So whats up with full time 4wd?

Jeepjeepster · **Posted:** Thu Dec 08, 2005 11:38 pm

Ive never really understood full time. It was raining tonight and it was around 36F outside so i stuck it into full time. About a mile later i had to pull out onto a highway and i gave it alot of gas to clear all the cars and not get into their way. Well the back tires started spinning and i never thought it was going to hook up. I thought full time put 48% of the engine power to the front wheels, but it did nothing in my situation. I stuck into part time on the way home just to make sure it was engaging and got no spinning what so ever.

Does full time suck that bad? I thought it had clutches in there to put power to the front wheels? Or is it just an open differential in full time? Either way, i found out it sucks. Im going jack the rear wheels up and see if it will pull its self around on the the floor jack with the front wheels in full time, hehe i wont go far.

Does anyone actually know how the full time system works? Is it like a limited slip in the rear end? does it ever wear out.

RespectMyLibertay · **Posted:** Thu Dec 08, 2005 11:50 pm

Full-time employs a transfer case differential which allows the front and rear axles to rotate at different speeds. When you want more wheels with power but you're on a pavement road, full-time is great, because you have 4 wheels with power, but you can still turn and not fear any binding (like you may get with part-time on hard surfaces). Because this allows the axles to rotate at different speeds, it also allows one end to spin out while the other doesn't. Part-time forces both axles to rotate at the same speed, so for either axle to spin out, the other has to do the same. This is why you've never spun out with part-time. I've done it in snow, on purpose, and you get all four tires spinning. Then it gets scary...

Jeepjeepster · **Posted:** Fri Dec 09, 2005 12:05 am

yea, i know how part time works. I know everything about transfer cases but i dont know how full time works. Weve got 3 jeeps and i think the 247 in the grand cherokee is the best, it will put power to the front wheels and never let the rear wheels spin. I know the viscous coupler is going out soon b/c it has 156k on it, but its great for the rain and snow. Its never spun a tire in the rain.

The thing i dont understand is how the full time system puts power to the front wheels. When my rear was spinning it was like it wasnt putting any power on the front wheels and my jeep was barely moving when i was hitting 3k rpms. Is it just an open differential and lets the power go to the front and rear as it likes? Thats pointless if it does.

dog_party · **Posted:** Fri Dec 09, 2005 8:56 am

It's supposed to be 48% front and 52% rear in SelecTrac full-time. Mine works great in the rain. I get a bit of binding when I try to turn really sharp (like when I'm backing into my parking space in the dry-concrete parking garage), but it works fantastic out on the wet roads. No spin at all even at full throttle in the CRD. If you're getting spin on the rear wheels in full-time, I fear there might be something wrong with your T-case.

Eddo · **Joined:** Thu Jul 07, 2005 7:01 pm **Posts:** 1214

The center diff is just like the diff in your axles. Lift one tire or put it on a slipery surface and it spins helplessly as the other just sits their. That is how and open diff works.

Also, I actually have to juice it pretty good to get wheels to slip in 4-fulltime even in the snow. So you maybe hit the gas too hard or the goodyear tires just don't do so wheel when wet.

FarmDiesel · **Joined:** Thu Sep 15, 2005 8:49 am **Posts:** 132

The full time 48/52 split in KJs is under perfect conditions (i.e. no slip at any wheel), just like an open differential in teh rear axle shifts power 50/50 L and R under perfect conditions. But as noted above, the center diff is open, without clutches or traction aiding devices, when in full time.

LibertyCRD · **Posted:** Fri Dec 09, 2005 9:49 am

I was in full-time this morning and got stuck going up a small snowy incline because one wheel lost traction. So yeah, you definitely can get into situations where the front end gets 0% torque.

But Full-Time is still better than 2-high when it's snowing outside because in 2-high the front driveshaft NEVER gets any torque. In Full-Time at least the front driveshaft does have the opportunity to help pull the vehicle along. So I wouldn't say Full-Time is useless. It's perfect for a light snow/ice condition or driving across a wet field or something.

Jeepjeepster · **Posted:** Fri Dec 09, 2005 5:12 pm

Im sorry to say, but if it is an open diff like everyone has said, that is pointless. Ill find out for sure today if it is.

RespectMyLibertay · **Posted:** Fri Dec 09, 2005 5:25 pm

I only use full-time on roads patchy with ice or snow. I use part-time when the entire road surface is covered in say, fresh snow. Seeing any really hard surface with part-time may result in binding when turning, but I have never binded when in part-time on snow, even in sharp turns.

bennybmn · **Posted:** Fri Dec 09, 2005 6:39 pm

Quote:

but if it is an open diff like everyone has said, that is pointless.

It's not pointless, it allows you to actually make turns on pavement! Otherwise you'd either skid or break something in a tight turn.

These aren't fanct AWD systems like Subarus or Audis (or the other cases in the GC)with anything active going on, they are purely mechanical systems, and physics is physics...

Jeepjeepster · **Posted:** Fri Dec 09, 2005 7:17 pm

it should at least have clutches in the tcase like a Limited slip in the rear diff.

Eddo · **Joined:** Thu Jul 07, 2005 7:01 pm **Posts:** 1214

It is about as pointless as the open diff in the axles. It does do ALOT better than just 2wd, it just snowed up here and in 2wd it was all over the place and almost got stuck a couple times. Pop here in to full-time and no problems at all. It dosen't grip as good as part-time, but then again you don't have to worry about shifting in and out all the time.

Jeepjeepster · **Posted:** Fri Dec 09, 2005 11:01 pm

Well, one good thing about it is it lets the front wheels spin. If your turning and the front looses grip then the tire will spin and hopefully pull you that way. I put the front end on some ice and it actually pulled me off of it. I figured it would just spin the front wheels and not move. I even pulled the e brake some to hold the back and it still pulled me off the ice. So it may not be that pointless.

Libby4Wheeler · **Posted:** Fri Dec 09, 2005 11:02 pm

I havent had any problems like that with mine when im in Full time is grips like it should. Even though my part time and 4 low arent working proparly. cant get the to engage when loss of travtion accurs...

Metrowest_Kev · **Posted:** Sat Dec 10, 2005 12:07 am

Today's storm was my first real test for Full time 4wd. A++++. The stock tires slipped a few times in tough spots, but always got me going. This afternoon was the nastiest I've seen the roads around here. Lots of traffic, and plenty of cars (and a semi) spinning out, sideways on the road, and abandoned. No problems for my KJ. She's a beauty! My driveway wasn't plowed yet when I got home, no problem. 10 inches of snow, and me, not shoveling.

So whats up with full time 4wd?

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