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NP242 - Why?

Wombat Ranger

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Joined
Oct 23, 2024
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8246
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Just came over after posting this on the old dead site. I was wondering what had happened. Haven't been on in close to 10 years. Anyway-



We all know an open differential sends power through the path of least resistance.

So why offer the powertrain the option to pick the worst of 4 traction options, instead of just 2?

If the NP242 and similar non-viscous/center-differential full time transfer cases really do simply use an open differential in the transfer case, the full time function would be less capable than using the vehicle in 2wd.


The variable that may be the answer to the question is this: Overcoming the rotating mass of the entire system spinning together in 4x4 may serve to "dampen" the drawbacks associated with typical open differential operation.


Reason for the question: I have an NP242 from an XJ, and am thinking about putting it in an AMC Eagle. I want to like the idea, but the 242 "open diff" full time does not make sense to me.





What do you think?
 
No, AWD sends power to the wheel/s with traction. Open diff does the opposite.

If the NP242 truly operates with an open differential in full time mode, you could have traction at 3 tires, one tire on ice, and the vehicle would not move. Doesn't make sense. There must be more to it.
 
They work fine for their intended purpose. I’d have no problem running one in a DD/snow car.

Just like a no203.

It still sends torque (not necessarily rotation) to 2 axles.. still better than one.
 
I swapped a 242 into our jeep for the purpose of having a pavement-friendly 4x mode (wet/slick conditions.)

Fulltime mode doesn't like an automatic locker with a manual trans. I took the locker out since I felt the backlash was abusive to the diff in the tcase.
 
Wombat Ranger said:
What do you think?
Click to expand...
That you don't understand the purpose and operation of a differential.

Wombat Ranger said:
If the NP242 truly operates with an open differential in full time mode, you could have traction at 3 tires, one tire on ice, and the vehicle would not move. Doesn't make sense. There must be more to it.
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Same could be said for any open diff.
Why bother having open diffs in any vehicle then?
 
I think none of the open transfer cases work like you think. I've never heard of any of the NP transfer cases only spinning one tire. In eagles or jeeps. Not even the NP 128 which is an open differential.
 
Did anyone ever mashed a lunch box style, or alike, into one of those thing? I often think this will be the bomb if can be made to work. Even better with a torsen/truetrac limit slip traction aid unit (transmit power to whichever outputs needs it most).

same goes with dropping one in a transaxle for a mid engine buggy rather than welding it locked.
 
Its not 100% on or off like your thinking. Keep in mind the differentiation at the t-case is far less than at the axle differential, which really isn't that much either unless your peeling out. Also your scenario would need to exist in that perfect classroom science lab with zero friction and zero drag on anything. Thats not reality and I'm guessing the drag from just the components in the rear axle would help drive the front through whatever ice there is.

If all 4 wheels are on ice and have equal slip, which wheel gets driven?
 
NP242 also has an overdriven rear in AWD mode so the rear tires are are more likely to drive. Only 52% or so from memory
 
I have a 242 in my XJ and back before I swapped a Ford 9 in the rear, I had a Lunchbox locker in the front D30 and the factory limited slip 8.25. I would run it in fulltime during snow runs and winter and it was point and shoot driving. Specially in heavy snow, ice....it did better than straight 4WD, but it depended upon the tires I would run. The diff in the 242 allow the front with the lunchbox to pull the Jeep whatever direction I steered it.

I remember driving to college in Boston in 16-18" of fresh powder on I89 through NH with this setup with little to now worries.
 
never looked inside the t case on my ZJ. buut, that thing in AWD mode in winter just straight up works! I'll just leave it in fulltime all winter. never need to think about it. can say is does much better then 2WD in snow/ ice. it never feels like only the front or rear is getting power at a time. feels like some sort of limited slip thing going on in there. :beer:
 
F***roads said:
never looked inside the t case on my ZJ. buut, that thing in AWD mode in winter just straight up works! I'll just leave it in fulltime all winter. never need to think about it. can say is does much better then 2WD in snow/ ice. it never feels like only the front or rear is getting power at a time. feels like some sort of limited slip thing going on in there. :beer:
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This is what people with 242s who use them in snow on paved roads typically report. Has the OP actually driven with a 242 in the snow?
 
^^ No. Didn't claim to. Just asked a simple question about how they work.


Thanks for the replies!
 
Wombat Ranger said:
^^ No. Didn't claim to. Just asked a simple question about how they work.


Thanks for the replies!
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Honestly, I wouldn't want one for my own vehicle....but for a one used on snowy roads.....they have a solid reputation. Some argue they are actually better than part time tcases for that use case. How much of that opinion is based on people who've driven both, I can't say.
 
F***roads said:
never looked inside the t case on my ZJ. buut, that thing in AWD mode in winter just straight up works! I'll just leave it in fulltime all winter. never need to think about it. can say is does much better then 2WD in snow/ ice. it never feels like only the front or rear is getting power at a time. feels like some sort of limited slip thing going on in there. :beer:
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Same. ZJ in winter, it's like a Subaru.

Pull your front driveshaft, or hang both tires in the air, it would probably suck. (Except for '4WD' or '2WD' modes)

Good discussion here: NP242 torque split, center differential..
Yes, it's true that when in Full-time mode the NP242 acts as a center open differential, meaning that if either end of the car has absolutely no traction, that's where the torque is going to go. BUT: in reality that would be a very rare occurrence and as previously written, a quick flick of the lever to Part-time will lock it up, or simply applying some brake with the left foot on the brake pedal while giving it gas will spread the torque to the front and rear.


I can tell you that I LOVE keeping it in the Full-time mode in wet or snowy weather. One can accelerate strongly from stoplights, stop signs and around corners without having the rear tires break loose and/or flaring out when cornering. I know it's a little bit sociopathic of me, but when I approach a stop sign or red light after being stuck behind the typical slow driver creeping along because they can't be bothered to buy proper winter tires and usually have just FWD or even RWD, I LOVE pulling up on their right and when the light changes, flooring or almost flooring it and pulling away as if I am on a dry surface. The best times of this are when they actually try to keep me from passing them. They stand no chance. I didn't even know about the NP242 when I bought my Jeep - I was lucky to end up with a Cherokee with it. And the optional and rare ABS brakes, too. Equipped with the NP242 and ABS, it's the perfect winter vehicle for Chicago.
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Google says:
288489d1463610673t-np242-bearings-image-54835350.jpg


242 Transfer Case Modes - Jeep Cherokee Forum

Stock XJ Cherokee Tech. All XJ Non-modified/stock questions go here - 242 Transfer Case Modes - I have read different things about this online, and I'm still confused. What is the difference between part time and full time on a 242? I know you can drive a 242 in 4WD full time all the time...
www.cherokeeforum.com
1999 NP/NV 242J differential internals:
304952d1501203430-242-transfer-case-modes-dscf2289.jpg


304954d1501203430-242-transfer-case-modes-dscf2296.jpg


I would expect that functionality-wise since full-time 4WD is an open diff, if one tire loses traction the Jeep will sit there and spin that one tire.
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Another case of "In theory, practice and theory are the same. In practice, they're not."

I like it better than the 249 permanent full-time my last ZJ had. No need for fwd most times.
 
Open center diff is NOT designed to be used where tires are coming off the ground. That would cause all the power to go to that one wheel. Hence why you have other modes like a locked mode for that center diff.

It is designed for when you are planted on the ground but encounter slippage. You can put power to the front and rear axles together giving you added grip but still can have a lot of speed differentiation. The worse the grip the worse it will work but now see the above statement. It's made for when you need more then just rear wheel drive but not four-wheel drive.

Some manufacturers like Audi and Toyota use a gear driven Torsen center diff which will send more torque front and rear without breaking free so to speak. It's the next upgrade over a center diff. At the end of the day when traction is that bad the center diff needs to be locked.

I used an NP242 on my S10 build for this specific reason. 2wd pickups suck in the snow. But most of the time I don't want to be going down the road in 4hi. Or if I give the truck to my wife I can lock the hubs and put it in AWD. Let the tcase do all the work.

When you hit the trail you still have 4HI locked and 4LO. To me it's the best of all worlds. Sure it's not bulletproof but I think the 242HD is one of the most underrated cases out there for an all around wheeler.
 
FWIW, the early 242s (say XJs) have the small 27 spline output while many of the later ones like WJ, Liberty, Durango have 32 spline. Some are 'dry' rear output, and some will leak fluid when the driveshaft is removed.
 
Yes you want to look for a WJ or Durango case to the the HD. It's quite a bastard as mine came from a V8 truck and was 32 spline it only had the 3 pinion low range planetary. Some have the whatever 5 or 6 planet version.

My guess is you are doing an SYE so it doesn't matter but you don't want one from an XJ for the above reason if you want 32 splines.

Note that the front yoke is a pain. Had to get some odd all yoke to fit a 1350 CV without hitting the shift linkage.

The WJ also has a bastard useless input gear which I found out after I already put my case together with the SYE so plan ahead depending on what tcase adapter you are using.
 
The binding from part time transfer cases and lockers causes slippage while driving on snowy/icy roads.

My understanding, like an open diff, the slipping tire has to spin twice as fast. The open transfer case split between the front and rear with open diffs imparts a resistance/drag causing friction preventing one tire from spinning. Apply braking to this also.

In the right conditions you could have problems, but iv never done the math. So
 
Gravy said:
The binding from part time transfer cases and lockers causes slippage while driving on snowy/icy roads.

Yes and no. I've run a lot of snowy and icy highways front-locked in part-time. In a straight line and gentle turning it's been fine. Of course it will bind hard steering further. With a manual trans however, you can lift the throttle to unlock the locker at any time.
Click to expand...
 
Wombat Ranger said:
The variable that may be the answer to the question is this: Overcoming the rotating mass of the entire system spinning together in 4x4 may serve to "dampen" the drawbacks associated with typical open differential operation.
Click to expand...

This was basically my interpretation of the answers given here. I had the idea, not the verbage. I will not hesitate to punch it into my Eagle when the time comes!
 
I think the fundamental flaw in a lot of argument here is that the open diff just peg legs one tire because it hypothetically in the air or doing a big power one wheel peel.

That isn't the case, the open diff is going to send power to the wheel with the least amount of traction but that isn't always zero.

So now take that concept and move it further upstream and jam it between the front and rear driveshafts.

So now the front and rear driveshafts are going to do one wheel peel unless you have one entire axle with zero traction which vast majority of the time would not happen with a street right in the snow unless one tire is on ice and you are in the throttle.

So then you would lock the center diff.

It's simply distributing power front and back over 4 tires instead of just 2.

Yeah in the scenario of someone in 4HI unlocked pulls one tire onto a patch of ice. Stops completely. Then mashed the throttle yes the truck would in theory not move but that's more theory then in principle.

You can take an H1, Land Rover, Land Cruiser, etc. pretty far on the trail unlocked and you really wouldn't notice the difference until you started lifting tires or it was pretty slick.
 
subybaja said:
Hah. First snowfall today, I put it in Full Time, and watched the Charger in front of me spinning 360s through the intersection. :laughing:
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Is that because of your transfer case, or the Charger has an HP to IQ ratio of 10 million?
 
Spiders will eventually warm up and starting melting together. That will get you some traction...till the pin comes out.
 
It's a stalled project, I plan to get back on soon.

I intentionally swapped the 231 out of our XJ and put a 242 in it for its AWD use in inclement weather and cattle pasture duty.

I've got a Torsen Tru-trac for the D30 front, and an 8.8 with a Ford Racing LSD for the rear. I have a Lock Right for an 8.8 too, but I intend to keep the ABS functional < pause for collective gasp > with the D35 tone rings swapped onto the 8.8 axles, so am leaning towards the LSD. It's not going to be a rock crawler, it'll be on pavement or gravel most of the time.

Give me a couple months and I'll let you know if it sucks or not.

Edit: well XJs suck, I think most folks know that, but I'll let you know how well the combo works for its intended purpose.
 
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