Center diff vs transfer case

[4runner]

I have a general 4WD question. It is related to towing so I figured this is a good place to ask. I currently have a 4Runner with a center differential that allows me to operate in full time 4WD, or part time 4WD or 2WD. I can also lock the center diff if need be. The multi-mode system has worked well for me, and I like running in full time 4WD in winter/snow conditions, which in western New York, is longer than I wish.

I'm currently looking at my "next vehicle" and the new 4Runners have part time 4WD, as does the Tundra and most other pickups.

Why don't bigger vehicles have a center diff? Is it because the center diff is a weak link?
How do you guys who drive in snow or mixed conditions like using a part time system? Does the additional vehicle weight help traction?

Thanks.
Bob

 

[timswi]

I might be lost in the differing terms, but here's my take:

I have a GMC 1500 4x4 that has AWD, high and low 4wd and 2wd. If the weather is snow and whatever mixed, I run in AWD. If the snow is really bad, I run it in 4wd. Otherwise, it's 2wd.

On the previous trucks I've had that didn't have AWD, I'd just change between 4wd and 2wd as needed. Never had a problem.

 

[hawkeye08]

I might be lost in the differing terms, but here's my take:

I have a GMC 1500 4x4 that has AWD, high and low 4wd and 2wd. If the weather is snow and whatever mixed, I run in AWD. If the snow is really bad, I run it in 4wd. Otherwise, it's 2wd.

On the previous trucks I've had that didn't have AWD, I'd just change between 4wd and 2wd as needed. Never had a problem.

Your AWD is actually "auto" 4WD which engages front axles at differential and when slippage is detected, engages transfer case for 4WD. We run this way in the winter months on '98 yukon since black ice is common.

GM pickup and SUV AWD (all wheel drive) has a full time center differential, no 2WD available, such as the Yukon XL Denali (since about 2000).

 

[timswi]

Your AWD is actually "auto" 4WD which engages front axles at differential and when slippage is detected, engages transfer case for 4WD. We run this way in the winter months on '98 yukon since black ice is common.

GM pickup and SUV AWD (all wheel drive) has a full time center differential, no 2WD available, such as the Yukon XL Denali (since about 2000).

True on the "auto 4wd"...Got the terms wrong. The wife's Hummer runs in awd all the time, can't shut it off...Even though I wish I could.

 

[Diamondpilot]

The way I understand it a center diff like on the Mercury Mountaineer we had with AWD only allows one range.

A transfer case, like on every 4x4 truck I have owned, has the ability to have multiple ranges. In most cases its High and Low. It also allows for the selection of which output shafts get power. In most cases its usually running power to the output shaft that is connected to the rear drive shaft and gives the drive the ability to control the engage/disengagement of the output shaft that is connected to the front drive shaft.

This would give your average transfer case the selection of 2 wheel drive, 4 high, and 4 low.

Chris

 

[4runner]

I guess I picked a bad title. My 2002 4Runner has a two range transfer case too. But with its center differential, it can be run in 4wd all the time, or 2wd. They call it multi-mode.

The new 4Runners and Tundras have a part time system with no center diff. (although the 4Runner Limited and Sequoia both have center diffs and the ability to run in 4wd all the time).

I did not know that GMC/Chevy had the ability to run in full time mode. I checked the Ford F150 brochure and didn't think it could.

My real issue is whether I will be satisfied with a part time 4wd system given my winter driving conditions, and I am looking for feed back from others.

It is also related to towing because when I tow, I typically keep it in 4wd all the time too.

Bob

 

[dodge man]

I've only had part time 4 wheel drive pick ups, no center diff. Its usually not a problem for me, once in a while I will end up coming onto a paved road in 4 wheel drive, and the front and back end are fighting each other while it comes out of 4 wheel drive, but its not that big a deal.

I'm sure a center differential is just one more thing to break down, but I don't know if they are ever really a problem area or not.

 

[LD1]

I'll give you my understanding of the terms, which may be wrong.

The center diff is just like the diff in the rear axle. In the rear axle it allows one wheel to turn faster/slower than the other when making turns (in theory). But when stuck, like on ice or in mud, you only have one wheel doing any work.

The center diff is the same only it splits the fronts and rears. Under normal driving conditions, All the wheels have power and it allows you to make tight turns in 4wd without the front and rear fighting eachother due to the front making a wider arc than the rear.

The problem with the center diff is the same as a rear diff, you may only have one axle turning if you get stuck. And if that axle doesnt have a locker or limited slip, it may only allow one wheel to turn.

So hypothetically, If you nose into a mud pit/sink hole with the front axle, ad are in 4wd but have a center diff, there is a chance that you wont be putting any power at all to the back tires that have traction. And if you have an open diff in the front, only one wheel will be spinning even though you are in 4wd. This is one of the reasons true 4wd'ers dont like them. As well as they are another weak link.

A lot of newer vehichles are running center diffs to give full time 4wd but an older example is the NP203 transfer cases. They were full time cases but they had 4 selections. hi, hi-loc, low, low loc. The loc settings locked the center diff to still be able to have true 4wd. I think this is probabally what your vehichle has, but the termonoligy has changed a bit.

 

[Diamondpilot]

Ford used to offer a AWD selection and my GMC I had in 05 had it. I did not care for it. I am smart enough to decide when I need 4 wheel drive and just prefer to do it myself. The wife is a different story so she gets AWD most of time.

Chris

 

[LD1]

Ford used to offer a AWD selection and my GMC I had in 05 had it. I did not care for it. I am smart enough to decide when I need 4 wheel drive and just prefer to do it myself. The wife is a different story so she gets AWD most of time.

Chris

I have an 03 chevy that has the "AUTO" as well. I agree, I am spart enough to decide.

The big thing I dont like about it, it has to see a difference in wheel speed from front to rear (slip) to engage. By that time, It may be too late.

Last winter while plowing, I decided to us it in my drive just to see what it was like. While it does detect wheel slip rather quickly, it might not be fast enough in a real spin-out situation on the road. And the other thing i dont like about it, once the rears are spinning and it decides to engage, everything is moving and under load. Engaging the fronts with the backs already spinning and under load cant be good on components. So I dont use it at all anymore.

 

[Diamondpilot]

I have an 03 chevy that has the "AUTO" as well. I agree, I am spart enough to decide.

The big thing I dont like about it, it has to see a difference in wheel speed from front to rear (slip) to engage. By that time, It may be too late.

Last winter while plowing, I decided to us it in my drive just to see what it was like. While it does detect wheel slip rather quickly, it might not be fast enough in a real spin-out situation on the road. And the other thing i dont like about it, once the rears are spinning and it decides to engage, everything is moving and under load. Engaging the fronts with the backs already spinning and under load cant be good on components. So I dont use it at all anymore.

This is the samething that happened to me. I was nearly in the ditch but had it corrected then the system decided to do its thing and it only made matters worse at that point. I now only buy full time AWD or just regular old 4X4

Chris

 

[Wis Bang]

I had a co-worker w/ an Olds Bravado w/ a full time AWD w/ traction control. The first fall he backed down the river bank to pull his dock. One wheel at a time spun until all four were buried in the sand. A neighbor w/ a Tahoe had to pull him out. AWD isn't always the answer.

Going back to my '62 Willys w/ Warn locking hubs, I use 4x2 until I get in trouble then I shift to 4x4 Snow covered roads needed 4x4 until the roads dried up, no shift on the fly in the old beast. That old CJ5 went sideways a few times on black ice...short wheelbases do that.

I'm glad my '05 Silverado is a 'work truck' and still has the floor shift 4x4 and posi traction instead of the auto setup.

 

[timswi]

I drive alot of country roads, interstates an ciity streets in my commute. I can honesly say that I am now a big fan of the auto 4wd given that I can go from full snow covered to dry and back again very quickly. The auto system works flawlessly. For off road use, deep snow and stuff like that, I do run it it full 4wd.

Also, the auto works good on wet tar and chip hills where that truck just spins in 2wd when trying to move from a dead stop and that's with a locking diff. PA loves to turn the local roads into skating rinks.

As for the Bravada example...Let's face it that was a supposed to be nothing more than a luxury vehicle and not an off roader.

As far as the center diff...I think it used to be called a viscus coupling. This is a memory from my import dealer days.

 

[dstig1]

A differential is essentially the same thing no matter where it appears on a vehicle: It allows the 2 output shafts to turn at DIFFERENT speeds.

The rear diff on all RWD vehicles allow the L&R wheel to turn at different speeds, such as around a corner.

A center diff allows the front and rear output shafts to turn different speed for corners, differences in tire size, whatever...

All vehicles will have a diff on any driven axles (FWD is at the front, RWD is at the rear and AWD/4wd will have one on both F & R axles) as you can't even really turn without one. You can "get away" without a center diff on 4wd vehicles as you are letting the road surface act as the "differential". You get tire slip from a loose surface to allow the tires to make up any difference F to R. Typically the difference in speeds F - R are not as dramatic as side to side. But this is not good to use for any period on a high traction surface like dry pavement as you get little slip. Then things break. True 4wd systems are generally considered off road only, or for other low traction surfaces (snowy roads) as they have no center differential.

And then you get into all manner of specialty diffs like viscous coupling, limited slip, lockers... These are common for center differentials on AWD vehicles.

 

[Ryan03]

AWD vehicles typicaly have a diffrential or fluid coupling in the transfer case. In the case of a diffrential in the transfer case, it can be a open type, like a power divider in a large class 8 tandem truck, or a clutch type diff. AWD vehicles typicaly can not equaly split torque between the front and rear drive axles, it usualy winds up being 70/30 or 60/40 etc. The exception to this rule is when the AWD transfer case has an inter axle diffrential lock mode, which couples the front and rear transfer case output shafts together to allow for a 50/50 torque split between the front and rear drive axles.

IMO, the AWD functions better for slick winter weather conditions, especialy when negotiating sharp corners, such as pulling into or out of a driveway or parking space. This is due to the fact that all wheel drive allows some varience between front and rear driveshaft speeds, which greatly reduces understeer and the kicking and bucking that you get with 4 wheel drive. On the flip side, in deep snow, mud, or pulling something out of a ditch etc. 4 wheel drive has the obvious advantage due to its ability to equaly tranfer torque to both axles under high load conditions.

 

[Diamondpilot]

These last two post were very informative and well written. Lots of good info guys. Thanks.

Chris

 

[4runner]

I agree with Chris, very helpful information. Thanks.

I understand the 50-50 torque split that occurs in a part time 4WD with only a transfer case. This same occurs when the center diff is locked on vehicles with a center diff.

In this way it seems like a 4wd with a lockable center diff is the best of both worlds.

Or is there a down side? Is there some reason/advantage to eliminating a lockable center diff? Is it just cost? Or does a center diff has limited ability to transfer torque? In 2002 the 4Runner had the lockable center diff. The current model 4Runner has a part time system on the SR5 version. And the Tundra only has a part time system.

Bob

 

[Diamondpilot]

To me the center diff thing with a locker is good for light vehicles and for casual, and I mean very casual use. Such as in something like a cross over SUV of small SUV for driving in adverse weather conditions, not off roading.

For pulling stumps, heavy trailers though the muck and mud, pulling stuck equipment from the ditch, ect its tough to beat a 2 range transfer case with good axles and real lockers. Owning three 4x4 trucks currently the one with lockers is hands down twice the vehicle that the other 2 are without in these type of condition's. When it also comes to 4 wheel drive a independent front suspension versus the solid front axle like that found in my 2 Ford Diesels is far superior. Its not as strong so its not good for all applications. Yes, the solid front axle is super strong and great for carrying a load like a snow plow but it has its downside also.

Once again, there is no perfect setup. You need to chose the right tool for the job and sometimes you have to sacrifice and get one that does 90% of what you want.

Chris

 

[Redneck in training]

I rented a GM SUV last winter. Forgot what it was but it had AWD system with a traction control that used ABS to prevent slippage of single wheel. I assume that it simply applied break to the wheel that had tendency to slip.
When disengaged I could do doughnuts on a snowy parking lot at will. When engaged it was near impossible to do so. Also climbing very steep hill on snow when disengaged I would get stuck not too far from the bottom. When engaged the car climbed up only with some hardly detectable slippage. In other words I was impressed how well it worked. The great thing about system using electronics is speed of response I suppose. I also read somewhere that it allows for an "active steering". When steering wheel is turned the system applies certain amount of breaking to the wheels inside of the turn.

 

[FlaDon]

I rented a GM SUV last winter. Forgot what it was but it had AWD system with a traction control that used ABS to prevent slippage of single wheel. I assume that it simply applied break to the wheel that had tendency to slip.
When disengaged I could do doughnuts on a snowy parking lot at will. When engaged it was near impossible to do so. Also climbing very steep hill on snow when disengaged I would get stuck not too far from the bottom. When engaged the car climbed up only with some hardly detectable slippage. In other words I was impressed how well it worked. The great thing about system using electronics is speed of response I suppose. I also read somewhere that it allows for an "active steering". When steering wheel is turned the system applies certain amount of breaking to the wheels inside of the turn.

Mom and dad bought a Pontiac Vibe AWD for those reasons. Last winter they were the only car to get out of the hotel parking lot after the massive storm in Dec. AWD does have its advantages if done right. The transfer case in it decides which way to send the power with 95/5 splits to 60/40 splits possible.