Part Time 4wd vs Full time?

Well I searched the forum but couldn't find anything (nor on the Web generally). When off road, say in muddy or wet grass conditions, what does Part Time 4wd give that Full time doesn't. Is Part Time giving better traction because it stops individual wheels from "free wheeling"? I saw a reference to "locking the diffs" but what does this actually do / achieve?

It's my understanding that full-time allows differentiating (slip) between the front and rear axles, allowing use on hard dry surfaces. The part-time locks the front and rear axles together, thus provinding more traction and the recommendation it not be used on hard dry surfaces.

"Part time" locks the differential, which means 50% of the power goes to the front axle and 50% goes to the back axle. "Full time" acts like an open differential (that's why you can run it on non-slippery surfaces without blowing the transfer case). The power will go wherever it can turn a dif most easily- meaning the one with least traction. The other dif will get less power, or no power, if the slippery end is really on slick stuff. "Full time" is great for highway driving on snow and ice, or changeable conditions. "Part time" for the most challenging stuff.

Many thanks, I thought that was the case but didn't want to make a fool of myself (well in public anyway) by trying it out and getting the wrong result. So in mucky stuff off road try part time and if knackered go low?

KiwiXJ said:

Many thanks, I thought that was the case but didn't want to make a fool of myself (well in public anyway) by trying it out and getting the wrong result. So in mucky stuff off road try part time and if knackered go low?

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Correct.

To give you an example of the difference between part time and full time, recently the U joint connecting my rear drive shaft to the rear differential broke, and the rear drive shaft fell out, so I had no power to the rear wheels
(thats similiar to what might have happened had the rear wheels been on a very slippery surface_

I was able to drive in part time 4 wheel drive because all the power went to the front wheels (in effect making my xj into a front wheel drive vehicle)

However the vehicle woouldn't move at all in full time 4 wheel drive, because all the torque was going to the nonexistent rear drive shaft,
which is what happens in full time (all the torque goes to the wheels with least resistance)

Full time works where you have some traction in both front and wheel drives, but if even one wheel front or rear has no traction, you lose all power to all wheels.

Hi Kiwi,
I had the same doubt for many month and tryied to found the answer in practice (jeeping). I couldn't find much difference until i drove in a a heavy snowed road without chains and only wit BFG A/T. I knew i needed speed, but in addition to 4wd part time it was incredible. Going from one side to the other (like in deep mud), but kept going on, even passed a military Unimog that was installing chains. Those guys couldn't believe that i made 3 miles ahead and backwards without chains.
Name of XJ was very well positioned on the very south of Chile, between woods and volcanos.

Mike

I Am Looking For Trail Rides In Southern Indiana...not Too Rough Just A Day Of Good Fun With My Sons

Go to the Jeep website. They have an animated explaination of the Jeep 4x4 systems and how they function. It is for the Liberty, but the Cherokee and Liberty both have the same 4x4 system and operating levers.
.
http://www.jeep.com/liberty/4wd_capability.html

I've also heard that the NP231 (part time only) transfer case is more rugged than the NP242 (part time & full time). Is that true?

So with all this being said does this mean that the full time transfer cases are crap? If so are there kits available to make them part time. Im assuming you would just have to get another transfer case.

The 231 is not more rugged, nor are the 242s "crap." In fact, the 242 is made with slightly more heavy-duty components in order to make up for the fact that it is a bit more complex design.

Neither is "better" or "worse" than the other. They are just different. If you feel you need the full-time 4WD option then you want the 242. If you can do without it then the 231 is simpler, less expensive, and has more aftermarket parts available for it. Both transfer cases, however, have the option of operating in part-time 4WD mode.

In general, in really serious off-roading you want to use part-time 4WD. Because it locks the front and rear driveshafts together (it does not lock the differentials!) it will give somewhat better traction. Full-time 4WD is for when you're on paved roads with mostly pretty good traction, but occasional slippery spots that leave you feeling the need for 4WD.

And to carry on the knowledge dissementation;

Where does All Wheel Drive fall into this? Is AWD just Full Time 4WD with a greater torque split front/rear. OR is AWD a non-off road system that has not option for part/full or 2WD, its just constantly running all drive to all 4 wheels with no options for changes?

Axles have Limited Slip Differentials (LSD) to help apply torque to both wheels in slippery conditions. Do any XFER Cases have LSD between front and rear? Seems like that would be an excellent compromise between Full Time and Part Time 4WD.

Finally, Torque Splits the Jeep Site talks about Full Time 4WD has a 48%/52% Torque Split between Front/Rear. How do they do that? Is there some sort of special equipment in the XFER Case to make the 48/52 % Split? OR Is it just the nature of the drivetrain, with the extra universal joints and steerable wheels up front, that it less efficient and looses some torque the rear does not, thus you end up with the 48/52 Split in Torque?

For all intents and purposes AWD and Full-Time 4wd are the same thing. However, in practice AWD systems don't offer low ranges, locked part time or 2wd modes. Also, many full-time 4wd systems don't offer options for 2wd like the np242 does. Mostly though if its a truck, call it Full-Time, if its a car call it AWD and you'll be fine

Yes, many AWD and Full-Time 4wd systems use a form of limited slip in the centre differential to modify the torque balance. For example. A 100% open diff in the centre will give you the theoretical 50/50 torque split - but it WON'T transfer any torque to slipping wheels whatsoever (np203 for example) so its really more of 100/100 split or not a split at all, if that makes sense.

In contrast to a locked part-time case which really is 50/50. If you add an limited slip, most common is probably the viscious coupling (np242 for example), it gives you SOME resistance to slip front/rear. Depending on HOW aggressive the LSD unit is, and its placement in the drivetrain they calculate the rolling "torque split".

Now some AWD systems get far more advanced and can actually dynamicly change the front/rear torque split. Automatic subarus have been doing this for years, amongst many others. They have a clutch pack, which under normal driving doesn't have much pressure at all on it making it virtually FWD. When the system detects slip it tighens the clutch pack creating a tighter connection to the rear wheels, on the fly.