4WD and AWD systems explained

jhs70 · December 2003

Yes, unfortunately, she did drive it around town, and on dry pavement (once the snow cleared). Exactly how many miles I do not know. I'm sure she got up to 55MPH at times as well. Like I say, it performed ok yesterday in the snow, so I'm keeping my fingers crossed. I will heed your advice and get the fluids changed soon. Thanks.

ateixeira · December 2003

Going straight it won't matter - all the tires are rotating at the same speed.

It's only around a turn that the tires would be forced to scrub off any difference in speed. Let's hope she didn't do too many U-turns!

-juice

jhs70 · December 2003

Can you tell me what you mean when you say the "tires scrub off any difference in speed"?

By the way, I doubt she did any u-turns!

Also, what fluids need changing? I don't want to look like a complete idiot (that I am) when I bring this baby in.

Thanks!

paisan · December 2003

Change the front and rear diffy (rear not absolutely necessary, but while you are at it might as well) and the transfer case.

-mike

ateixeira · December 2003

In 4WD, the front and rear axles are locked together, so they turn at the same rate. Around a curve, the axles normally spin at slightly different speeds.

But...locked in 4WD, they mechanically must spin at the same speed. That's called "binding". The tires lose grip by force, and scrub off any difference in the diameter of the turn.

That's why many systems are for slippery surfaces only. Permanent AWD systems that send power to both axles at all time usually have a center differential to limit slip, but still allow some slip around the curves.

-juice

jhs70 · December 2003

Thank you all.

sebring95 · January 2004

Does anyone know what exactly is engaged when the GM vehicles (mines a Tahoe) are in 2wd? Are the axles still spinning with the wheels or is everything disconnected? I know when I switch to Autotrac I can hear something engage and feel a little drag, but I don't know if it's locking in something to the transaxle or just locking in the hubs. My fuel mileage seems to be very good for a 4x4, but when running with Autotrac it seems to drop slightly.

renco · January 2004

How does a center diff attach to the T case and both driveshafts?

Whats the difference between a locking center diff and locking t case? Is one better than the other

russla · February 2004

Think of it this way.

A Transfer case is a box that takes in power from the transmission, (from an input shaft) and sends it out two ways, (front and back)
Transfer cases, may have any of the following functionality

Locked Hi and low range (not center diff)
Neutral
2wd
Full time 4wd(means the t-case has a center diff built-in, like Jeep select trac or discos)

however, the T-case doesn't have to have all settings. the LR for instance doesn't have a 2wd setting. AFAIK(not sure about defenders)

Some t-cases with diffs contain Automatically locking devices such as(haldex or quadradrive or some funky electric clutch thingamabob)or can be a VC rather than a true diff,

Some systems don't have t-cases, as the output of the tranny is split and goes straight to the back, and front (like a pilot or Audi quatro), so a separate box to split the power out of the tranny is not needed

The honda pilot system could be characterized as an auto engaging part time system, as there is no center diff. It is a front drive vehicle that locks into part time 4wd for short bursts

However, on most of the audi's, they have a center torsen diff. so all the wheels are getting power all the time.

Nomenclature is confusing, but it can be simplified this way,

Forgeting about hi and lo range for a second and whether there is a t-case or not.

a part time system locks the front and rear axles together, should be only used part time when slippy.

a full time system has a center diff or VC to allow for turns, etc and all wheels get power all the time, 50/50 split, 60/40 etc

a auto engaging part time system usually powers one set of wheels, and has no center diff, and when too much slip occurs, locks in the power to the other axle. a good example is a VC system, where one axle gets 90% torque and the other 10, it is essentially driven by one axle, and when slip, the vc locks and it becomes a temporary part time system because the front is locked to the back.

What adds to the confusion, is that full time systems can also be auto engaging part time, when they lock.

And the technologies used to transwer power between the directions are the same. VC's, electric or hydraulic clutches. (when auto diff locking is used)

So For me there are only three types of 4wd.

Manual part time (2wd, unless manually engaged)
Full-time,(with or without auto part time)
2wd with auto engaging part-time

I'm waiting for the system that has
Neutral,
Driver adjustable torque split center diff 5/95 to 50/50 split with an auto locking center diff

I don't know what Drew will call that.

Sorry for the long post

renco · February 2004

Ok, both the Hummer and G wagen are full time 4wd, both have a t case. The Hummer has a locking T case. The G has a T case with a center locking diff.

Whats the difference between the 2 and how does the G's center diff attach to the t case? Thanks.

paisan · February 2004

They both essentially are the same thing, just different systems, different names.

They both "lock" the output shafts front and rear to the same speed, same as most part-time systems and other "real" 4wd trucks.

-mike

renco · February 2004

How are they different systems? And how does the center diff attach to the t case and both driveshafts, or does the center diff not look like a normal diff?

paisan · February 2004

And the H2 probably has a Borg Warner T-case unit.

Essentially any t-case that has 4-hi/4-lo is a "locking" t-case or locking center diffy.

Full-time t-cases or center diffys have a mechanism (electronic or mechanical) that allow speed differentials between the front and rear drive shafts, this allows them to be driven on dry pavement. Part-time systems that can only be used on slippery/offroad situations, do not allow speed differentials between the front and rear drive shafts thus you need to have slippage in the system and in the part time case it's the ground-wheel point that is the slippage point.

-mike

renco · February 2004

Actually the G is made by Steyr Daimler Puch in Austria. And to clarify, I'm talking about the original Hummer.

Anyways I guess what I'm asking is what does a center diff look like. Does it look like a normal rear or front diff or does it look like something else because I can't see how something that looks like a rear diff can be attached to a t case and 2 driveshafts. Thanks.

paisan · February 2004

A Front or Rear diffy changes the direction of travel of the power by 90 degrees, the center diffy in something like the subaru or G-wagen controls the power in a 180 degree direction.

-mike

renco · February 2004

Have a pic?

paisan · February 2004

They look like a transfer case or internal to the tranny, varies.

-mike

renco · February 2004

So the locking mechanism for both the locking t case the t case with locking center diff are built into the t case?

Whats the advantage/disadvantage of each? Thanks.

paisan · February 2004

As far as I know they really are just marketing terms. Hummer calls it a T-case, MB calls it a Center Diffy....

-mike

russla · February 2004

Here are some pics of t-cases
http://www.rubicon4x4.com/transfercases/
some have built in diffentials(full time), some don't(part time)any t-case for a part time only system is locked by default, there isn't a center diff.

when the t-case contains a center diff, it may or may not be auto locking, for example the NV242 in some jeeps, and the system in Discos'needs the driver to shift from full to part time. when that happens, they've locked the center diff, and it is now a part time system.

so, if the vehicle can drive in 4wd on street, it needs a center diff. if the vehicle can be shifted into part time 4wd, then the center diff can be locked,

You need to consider what the mechanism is for locking the center diff.

Is it manual? is it hydraulic or electric, is it auto locking? Does it have lo range and neutral? those are the things to compare constrast.

I prefer fulltime systems that auto engage the center lock hydromechanically, like the jeep quadradrive, rather than electric systems, but if I was in the outback, I would want a manual system,

With that said, the electric locking systems and open center diffs with traction control work fine for the road, and as long as you're under warranty, it will be fixed if it needs to be.

cho7819 · March 2004

Anyone here know the specifics about honda/acura's VTM-4 AWD technology? I purchased a Pilot and truely am hoping that this system is worth the extra money over getting a FWD minivan I was going to get.

Thanks

paisan · March 2004

is better than FWD that's for sure. What it does is put some power to the rear axle below 19mph, then it puts 100% to the front axle until there is significant slippage on the rear wheels then it shifts power to the rear. While not the best AWD system out there, it's far better than FWD IMHO.

-mike

ateixeira · March 2004

It's worth while if you get snow often, or if you drive on gravel/dirt roads once in a while. I doubt you'll regret it.

-juice

ims · March 2004

Can you explain the Expedition 4WD, especially the switch setting of A4WD, which cannot really be an AWD, if the Expedition also has a switch setting for 2WD?
Also, is it a good tracking/traction 4WD when it is not in high mode? My point is, when in high mode, I assume all cars have the wheels locked together, so there aren't major differences one car to another. The issue is driving along the highway at 60mph in A4WD on the Expedition...

paisan · March 2004

It is a common misconception that 4-wheel drive hi/low locks all 4 wheels. In actuality it only locks 1 wheel per axle when in 4-hi/4-low on *most* vehicles.

Anyway Auto-4wd is not AWD. It is Rear wheel drive, once the system senses a wheel phyically slipping it will engage the 4-hi mode temporarily to regain traction. This is a re-active system which helps get you un-stuck but doesn't help much until you've already skidded or are partially stuck. A system that puts some power to both axles at all times is a better syatem IMHO.

-mike

carguy1234 · June 2004

Does anybody have details on how the Dodge/Chrysler AWD systems work in vehicles such as the new Durango, and the 300/Magnum?

Obviously they are rear biased, but I'm wondering if they are 100% rear unless slip is detected, or if they are always some percentage front, or variable by speed, etc.

Thanks

paisan · June 2004

Yes I'm interested in the system in the 300c/Magnum as well.

-mike

carguy1234 · June 2004

The only thing I've been able to dig up is that the system will be based on the MB 4matic. So that would put it at roughly 65% rear/35% front. I'd like to find some details direct from Chrysler though, but so far no luck. Anybody else?

ateixeira · June 2004

Let's see, from what I recall, M-B uses 3 open differentials and then lets the traction/stability control manage the power split. So it's pretty high tech and can send most of the power to a single wheel.

That'll be pretty tough to get one stuck. Unless you high center. The wheelbase is very long so the break over angle won't be that good, either.

-juice

steve90501 · July 2004

I have a Suburban LT 4wd with Stabilitrak and locking rear differential. How is the 4wd performance in snow conditions versus the truck with autotrac?

ateixeira · August 2004

Isn't Stabilitrak their stability control system by itself? I think it might be in addition to the regular 4WD system in use in the standard trucks. A nice addition, too.

For instance, the RWD STS has Stabilitrac, but not 4WD, obviously.

-juice

paisan · August 2004

Any kind of reactionary items such as stabilitrak will be less likely to save you from a skid than an AWD system sending power to both axles at all times...

-mike

vlad · September 2004

Hello,

I really enjoy reading your messages and I truly admire the level of expertise coming here from all corners of life. Recently I purchased my first SUV - a used 2001 Dodge Durango SLT with 77,000 km and 2WD, 4WD-high and 4WD-low driving modes. So, finally I also have some questions to ask:

1/ What goat-like surprises can I expect from 2WD Durango in the winter? Any tips how to improve its behavior?

2/ Since I live in the area of sometimes questionable winter driving conditions (Ontario), I wonder whether AWD mode couldnt be a better choice (in the summer, I often pull a camping trailer, nevertheless my questions relate to winter driving only).

3/ I would also like to receive your advice about when I should safely engage the 4WD-high mode while driving on-road (rain, heavy rain, snow, snow storm, ice?)

4/ And when exactly to apply 4WD-low

5/ Are skid plates really needed for a heavy snow? When mounted, do they create annoying obstacles for future maintenance of the vehicle?

Thanks in advance!

Vlad

paisan · September 2004

Skid Plates- Only need em if you are offroading, don't bother adding unless you regular drive through severe icey chunky conditions.

4-wd Hi- Engage it when the road is snow covered or ice covered, you could use it in rain, but I wouldn't suggest it. I would also get some good tires, they make all the difference in snowy conditions

4-wd Lo- Use this mode if you are stuck in a rut or need to break through a snow drift or pull out of a parking spot where you got snowed in by a plow etc. Don't use this mode above 25-30mph.

Hope this helps.

-mike

vlad · September 2004

Hello Mike,

Many thanks for your practical info, I really appreciate it!!!

Best,

Vlad

ateixeira · September 2004

If you have good tires on there, your truck will do better if only because of its extra clearance. I'd engage 4WD if the ground is wet or snowy, though, for the extra traction.

AWD takes the guess work away, because usually it's full-time or engages automatically.

Tips? Drive smoothly, no sudden inputs, respect that 4WD helps you go but doesn't help you stop. Maybe even practice a bit in a snowy parking lot to get used to the dynamics of that truck.

Edmunds Live (in 2000) had one and it walked right through their muddy hill off road section even in 2WD with good tires mounted.

-juice

vlad · September 2004

I should have mentioned that my 4WD is a part-time system only. That is why I'm asking about the appropriate road conditions. I don't want to damage it. I guess a heavy rain or light snow are not enough reasons to engage it, right?

Vlad

bp3959 · September 2004

Being a part time system you only want to engage it when needed, if you are driving on a snow covered road in 4wd, and turn onto a clear road switch out of 4wd right then and there. You must also be careful turing corners when in 4wd as if you don't have enough slippage you may damage something

ateixeira · September 2004

Yep, basically think of it as for slippery surfaces only. It should not be used at all on bone-dry roads, a U-turn will create some nasty binding.

-juice

paisan · September 2004

Yup, use 4-hi on any snowy road, flip it off when you hit dry road. Never use 4-hi or low on dry pavement.

-mike

one_post_guy · November 2004

What would a Jeep Wrangler be? And if I used a Wrangler for off-road 30% of the time and on-road for 70%, would it be one of the ones that would end up giving me complications.

ateixeira · November 2004

Wrangler has locking 4WD for dry surfaces only, I believe.

If you are on-road 70% of the time, and it's dry, you should disengage 4WD. You probably don't need it anyway.

-juice

twinmom04 · January 2005

After reading all of this I am nervous. My husband and I purchased a 2004 Expedition 2wd yesterday. Is this a really bad decision???

stever · January 2005

People don't usually write in when everything is going well with their cars - human nature to complain I suppose. There wouldn't be so many Explorers on the road if they were all junk.

So relax and enjoy your new ride!

Steve, Host

ateixeira · January 2005

Expeditions are very heavy, so they're a lot better than, say, a RWD pickup with an empty bed.

-juice

bcmalibu99ls · January 2005

Hi, all!

Is the locking differential in an AWD Chevy Express Van activated manually by a button on the dash, or does it engage automatically? If automatically, in what conditions? Is it located on the front axle, rear axle, or in the middle?

Thank you!

pjb3 · January 2005

Hi all,

I live in Denver with 3 kids and need to know the best buy for a used vehicle. We currently have a Mountaineer but it doesn't have 3rd row seating so it's too small but it is AWD. I am looking at the Navigator but my husband wants a mini van. I'm not a very experienced snow driver so is AWD mini van better than 4W Navigator? Any other thoughts for us?

Thanks!

ateixeira · January 2005

A 4WD SUV will have heavier duty mechanicals, low range gearing, and skid plates. Plus ground clearance is generally higher.

An AWD minivan would likely suit the needs of most folks, light-to-medium duty stuff. And it's easier to live with the rest of the time.

The Sienna has an AWD option, so do the Grand Caravan and the Montana.

Then there are cross-overs that try to blend the two, cars like the Freestyle, Pacifica, and the upcoming Subaru B9 Tribeca.

-juice

cromero · February 2005

I'm beginning to experience my SUV "slipping" on wet pavement. Its kind of an strange feeling--as though I don't have control especially going around corners. I'm taking it to the shop tomorrow. Can anyone tell me what to expect?? Should I take it to the dealer or a good mechanic (the shop).

Thank you in advance -

cromero

ateixeira · February 2005

Presuming the tires are good, it could be a faulty RT4WD system. I believe when they overheat or otherwise malfunction, they basically are disabled. So you might be operating with just the front axle.

I believe the fluids are serviceable, so let's hope it's just that simple - a fluid change.

-juice

4WD and AWD systems explained

Comments

Helpful Tools

Most popular vehicles