4WD Auto questions

[Cutterone]

Please correct me if I'm wrong, but in these Wranglers with 4Auto, they have CVs up front, so could you in theory leave it in 4Hi (not $Auto) on dry roads and not get binding as you do with U-joints?

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[TheRaven]

The “full time” TCase is able to vary clutch engagement in 4 auto
From what I’ve read.

I do see why you might think that...as here's how it's officially explained:

"The 4WD Auto option is part of the the new Selec-Trac III Full-Time Four-Wheel-Drive System. It includes a 2.72:1 low range. This two-speed, active full-time system includes a 4H AUTO mode that requires no driver input for any driving conditions. In 4H AUTO, 4WD is automatically engaged and disengaged as needed to maintain traction."

However to me, that last line means it works like every "Auto 4WD" system i've ever known, and simply engages and disengages the front axle as needed.

 

[GATORB8]

I do see why you might think that...as here's how it's officially explained:

"The 4WD Auto option is part of the the new Selec-Trac III Full-Time Four-Wheel-Drive System. It includes a 2.72:1 low range. This two-speed, active full-time system includes a 4H AUTO mode that requires no driver input for any driving conditions. In 4H AUTO, 4WD is automatically engaged and disengaged as needed to maintain traction."

However to me, that last line means it works like every "Auto 4WD" system i've ever known, and simply engages and disengages the front axle as needed.

Not from that, I’ve read a technical operation description on the case. I remember being surprised.

 

[TheRaven]

Not from that, I’ve read a technical operation description on the case. I remember being surprised.

How confident are you in the correct-ness of that description? Because FCA had a video on Youtube explaining how BLD works a couple years ago, and it was wrong. It's long gone now, probably the product of whatever marketing agency they tasked with the project not bothering to sufficiently research the subject.

I'm certainly not trying to say that the marketing blurb I posted should be trusted over whatever it was that you saw...i'm just saying that since every other Auto 4WD system ever has operated by simply connecting and dis-connecting the front axle, and the marketing blurb that I posted seems to be a very wordy and marketing-speak way of saying that same thing, i'm inclined to believe that that's how it works.

 

[GATORB8]

How confident are you in the correct-ness of that description? Because FCA had a video on Youtube explaining how BLD works a couple years ago, and it was wrong. It's long gone now, probably the product of whatever marketing agency they tasked with the project not bothering to sufficiently research the subject.

I'm certainly not trying to say that the marketing blurb I posted should be trusted over whatever it was that you saw...i'm just saying that since every other Auto 4WD system ever has operated by simply connecting and dis-connecting the front axle, and the marketing blurb that I posted seems to be a very wordy and marketing-speak way of saying that same thing, i'm inclined to believe that that's how it works.

Ha, I understand the scepticism. I’ll track it down when I get a chance.

 

[jac04]

Please correct me if I'm wrong, but in these Wranglers with 4Auto, they have CVs up front, so could you in theory leave it in 4Hi (not $Auto) on dry roads and not get binding as you do with U-joints?

In 4H part-time (not 4H Auto) you will still get drivetrain binding when going around corners on dry roads because the front & rear driveshafts are forced to turn at the same speed. The CVs on the front axle shafts do nothing to prevent this.

 

[jac04]

Auto 4WD is not the same as AWD. AWD is an "always-on" system...it vectors power and clutches are allowed to slip but there is always a mechanical connection with AWD. Auto 4WD can completely disconnect the front axle. It will only re-connect when rear wheel slip is detected and when it does, it's not a slip, it's full-on...requiring the front diff to vector torque between the front wheels.

Because of the way Auto 4WD disconnects and reconnects the front axle, it's not advisable to leave it on all the time...that would subject the front axles and diff to additional unnecessary stress. Will it actually make a difference over, say, 100k miles? Dunno. But it could...so it makes sense to leave it in 2H on completely dry roads and only use Auto when the roads are at least a little wet.

The generic term 'AWD' means different things to different people. But, it does not always mean that there is a mechanical connection between the front & rear axles. In the quest for better fuel economy, modern AWD systems are able to de-couple an axle when it is determined appropriate to do so.

For example, Audi's "Quattro with Ultra Technology" is still considered an AWD system. However, when the computer decides that front-wheel drive is sufficient, it electromechanically disengages a clutch positioned right behind the transmission. A clutch in the rear differential also decouples to stop the prop shaft from rotating in order to reduce friction and losses. It is only when the predictive software specifies that rear drive is needed that in re-engages the rear axle.

Selec-Trac is not an on/off system for engaging the front axle. It has the ability to slip the clutch to apply varying power to the front axle.

 

[TheRaven]

For example, Audi's "Quattro with Ultra Technology" is still considered an AWD system. However, when the computer decides that front-wheel drive is sufficient, it electromechanically disengages a clutch positioned right behind the transmission. A clutch in the rear differential also decouples to stop the prop shaft from rotating in order to reduce friction and losses. It is only when the predictive software specifies that rear drive is needed that in re-engages the rear axle.

Yes thats Haldex AWD. Cadillac uses it too. It has clutches on each end of each driveshaft...however they do not completely disengage...which is why it qualifies as "AWD" and not "Auto 4WD".

https://en.wikipedia.org/wiki/Haldex_Traction

Also I want to clarify - I was not saying that the Auto 4WD transfer case cannot slip AT ALL. I was saying that it doesn't slip to "disengage" the front axle. It actually disengages it, unlike, for example, Haldex, which is never 100% disengaged. I just added an edit note to my first post to clarify.

The question here is whether or not the Jeep Auto 4WD system is actually different from the classic Auto 4WD concept.

 

[GATORB8]

Yes thats Haldex AWD. Cadillac uses it too. It has clutches on each end of each driveshaft...however they do not completely disengage...which is why it qualifies as "AWD" and not "Auto 4WD".

https://en.wikipedia.org/wiki/Haldex_Traction

Also I want to clarify - I was not saying that the Auto 4WD transfer case cannot slip AT ALL. I was saying that it doesn't slip to "disengage" the front axle. It actually disengages it, unlike, for example, Haldex, which is never 100% disengaged. The question here is whether or not the Jeep Auto 4WD system is actually different from the classic Auto 4WD concept.

So I spent a minute looking, and although I thought I had seen it in a white paper, I couldn't find that again. There is confirmation on the 4xe forum with live data identifying variable clutch pressure in 4Auto. The 4xe has the same MP3022(OR) as other JLs with the optional full time case, and the 392, although 2wd is locked out on the 392.

If it will let me link it:
https://www.4xeforums.com/threads/mp3022-transfer-case-operation-and-monitoring-with-j-scan.2602/

 

[jac04]

Yes thats Haldex AWD. Cadillac uses it too. It has clutches on each end of each driveshaft...however they do not completely disengage...which is why it qualifies as "AWD" and not "Auto 4WD".

https://en.wikipedia.org/wiki/Haldex_Traction

Also I want to clarify - I was not saying that the Auto 4WD transfer case cannot slip AT ALL. I was saying that it doesn't slip to "disengage" the front axle. It actually disengages it, unlike, for example, Haldex, which is never 100% disengaged. I just added an edit note to my first post to clarify.

The question here is whether or not the Jeep Auto 4WD system is actually different from the classic Auto 4WD concept.

Ultra Quattro is not Haldex.

Selec-Trac does indeed slip. Even standing still in 4H Auto, Selec-Trac applies torque to the front axle - it is not actually disengaged.

 

[Chief Toby]

This drive train is called "Selec-Trac", and I have it in my High Tide. 4wd-Auto is great in the snow! To shift from 2WD to 4WD-auto just pull the transfer case lever back. You will see this on your dash lights. This setting sutomatically shifts power to the front wheels when needed and IMHO is the best 4WD system I've ever seen, especially in snow. Now, if you want full time 4WD basically 50/50 front and rear then just slide the transfer case lever to the right from the 4WD-auto mode and you will see this on the dash lights. Still in high range. I don't think this is as good as the auto mode on snow and ice. I do not go to 4WD auto unless I think I need it. I do not leave it in 4WD-auto all the time. I very seldom if ever use the full time 4WD high range. Low range is for really tough off road stuff and pulling out tree stumps, etc. I notice a clear difference in the transfer case and tranny sounds in 4WD-auto and the idle is rougher so I know that keeping that transfer case engaged with 4WD-auto is causing some extra parts to move. Just saying.

 

[TheRaven]

Ultra Quattro is not Haldex.

I'm not getting into a debate about Audi's AWD systems. In the context of this conversation, there is no effective difference.

Even standing still in 4H Auto, Selec-Trac applies torque to the front axle - it is not actually disengaged.

Can you provide a source for this claim? The only source I have seen so far seems to say it is in fact disengaged, just like every other Auto 4WD system ever.

The really confounding part about this is - if it is true that the front axle is always engaged in Auto 4WD, then it's actually AWD...so why would FCA not just call it that?

 

[Chief Toby]

I'm not getting into a debate about Audi's AWD systems. In the context of this conversation, there is no effective difference.



Can you provide a source for this claim? The only source I have seen so far seems to say it is in fact disengaged, just like every other Auto 4WD system ever.

The really confounding part about this is - if it is true that the front axle is always engaged in Auto 4WD, then it's actually AWD...so why would FCA not just call it that?

It is just a question of semantics. As in would you rather be called Richard, or Rick, or Dick?

 

[jac04]

^^^ Exactly.

 

[jac04]

...The only source I have seen so far seems to say it is in fact disengaged, just like every other Auto 4WD system ever.

Can you provide a source for this claim?

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