Difference between 2H and 4H AUTO on dry roads?

[Zandcwhite]

Oh, I know how impractical the 392 is.

It's not that this is a deal-breaker but this is all new to me and I want to know what you guys know and I am here right now and y'all are so helpful and I am analytical to a fault...

My delving curiosity always makes me look overly-concerned. LOL.




You're the second person who has said this. I can check into it to educate myself but my first thought is how I would shift into 2H given the pattern portrayed on the shift knob itself:

I don't own a 392, but the tazer uses the cruise control buttons for its features like engaging just the front locker, disconnecting the sway bar in 2wd, etc so I'm sure you'd leave it in 4A and bypass it via the cruise control buttons.

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[sunset]

I don't own a 392, but the tazer uses the cruise control buttons for its features like engaging just the front locker, disconnecting the sway bar in 2wd, etc so I'm sure you'd leave it in 4A and bypass it via the cruise control buttons.

I just messaged the Tazer folks to confirm this as my amateur eyes did not see this in their documentation. I'll post here what they say about Tazer converting 4H-AUTO to a true 2H.

Not that I would do it, but I'd like learning about it. Thanks.

 

[HungryHound]

On dry paved roads there is pretty much no difference between 2H and 4Auto. 4Auto is 2H, unless the rear wheels lose traction and 4WD engages. However to have 4WD at the ready like that there is a small hit to MPG due to more drivetrain parts being engaged.

I'm pretty sure the reason why there is no 2H on the 392 is because the engine is strong enough to break the rear wheels free. Only the old-timers know what it's like to drive a vehicle like that (hooray '60's muscle cars!). There are good and very, very bad times to spin your rear tires.

So to avoid litigation from people who don't know how to drive a powerful vehicle, there is only 4Auto so the front end engages when the engine breaks the rear tires loose.

On both the 392 and 4xe, there's enough torque to twist the splines out of the Dana rear axle. To avoid this, the 392 is always in 4HA and the torque is reduced on the 4xe unless it's in 4HA in 1st and 2nd gear. Sucks if you wanna do burnouts.

 

[Yawnie'sPapa]

On dry paved roads there is pretty much no difference between 2H and 4Auto. 4Auto is 2H, unless the rear wheels lose traction and 4WD engages. However to have 4WD at the ready like that there is a small hit to MPG due to more drivetrain parts being engaged.

I'm pretty sure the reason why there is no 2H on the 392 is because the engine is strong enough to break the rear wheels free. Only the old-timers know what it's like to drive a vehicle like that (hooray '60's muscle cars!). There are good and very, very bad times to spin your rear tires.

So to avoid litigation from people who don't know how to drive a powerful vehicle, there is only 4Auto so the front end engages when the engine breaks the rear tires loose.

The 4xe has SelecTrac and limits torque in 2H to avoid traction issues - the 4xe has more torque and HP than a 5.7

As far as 4H auto - it's engaged when you are stopped and as you apply throttle - it slowly disengages or relaxes the clutch pressure as vehicle speed increases. So you take off in 4 wheel drive regardless if it's in 4HA

Going back to the 1980s, AMC introduced the Eagle with full time 4 wheel drive. No FAD, no 2 wheel drive mode. In 1981 model year in an attempt to meet fleet CAFE standards, the introduced SelectDrive - a FAD and a switchable 2wd or 4wd transfer case.
A few years later they found that the extra cost of the FAD and switching and vacuum motors made only fractions of an MPG difference, and went back to full time.
So for that vehicle with those engines in that era, it made little difference.
Wear and tear? Not really. They did recommend for those of us with the SelectDrive to run in 4 wheel drive mode now and then to keep the carrier and spider gears lubricated as they spun inside the carrier while in 2 wheel drive with FAD disconnected but the ring gear was not splashing oil up to lubricate them as in 4 wheel drive.

 

[sunset]

I just messaged the Tazer folks to confirm this as my amateur eyes did not see this in their documentation. I'll post here what they say about Tazer converting 4H-AUTO to a true 2H.

Not that I would do it, but I'd like learning about it. Thanks.

Z Automotive was quick to respond.

My question to them, followed by their reply:

Q:

Hi,
As you may know, the factory Rubicon 392 transfer case does not have 2H but does have 4H-AUTO. The guys on a Jeep forum are telling me that the Tazer can change the 4H-AUTO to act as a true 2H if desired.
Is that true? I didn't see it mentioned in your documentation. Where would I look? I am brand new at this stuff.
Thank you.
Jay

A:

Hi Jay,

That would be the "Force RWD" setting, which does just that.

Thanks,
Ian S
Z Automotive


Ha, I had been looking for "2H". Pretty cool option. That Tazer is something, isn't it? I've added it to my growing list of Accessories To Ponder When I Buy My Jeep.

Is performing that option really a harmless thing to do, except maybe to the Warranty?

 

[Dyolfknip74]

I don't live where it snows and if/when I head to the mountains looking to go snow wheeling I know I want full time 4wd, thus I've never owned a wrangler with the 4A. Extra complexity and wear parts that I have no need for.

Honestly, mine was bought off the floor and I would have never gotten it. A Canadian learns pretty quickly how to drive 4wd in the winter without it.

 

[Ratbert]

In my opinion/experience if you drive aggressively in a non-392 JLU on big tires your mileage is going to be pretty close to that of the 392 anyway. Our 2019 on 38s with the 2.0t got mid 16 mpgs on average and closer to 15 at freeway speeds. This mileage didn't change much when we regeared from 4.10s to 5.38s. That brick into the wind thing needs all the power it can get. Out 2022 with 35s and a 3.5" lift averaged low 14 mpgs and would drop into the 13s at freeway speed with 4.56 gears. Now with 39s is averaging high 13s. Now granted we do drive on the faster/more aggressive side, but with the 392s power to spare I'm guessing you'd actually be easier on the skinny pedal. Most guys seem to average in the 12-14mpg range on their 392s which is right in line with our 3.6L mpg. If you're concerned with mpgs I would not but a wrangler period. Can you hypermile an ecodiesel into the high 20s driving 10mph under the speed limit and drafting semi's everywhere you go? Sure. My buddy with an ecodiesel on 37s who drives like I do averages 14-15mpg on the way up to Tahoe. I'll just but gas and get to the wheeling destination asap, but to each his own.

That's really weird. I consistently get 19 to 21 in my significantly over 6k pound EcoDiesel with 37s, 3" lift, 4.63 gears and a massive exoskeleton roof rack with awning. That average, of course, includes trips across Utah where I set the cruise at 88 mph.

 

[txj2go]

The fad doesn't control when 4wd engages if 2wd is slipping. That's a function of the tcase.

The FAD controls the pieces within the front differential. If the FAD is doing its thing and disconnecting the axle shaft on one side, the ring and pinion don't turn and the front driveshaft doesn't turn. The axle shafts turn and the spider gears spin the stub axle shaft even faster. If the FAD has engaged the 2 halves of the axle to behave as 1 then the ring and pinion and the front driveshaft turn. Apparently it takes more poser to spin the ring and pinion than to spin the spider gears, probably due to the ring and pinion spinning in the heavier gear oil.

I can't imagine that spinning the ring and pinion would change the fuel economy very much but if you are trying to get corporate fuel economy up a few fractions then every little bit helps. Checking fuel economy in order to do comparisons is very tricky. It requires careful methodology, matching conditions, etc. and do this over a longer period of time. I've done this with my other car over similar conditions and 5 tankfulls. I've played with different things on my Jeep on the highway and it is hard to draw conclusions. Even using the instantaneous fuel usage mode on the dash I would drive at constant throttle on flat ground and it seemed to jump around a lot.

 

[Zandcwhite]

That's really weird. I consistently get 19 to 21 in my significantly over 6k pound EcoDiesel with 37s, 3" lift, 4.63 gears and a massive exoskeleton roof rack with awning. That average, of course, includes trips across Utah where I set the cruise at 88 mph.

The trip up to Tahoe includes 8k feet of elevation gain and almost no downhill. Kind of wrost case scenario for fuel economy. But in those worst case conditions I find that pretty much all lifted JLs get the same shit fuel economy.

 

[Reinen]

As far as 4H auto - it's engaged when you are stopped and as you apply throttle - it slowly disengages or relaxes the clutch pressure as vehicle speed increases. So you take off in 4 wheel drive regardless if it's in 4HA

I doubt that. There is no logical reason for it. If the rear tires aren't slipping then engaging 4WD does no good and only causes wear of tires and xfer case clutch when binding happens.

It's rear tire slip which engages 4WD and it happens fast enough to be imperceptible.

It sounds like you're mixing up 4Auto with AWD which are two different things.

 

[Ratbert]

Don't forget that auto 4wd has CV joints. Just another delicate suspension component that can get damaged on the trail. All it takes is a stick at the wrong location.

 

[HungryHound]

Z Automotive was quick to respond.

My question to them, followed by their reply:

Q:

Hi,
As you may know, the factory Rubicon 392 transfer case does not have 2H but does have 4H-AUTO. The guys on a Jeep forum are telling me that the Tazer can change the 4H-AUTO to act as a true 2H if desired.
Is that true? I didn't see it mentioned in your documentation. Where would I look? I am brand new at this stuff.
Thank you.
Jay

A:

Hi Jay,

That would be the "Force RWD" setting, which does just that.

Thanks,
Ian S
Z Automotive


Ha, I had been looking for "2H". Pretty cool option. That Tazer is something, isn't it? I've added it to my growing list of Accessories To Ponder When I Buy My Jeep.

Is performing that option really a harmless thing to do, except maybe to the Warranty?

Dana rear axle has left the chat....

 

[Wbino]

Can someone tell me why my old RWD Benz will flash an indicator with rear wheel slippage but my 2021 jeep does not?
It would be nice to know when the SelecTrac engages.

 

[jav_eee]

Why does the 392 not have/need 2H?

because FCA does not want you having THAT much fun

 

[txj2go]

because FCA does not want you having THAT much fun

I always wondered what the thinking was for giving the Sahara a different transfer case.

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