Full time vs part time

[itbrokeagain]

Just picked up a xj with full time and part time 4wd. What is the difference? If I am wheeling in full time what will happen?

[KargoMaster]

Where are you getting the part time and full time part??


Here's my take on this generic idea... (and I'm no XJ guy)

Full time 4wd means the front driveshaft always spins.

Part time 4wd means the front driveshaft will not spin in 2wd (usually accompanied by lockouts)




You can have one or the other... not both.. but there are also wacky things like a tcase locking device that will force a 50/50 split of power front/rear.... guess I'm still confused on where you're getting the idea that it has both...

[jeeptech77]

full time

in a xj the transfercase has what is known as a viscus coupling it is suppost to allow limited slip between front and rear driveshaft
ideally for snow and on pavement use
this is known as part time use

full time operation is when the differential in the transfercase is locked together ( kinda like a spool ) but disenguageable

the older chevy ford dodge had lockouts so the vehicle would not move in part time just like a open diff with wheel off the ground

[itbrokeagain]

But does it make a difference in a offroad situation? Now I would normally leave it in low but I am just curious.

The t-case positions are

2wd
4full time
4part time
N
4lo

[Hope Springs Hauler]

From my test drives of XJs, it seems to me that the viscous coupling is active when it is in 4wd Fulltime. This would be the setting to use for slick onroad situations.

Switching to 4wd part-time locks the coupling and it becomes like a traditional transfer case.

The danger in wheeling in "fulltime" is placing a lot of stress on the t-case if 1 wheel were to spin and the rest have great traction.

[WrenchMonkey]

in a xj the transfercase has what is known as a viscus coupling it is suppost to allow limited slip between front and rear driveshaft
ideally for snow and on pavement use
this is known as part time use

full time operation is when the differential in the transfercase is locked together ( kinda like a spool ) but disenguageable

the older chevy ford dodge had lockouts so the vehicle would not move in part time just like a open diff with wheel off the ground

Close... Just exactly backwards.


The standard XJ case is the NP231. 2WD-4HI-N-4LO.

Engaging 4HI or 4LO locks the front and rear driveshafts together (actually chains them together, whatever...) Great for off-road, not so much on-road.


The optional cases (NP242 et al) have the viscous coupling, and the positions you list: 2WD-4FT-4PT-N-4LO

The 4PT works like the 231's 4HI: Positive engagement.

The 4FT is the one that allows some slip between the 'shafts. Great for on-road, iffy-traction situations.

You already know to use 4LO on the trail.

Robert

[blazerbrad]

As mentioned above, 4 full-time is the same basic idea as AWD and you can use this anytime on any condition. However, there is no viscous coupling or any type of limited slip in regards to transferring power between the front and rear tires. The transfer case functions as an open differential between the front and rear axles (we did testing on vehicles equipped with this transfer case for years, and I just glanced through a factory service manual to verify)......no clutches, no viscous coupling, just an open diff....at least for the Jeep/Dodge/Chryler versions.

This means that you can have a "one wheel peel" going on when in full-time 4wd mode, and I've personally seen this. Again, we intentionally performed a durability test on a wetted uphill grade where a vehicle took off at wide open throttle from a stop, which caused only the left front wheel to spin like mad. Also, if you pulled either the front or rear driveshaft the vehicle would not move unless in 2wd or locked in 4wd.

Several versions of this transfer case were used over the years. Included are "4 position" and "5 position" versions in both manual and electronic shift models. The 5 position versions had 2 hi, 4 FT, 4 PT, N, 4 lo. The 4 position versions eliminated the 2 hi option, normally electronic shift, and where used in heavier vehicles like the V-8 Durangos. This version was called the 244. They also tried a 144 version that eliminated low range, but I don't know if that ever made it into production. (Note the name changing from "2" to "1" which denotes 2 gears, hi and lo range, to just 1 gear).

Jeeps did use other versions of transfer cases with various forms of limited slip and viscous couplers including the 249 (optional in the ZJs) and the 247 (optional in the WJ's). Both of these had a direct drive to the rear wheels, and the viscous coupling applied power to the front as needed.....meaning you could pull the front shaft and they would deliver power to the rear wheels with no slippage.

[itbrokeagain]

Ok sounds good. So how does this t-case hold up?

And blazerbrad got me thinkiing about something else. Does this mean that in 4fulltime because of a center diff, that it will be either front wheel or rear wheel drive depending on traction?

[KennyTJ]

do you know if it is 228 or 242 tranfer case, should be on the tag. Sounds like it's a 242 if it has full time and part time. The 242 in part time "locked" position should be as strong as a 231. You still have the expensive coupling in there if it does take a dump on you.

[blazerbrad]

Ok sounds good. So how does this t-case hold up?

And blazerbrad got me thinkiing about something else. Does this mean that in 4fulltime because of a center diff, that it will be either front wheel or rear wheel drive depending on traction?

The 242 seems to hold up pretty good. The only failure we ever saw was grenading the center diff but ONLY by repeating a very specific condition that was known to cause issues numerous times (basically during a burn-out on a hill)....basically, in real life it is extremely unlikely you would see those conditions at all and little less repeat them enough times to cause failure.

Will it be either front or rear wheel drive in 4 full-time...well, yes and no. Take a regular open rear diff and at certain times it will only spin one tire. Now while you can only see one tire spinning and the other looks like it is just dead weight, there is in fact torque going to other wheel. So an axle (or transfer case) with two outputs (tires) with an open diff does indeed put more torque to the ground than if you just had one tire period (for example, if one tire was not connected to an axle shaft at all). Have you ever seen somebody with a front locker blow a shaft so they just unlock the hub on the one side and keep going in 3 wheel drive? At least in my opinion they always seem to have less capability than a similiar vehicle with just an open front diff in most situations. This holds true for an AWD transfer case with just an open diff. Put the vehicle on flat ground with the t-case in 4 full time (open diff) and it will have more drawbar torque before tire spin than if you put the same vehicle in 2wd. All 4 tires will be getting equal torque.

Now that everybody is asleep, let me explain the comment about all 4 tires getting equal torque with an open diff. By definition an open differential is "torque equalizing". So on an axle with an open diff both tires are always getting equal torque. Before somebody calls BS let me explain. Take a vehicle with an open diff rear axle and jack one tire off the ground. You can pretty easily spin that tire by hand, meaning it takes very little torque to spin it. The tire planted on the ground sees that same "very little torque" as the tire in the air, but you obviously can't spin the tire sitting on the ground by hand.....that's what I mean by torque equalizing.

A limited slip or locker is "torque biasing", basically meaning that they transfer more torque to one side or the other. Take the above example with one tire in the air. Let's say it only takes 5 ft-lbs. of torque to spin the tire in the air. A limited slip may have a bias of 4:1, meaning it can transfer 4 times the torque to the other tire. 4 x 5 = 20 ft-lbs., however that much torque is still not sufficient to spin the tire on the ground (but it may be enough to pull the vehicle off the jack). A true locker basically has an infinite torque biasing ability, with the upper limit being the total torque going into the diff from the engine and gearing.