NP231? what does this part-time stuff mean?

I have a 1991 Jeep Cherokee Limited with a 4.0L I6/5-speed. I think it has the NP231. What does it mean that it has part-time 4Low? Does this mean it has planetary gears in the t-case like a NP203 like for an all-time 4WD unit?

It means you should only use the 4wd on surfaces where it can get wheel slipage. If you run it in 4wd on dry cement or asphalt, it causes binding and can cause breakage in the drivetrain.

Thanks. Why do they say that? Is it just kind of a disclaimer because the D30 and D35 axles are so weak?

charlesarthuriii said:

I have a 1991 Jeep Cherokee Limited with a 4.0L I6/5-speed. I think it has the NP231. What does it mean that it has part-time 4Low? Does this mean it has planetary gears in the t-case like a NP203 like for an all-time 4WD unit?

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Ha ha--I found this counter-intuitive when I first got my XJ. The 242 (Select Trac) has planetaries to allow both Part time and Full time operation. Full time you can run on any surface, at any speed. Your 231 (Command Trac) only allows operation of (as Old Man points out) on slick or soft surfaces.

Even "strong" axles will break if they are locked and can't roll on the different radius turns left vs right. When doing this on asphalt, one tire will turn more revolutions than the other - which it can't - and so slippage must occur - which it can't on a high friction surface. Boom, axle or joint failure.

On dirt, ice, snow, mud, the tires get to turn slower or faster as necessary, which a lot of times isn't even noticed, other than kicking out a little driving surface all over the bodywork . . .

When you turn, the wheels spin at different speeds. Since the front tires turn and the rear don't, there is speed difference between all 4 wheels in turns.

Now, between left and right wheels on a axle, you have a differential, that allows the wheels to spin at different speeds for turns. The same open differential will allow the torque/power to go the path of least resistance when one of the wheels slip, so if you want to prevent this you need expensive Limited Slip Differentials (LSD).

Now, in 4WD, in turns not only do you need a differential at each axle, for speed differences in the wheels left and right, you also need a differential in the transfer case for speed difference between the front and rear axles. Again, a simple open differential can allow the torque/power to go the path of least resistance if a wheel is slipping in the mud, and you get stuck.


SO, many jeeps have PART TIME 4WD, meaning there is NO Differential in the Transfer Case, thus no slippage between front/rear axles, also no way to allow speed differences between front/rear axles. So great off-road, because if a wheel slips on an axle, it won't automatically divert all the torque/power to the slipping wheel and leave you stuck. But, since there is no differential, then in turns where the tires need to spin at different speeds, it won't allow the tires to do so, they will be forced to slip on the dirt/mud/gravel off-road. On the street, the tires will have too much traction, and it will cause binding/wear/damage to the drivetrain.

Planetary Gears are only a different way of arranging your gears, gears just being planetary doesn't indicate anything or any special purpose. In the transfer cases, there are planetary gears used for reduction gearing, for 4Low, and also as differentials in transfer cases that have differentials. Having a differential in the XFER case means it can be used in Full-Time 4WD and can be driven on the street without binding/wear/damage to the drivetrain.

DOH! Double Post, don't know if its my proxy at work or the server is slow today, the latency was so great, I must have done something wrong while waiting.

Good info on this thread. here is a LOT of info on this topic everywhere. The one thing I still trying ot understand is- on heavy snow, slush, or ice or partly-icy+wet conditions, is it MORE dangerous to drive in part-time 4hi as is neccessarily needs the wheels to slip a bit and lose traction? thx

The line I use in my head is loose surface versus slippery surface. Part-time will force one of the powered wheels to break surface contact. This is okay on a loose surface like rocks, dirt and fluff snow, since the surface material will work around the wheel. This is no good on a slippery surface like water or ice, since there is nothing to hold the wheel, and there is a high risk of not regaining traction on a slick surface. IE slippery you will break surface contact and not get it back

OTOH if you are already stuck you will probably get unstuck faster in 4WD

Lol I asked this like 3 days ago

http://naxja.org/forum/showthread.php?t=1025778

Well, you sort of asked that question. I think the explanation of slippery vs. loose ground is what I was looking for. Ya I was wondering if, since in 4hi part-time the wheels must NECCESSARILY slip a bit this would be MORE dangerous on ice, slush and wet roads, since it forces the wheels to lose traction and thats usually hard to regain, as opposed to dirt and deep snow type stuff where the traction comes right back. So The answer I'm guessing is YES, it is harder/worse to drive. This is too bad I guess because thats the conditions I encounter where I feel I need it most of the time. Usually driving up an ice+slush-covered road is when I need it most, or on ice and snow covered hwy with dbag hummers and yota's driving like 80mph past me. So really I should stay in 2wd, the worst choice for ice..a big torquey rear-wheel-drive vehicle. Mind you I've never gotten stuck or had a dangerous spin, I consider myself a very good driver. But I'm thinking for the conditions up here in winter maybe I don't have the right vehicle. Better woudl be a 242, or a little awd, or even a little 2wd-frontwd even? Well I luv my heep so I'll deal. There was a post on here from a guy in CO who explained how he drives his 231 in the CO mountains and I'm gonna have to follow his advice: 2wd until its real bad, the 4wdhi. sometimes in-n-out. And drive well. Love to hear more comments on how people tool around in ice and snow on their np 231's. even though the 231/242 discussion has like a zillion post already..naxja rules! cheers- snow comin this week! yes! -TJ

charlesarthuriii said:

I have a 1991 Jeep Cherokee Limited with a 4.0L I6/5-speed. I think it has the NP231. What does it mean that it has part-time 4Low? Does this mean it has planetary gears in the t-case like a NP203 like for an all-time 4WD unit?

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it doesn't mean that at all. if you have "full time" listed beside your t case shifter, you have the 242. if you don't, then you have a 231. only with a 242 should you drive on hard surfaces, such as pavement, with the t case engaged. A 231 will bind and could break something. I do not believe there is a "part time" 4 lo option.

benXJ said:

I do not believe there is a "part time" 4 lo option.

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No. 4LO is exactly that - Part-Time 4WD, low-range. Same rules apply as for Part-Time 4HI and slippery/solid surfaces.

Now, if you meant that there is no Full-Time 4LO, you're right on the money

Why do people insist on "over thinking" this?

Snow, ice, dirt, sand, and mud allows a differentiation in the turning rates of the individual wheels--"slippage", yes, but not a loss of traction.

If the surface doesn't allow for ENOUGH differentiation in the turning rates, well, don't use part-time 4wd--high or low. Asphalt, cement, HARD road surfaces, dry or wet, usually don't allow ENOUGH differentiation.

Snow, ice, dirt, sand, and mud allow for ENOUGH differentiation, so have at it with part-time 4wd.

Don't make me get out my "Smart Stick"! :twak:

So i'm not suppose to run my 231 on wet pavement that might be slippery, thats gay, i always though the 231 was the better of the two, now i think i might prefer the 242.

yes. people swap in 242 for 231 to gain the ability to have '4wd' on pavement.

imprezkid said:

So i'm not suppose to run my 231 on wet pavement that might be slippery, thats gay, i always though the 231 was the better of the two, now i think i might prefer the 242.

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231 is better "when the goin' gets tough". The 242 will let one wheel spin by itself and could get you stuck easier. The 231 won't spin a wheel until a wheel (usually the opposite wheel) spins on the other axle too.

242 is better for a mall cruizer, I guess. That way you can keep it engaged to get across ice in the parking lot. A Cherokee with a 231 can easily out accelerate an AWD sport ute with twice the horsepower stop light racing in snow. (If I ever did that. LOL)

Yeah i will say i can just mash the cherokee in the snow in 4wd and there is no slipping it just hooks

winterbeater said:

231 is better "when the goin' gets tough". The 242 will let one wheel spin by itself and could get you stuck easier. The 231 won't spin a wheel until a wheel (usually the opposite wheel) spins on the other axle too.

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This is absolutely not correct: a 242 transfer case in Part-Time is functionally-equivalent to a 231 transfer case in Part-Time. The main difference between the two is that the 242 has a locking centre diff that is open in Full-Time (AWD) mode; the 231 does not have this. As for 'spinning a wheel', this is a function of your axle differentials and whether they're open or not.

Ya I was going to mention that about the 242 in part-time. I think the attitude around here is that the 231 is a bit stronger and just plain simpler..? Also I think Benxj has got is backwards? In any case, get out yer smart stick..doesn't "slipping" pretty much mean "no traction?" Even if its minor. The question is how quick do you get it back.. I was just wondering if the slushy/slightly snowy roads I drive a lot around here (Tahoe) are letting my wheels slip enough to keep from tweaking the transfer case, and if using my 4wd part time on ice is a bad idea because the wheels WILL SLIP because of the locked axles, and maybe that's more dangerous than just trying to 2wd it..especially like sharpish, downhill turns? I've learned a lot about this on these forums..
thx -TJ