheels was so strong on even a little bit of gas it just spun in the grass?

Whereas when I switched 4wd hi, backed up a yard and moved forward smoothly out to the street. This is what I was expecting from 4wd lo.

I'm trying to learn more about this.... maybe it's as someone suggested the 'clunking' was lack of gear engagement and I should have done reverse a bit and then it might have moved forward smoothly? Someone else seemed to suggest I should have done L1 manually?

Functionally - 4wd lo failed but 4wd hi worked in this case and I'm just trying to understand why as I suspect I used it incorrectly.

 

[blanchard7684]

Normally I just 2WD over the grass to the street but due to tons of rain, the left rear tire slipped in the grass->mud and I wasn't moving. Put in 4WD-Low and slipping (left rear) tire spun a bit -> 'clunky engage' -> more clunks -> general spinning / could not move forward and tore up the grass a bit more.

When you engaged 4 Lo, did you feel a lurch forward when going into drive?

If not then you weren’t in 4Lo and were stuck in between modes.

A 5500 lb hanging on the back likely complicated the shift in the transfer case.

To get into 4Lo: stop totally and completely. Engage neutral. Then hit 4 lo. Let system engage ( you will feel and hear clunking). Then engage drive. Then release brake pedal and move forward or rearward. There should be a das indicator and some messages about traction control being turned off as well.

When engaging drive you will feel a big surge forward.

Some systems need some coaxing while engaging the low gear in transfer case , it helps to take foot off brake. This is in in level ground.

With suv at an incline and with a large mass in back of it it may take a couple of attempts.

So you go into 4 hi and move forward a bit . If you can’t then go rearward. Try again.

You are trying to get the internal synchronizer to engage the teeth in the low gear.

You might check your fluid level in the transfer case as well.

Edit: the front differential has an ADD actuator that engages another set of synchronizers to lock the from axles together. If you were already in 4 hi this process already took place. In 4 lo engagement the transfer case has to select the low gear.

 

[tagexpcom]

When you engaged 4 Lo, did you feel a lurch forward when going into drive?

I hit 4lo button with rig idling and still - there was not lurch or noise at that point. When I gave it a bit of gas I got the spin/clunk - e.g. I was imagining the spinning wheel caused the 4lo to kick in - e.g. the clunk.

But may I wasn't properly in 4lo? This is what I'm trying to better understand - how should it work. Is the spin/engage normal? In 4hi it just moved forward - no spin/engage.

If not then you weren’t in 4Lo and were stuck in between modes.
A 5500 lb hanging on the back likely complicated the shift in the transfer case.

These 2 together may explain things. How does one ensure you're in 4Lo? Is the 'put in reverse, then forward' method recommended?

 

[blanchard7684]

I hit 4lo button with rig idling and still. This might be the spin/clunk I'm describing.


These 2 together may explain things. How does one ensure you're in 4Lo? Is the 'put in reverse, then forward' method recommended?

See edit above.

Need to be in neutral with vehicle stopped. Then engage 4Lo. Then engage drive .

You should get a dash indicator and some messages related to traction control.

Nothing wrong with “practicing” in your drive way. It is good maintenance to engage this system once or twice a month.

I just engage 4 lo , roll forward in drive about 100 ft straight line, reverse, then engage 4 hi, repeat, then 2hi.

 

[blanchard7684]

This video will explain the two speed transfer case in gory detail.

 

[Doubeleive]

As far as driving forward - yes.


My question is not so much traction but how the 4wd lo works. I was surprised that 4wd lo failed to help me move forward whereas the 4wd hi worked smoothly.

I was stuck due to left rear spinning in 2wd. Backed up a bit but when it hit the same spot, the left rear spun and I was stalled again. Choose 4wd lo, and gave it light gas. The slipping wheel (left rear) spun/then clunked in a 'rough manner' and then one or more other wheel(s) spun (did not get traction) and I was still stuck but it tore up more yard. The mechanical operation was rough - not smooth.

And as others describe, 4wd lo is max torque but this was not a case of needing extra torque - and maybe the extra torque actually caused my failure to move as the power to the wheels was so strong on even a little bit of gas it just spun in the grass?

Whereas when I switched 4wd hi, backed up a yard and moved forward smoothly out to the street. This is what I was expecting from 4wd lo.

I'm trying to learn more about this.... maybe it's as someone suggested the 'clunking' was lack of gear engagement and I should have done reverse a bit and then it might have moved forward smoothly? Someone else seemed to suggest I should have done L1 manually?

Functionally - 4wd lo failed but 4wd hi worked in this case and I'm just trying to understand why as I suspect I used it incorrectly.

if you have no traction it doesn't matter what mode you are in, all you can do at that point is create traction, if a wheel is spinning and hits approx 15mph, the g80 should kick in and then apply traction to the other wheel.
one thing you could try at that point is to pull the abs fuse, it may or may not help depending on wheel spin
or apply gas and brake at the same time, this can help make the g80 kick in
noises and clunks are not entirely abnormal when switching modes
in the case of no traction on wet grass 4lo serves no purpose, you need wheels that are not spinning to spin it is a inherent problem of having a open diff..
seems "tearing up the grass" was more of a issue than anything, in that case perhaps it is time to put in gravel/cement/blacktop
there are lots of videos and reviews of what the t2 4w independent suspension system can and cannot do on youtube

 

[tagexpcom]

This video will explain the two speed transfer case in gory detail.

This was really helpful - it gives me some idea of what's happening and why one needs to be idle/stopped for 4lo but not for 4hi.

Also, I appreciate the idea of just trying 4lo for short distance on pavement to see how it operates - Duh to me!

 

[Scarey]

4 low is the only way I can pull a heavy boat up a steep dirt/gravel ramp. I’ve tried many times 4 high and the wheels break loose. 4 low is amazing in steep marginal ground.

 

[DoubleDingo]

4 low is the only way I can pull a heavy boat up a steep dirt/gravel ramp. I’ve tried many times 4 high and the wheels break loose. 4 low is amazing in steep marginal ground.

^^^^^^^THIS^^^^^^^

 

[blanchard7684]

This was really helpful - it gives me some idea of what's happening and why one needs to be idle/stopped for 4lo but not for 4hi.

Also, I appreciate the idea of just trying 4lo for short distance on pavement to see how it operates - Duh to me!

Here is an even better video where the guy actually shows the operation of the transfer case.

If forgot to mention that the main reason 4 Lo is not shift on the fly is the shifting sleeve does not have a synchronizer pack on it. So the shifting process has to take place with no drivetrain movement at all. Thus neutral and vehicle stopped.

4 Hi has not gear reduction but instead splits power front to rear.

when 4Lo is engaged the shift sleeve engages a specific area on the planetary gear set that enables the gear reduction.

What isn't shown here is the front diff has the same shift fork and syncrho mechanism in it. It is called an Automatic Disconnecting Differential.

There are two shafts that are disconnected in the front diff. In 2wd they spin freely inside of their bearing housings. When 4wd is selected the ADD motor moves a shift fork into a position that locks the two front shafts together. This is the second part of the whole process of getting truck into 4wd.

Here is video showing front diff operation in 4wd process. 3:00 mark shows the ADD operation.

 

[Doubeleive]

4 low is the only way I can pull a heavy boat up a steep dirt/gravel ramp. I’ve tried many times 4 high and the wheels break loose. 4 low is amazing in steep marginal ground.

that area pictured is pretty flat, nothing like a boat ramp and he had wheel spin on wet grass, all that is needed is traction.
4lo can be useful for a incline yes, but nobody is towing there boat around in 4lo if you are then you need a bigger truck.
try pulling that boat out in a steep area with nothing but wet grass instead of gravel, not gonna happen no matter what mode you are in.
comparing wet grass to gravel is why I suggested the parking area be addressed, otherwise the same thing is going to happen every time it's wet

 

[tagexpcom]

To recap one aspect that's becoming clearer to me....

4lo gives significantly more torque to the wheels compared to 4hi with the trade-off of lower speeds. And one reason for this extra torque that under steep enough conditions (for example) 4hi (the rig) doesn't have enough torque to move the wheels/vehicle forward? where the extra 4lo torque can overcome this.

*I guess I was thinking (not very well) that 4lo gave a different kind of traction control and didn't even think about the torque aspect as I assumed the rig was 'all powerful' in any mode.

So in my front yard, minimal slope, 4hi is fine / no need for extreme torque to move forward. In fact, the extra torque + clunk/engage might have been why the wheels broke free/spun on the grass and actually caused me to fail.

 

[Doubeleive]

To recap one aspect that's becoming clearer to me....

4lo gives significantly more torque to the wheels compared to 4hi with the trade-off of lower speeds. And one reason for this extra torque that under steep enough conditions (for example) 4hi (the rig) doesn't have enough torque to move the wheels/vehicle forward? where the extra 4lo torque can overcome this.

*I guess I was thinking (not very well) that 4lo gave a different kind of traction control and didn't even think about the torque aspect as I assumed the rig was 'all powerful' in any mode.

So in my front yard, minimal slope, 4hi is fine / no need for extreme torque to move forward. In fact, the extra torque + clunk/engage might have been why the wheels broke free/spun on the grass and actually caused me to fail.

one thing that could help although not very practical in every situation is sandbags in the back (over the axle), the added weight helps with traction. If you feel like working out before going fishing lol