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4WD and AWD systems explained



  • wwestwwest Posts: 10,706
    edited August 2012
    Since turning off TC results in the car reverting to a simple ONE-WHEEL drive he got exactly the results one would expect.

    With TC on the engine dethrottling would have been so severe the engine torque wasn't enough to pull that wheel over the "hump".

    TC off functionality is pretty much restricted to "feathering" the throttling right up to, but not beyond, the point of loss of traction. Wheelspin, rocking the car back and forth, to get unstuck, can also be used constructively.
  • dcm61dcm61 Posts: 1,457
    edited August 2012
    Did you turn off the traction control?

    Wake up before you post, Bob :P ... 2005 Outback 2.5L does not have traction control.
  • rshollandrsholland Posts: 19,661
    What's the little yellow light that shows a car skidding on the instrument cluster? I can turn that off in my car, and you're saying that can't be turned off in the Legacy 2.5?

  • xwesxxwesx Fairbanks, AlaskaPosts: 8,391
    VDC wasn't available on the 2.5L models until 2008 as an option, then standard in 2009.

    Even so, the car should have had traction in the front axle unless the center differential had gone out. It's unusual that he would have power to the rear axle under that circumstance, though, as typically it is the rear axle that would stop working as opposed to the front axle. In 2007, 08, and 09, the rear axles on the Outback were limited slip, but they were open prior to (and after) that, so under limited traction conditions they are generally 2wd (one on each axle).

    I managed to get my '96 Outback stuck more than a few times in deep snow by taking too much weight off the wheels, and it would always default to spinning the left front and right rear tires. :blush:
  • wwestwwest Posts: 10,706
    "..does not have..."

    That would be strange indeed. Even as far back as 2001 it ws well understood industry wide that TC could prevent a LOT of loss of control accidents rising from the use of to much engine torque applied to the drive wheels.

    Prior to that period TC was found mostly only on "pure" FWD vehicles, non-AWD versions.

    That was one of the aspects facilitated by DBW, the engine could be dethrottled without compromising the catalytic converter.
  • wwestwwest Posts: 10,706
    VDC probably refers to stability control INCLUSIVE of TC, the 2008 timing would be about right for that happenstance.

    Not knowing anything at all about the specifics of this particular "AWD" system in question the description, only the wheel with traction spinning freely, indicates a fully open center diff'l with the TC turned off.

    "..were limited slip..."

    But more likely than otherwise "simulated" limited slip using TC braking.

    Sounds as if your '96 had some method to automatically "lock" the center diff'l or PTO, VC's were commonly used for that back then. Once TC was implemented those methods because useless, non-functional.
  • xwesxxwesx Fairbanks, AlaskaPosts: 8,391
    But more likely than otherwise "simulated" limited slip using TC braking.

    No, they weren't, they were viscous LSD. After 2009, however, they did go to open front/rear differentials using the VDC system to compensate. It is a poor compensation, at best.

    The WRX retains a rear LSD, while the STi has both front and rear LSD.
  • dcm61dcm61 Posts: 1,457
    edited August 2012
    In 2007, 08, and 09, the rear axles on the Outback were limited slip, but they were open prior to (and after) that,

    2000 (w/AWP) and 2001 to 2006 2.5L also had rear LSD. Not sure which years/models of H6 had rear LSD.
  • wwestwwest Posts: 10,706
    I know of a few high end marques that use VC(s) to implement a form of rear diff'l LSD but I suspect in Subbies case you mean center diff'l or PTO drive "locking" is done using a VC.

    "..The WRX retains a rear LSD,..."

    That, an actual mechanical LSD, would be a bit of foolishness since the current WRX model has no means for driving the rear diff'l except under TC braking, along with full engine dethrottling, of the front wheels. Probably not even then unless you use an extended period of TC activation.

    The above assumes the WRX's VC center "locking" is at least minimaly functional, not something I have any faith in at all.

    "..STi...has both..."

    I'm willing to bet you good money that if the Sti has a rear LSD it is of a "virtual" nature using "differential" rear wheel braking.

    And front LSD...??!!


    Those are found ONLY on the most robust off-road vehicles (Hummer, etc.)wherein the driver is expected to be fully aware, expectent, experienced, with what a front LSD will do to your fingers, thumbs, etc.

    I think you will find that the Subbies version of front LSD is via TC braking and of BOTH wheels simultaneously even if only one wheel is slipping. To do otherwise would often result in yanking the stearing wheel right out of the hands of an unwary driver not expecting the resulting hard "TUG".
  • wwestwwest Posts: 10,706
    "Virtual" or "hard" LSD...?
  • xwesxxwesx Fairbanks, AlaskaPosts: 8,391
    If you had any clue, aside from the informing of your extreme bias, you might actually develop some credibility. As it is, you shot it long ago on this topic. ;)

    I feel for the original poster. He thought he might actually receive some assistance when he posted on this thread instead of your strongly opinionated blathering. :sick:
  • wwestwwest Posts: 10,706
    So, what's YOUR answer/solution/explanation for the situation the Op experienced...?
  • wwestwwest Posts: 10,706
    You said....

    "..Even so, the car should have had traction in the front axle unless the center differential had gone out..."

    If we take the Op at his word then the alternate explanation, and the most likely one by far, is that his Subbie had a standard open differential, and if TC was available it was turned off.

    Otherwise, like you say, the "car should have.."

    Awaiting a better explanation.....
  • larryvlarryv Posts: 9
    So I'm the original poster and have reviewed the various replies. And, yes, sometimes I enjoy snarky responses, but in this case, I'm looking for some serious thoughts and info from people more knowledgeable than me. My original purchase material indicates this '05 2.5L basic OB has "full-time AWD" plus a "limited slip REAR differential," but I see no reference to traction control or other features that some of you mention, nor any buttons or switches for same in the car. From some of the Sub websites, I found these demos and comparisons of 4WD vs variations of AWD:, and Plus, a drawing that shows a center differential and a rear one in the typical Sub AWD system. So, perhaps the videos are of newer models that have additional AWD capacities that mine lacks, but who can confirm this? Also, a Sub mechanic told me "off the record" that the solution to my predicament would have been to open the fuse box under the hood and to put a fuse into the vacant slot marked "FWD." He said that doing so would have turned off the "AWD" and made the car only FWD. So, in my situation, with the front wheels firmly on the ground, they would have pulled the car out because no power would have been sent to the rear wheels. Don't really want to try this, but is this mechanic blowing smoke or legit? Thanks. LRV
  • wwestwwest Posts: 10,706
    "full-time AWD" "limited slip REAR differential"

    That is virtually the same wording that "came" with my '01 R/awd RX300.

    Technically speaking you and I both actually do have "full-time AWD".....

    A fully open center diff'l will provide equal drive to both front and rear axles just as long as both are encountering roughly equivalent traction loads. But once a single wheel loses traction it will spin freely thereby limiting the torque level that is available to "other" wheels.

    For my RX that where TC "steps in", activates and adds artificial traction, via braking, to that slipping wheel. Regretably it also instantly dethrottles the engine.

    These days not even Porsche normally uses a mechanical LSD, TC braking only.
  • xwesxxwesx Fairbanks, AlaskaPosts: 8,391

    The mechanic was not blowing smoke. The car as a FWD fuse that engages a solenoid in the center differential which should, if working properly, disengage the center diff's clutch pack to send all the power to the front differential only. I think this fuse is available on both the manual and automatic transmissions, but I've only ever looked for it on automatics. This may be an important point because the two transmissions engage different AWD systems.

    I think you said your car was a manual transmission? If so, it has the "continuous AWD system," and the default power distribution is 50/50, with the ability to send power up to approximately 90/10 to either axle depending on traction conditions. The center differential has a viscous coupling, which is not the equivalent of an open differential, but instead will transfer more power to the axle with most traction should the other axle be spinning faster.

    The automatic transmission employs the "active AWD system," which is a more complex unit in that it uses the input of multiple sensors to the TCU to determine the power distribution front and rear. The default is 90 front, 10 rear.

    If you do have a manual transmission, you have one of two things going on. Your fluid(s) may be of a type or age that is allowing the viscous couplings to slip, meaning the power is going to take the path of least resistance, or your viscous couplings are both (center diff and rear diff) worn out. I'm going to vote for the first option as the most likely given the age of the car.

    You could test the system's operation by putting the car up on blocks (all four corners) and running some resistance tests on the various tires/axles to see how the car responds. What you should see if it is running properly is all four tires spinning initially, the non-resisted front tire spin when you apply resistance to the opposite front (it doesn't take much resistance to get an open differential to transfer power), and resistance of the tires to stop spinning on the rear axle when you apply resistance to one tire on that axle.

    More reading:
    AWD System Summaries on

    I hope that helps you to diagnose your situation, Larry.
  • larryvlarryv Posts: 9
    Thanks xwesx. Yours was the most detailed response and best suggestions. Unfortunately, I don't have any means to put the car up on blocks to run the suggested resistance tests. But I could take it to the dealer/shop. Is the age and condition of the fluid in the center VC supposed to be checked with regular servicings? And, first apologizing for being such a car mechanic peasant, exactly how is the VC supposed to send more power to the wheel(s) with the greater traction? If the VC fluid is shot, how does that prevent sending more power to the wheels with greater traction? I don't believe my car has traction control. Nothing in my paper work mentions it and I don't see any button on the dash or console marked as such. I do a lot of fishing and drive on non-paved roads a bit and so want to make sure all the AWD features of this car are working properly and what to do if I ever fall into a similar situation again. Thanks. LRV
  • xwesxxwesx Fairbanks, AlaskaPosts: 8,391
    This is a good explanation on the operation of a viscous coupling LSD: Viscous Coupling on How Stuff Works

    Now, this says it is in a sealed housing; I am not sure whether that is true or not for the Subaru center differential on the manual transmission. The car calls for GL5 gear oil in both the transmission and the rear differential, and the transmission is housed inside the transaxle, which includes the transmission, center differential, and front differential. I know the front diff and transmission use the same oil and are open to each other in terms of fluid transfer, and I think the center is as well. The VC in the rear axle is open to the fluid within the main housing.

    Basically, if the friction qualities of the oil are too low, you'll get excess slippage. When it is on the edge of "good enough," one might notice the slipping through a "chattering" feeling/noise within the car due to it engaging/releasing in quick succession. If this is due to the oil qualities rather than age, one can often simply add friction modifiers to the oil to get it to stop slipping. I think the service interval calls for 50,000 miles or more between fluid changes, and often people let it go much longer than that. The older the fluid, the more thermal breakdown the oil has experienced and the less it is able to deal with high-shear-force situations. In my opinion, even 50,000 miles is a questionably long period of time for the stock fluid, especially if you drive it in conditions that are likely to engage the VC frequently.

    For instance, the oil in my Forester, which is a synthetic that calls for 75,000 miles or five year intervals, was obviously well-used in the transmission/center/front diff when I changed it last month. I have 45K on the car now, and installed the oil at 2.5K. I didn't feel bad about changing it, that's for sure! I live in Fairbanks, Alaska, and slippage situations are frequent during the winter months (about half the year, Oct to April). The fluid in the rear differential, in contrast, looked like I had just installed it even though it was the exact same age. On my '10 Forester, the rear is an open differential.

    Now, if the VC isn't engaging, it doesn't necessarily mean that the VC is bad, it simply means it isn't working with the fluid it has available. Fresh fluid, with, possibly, friction modifiers added, and it is good to go again. If you've never had the oil changed in the car, you might try that first just to see if it helps. It is a terribly easy chore to replace the oil, and takes about 1.5 gallons total (check your owner's manual for transmission and rear diff capacities). You will need a Torx T60 or T70 socket in order to pull the transmission plug (it is a T70, but the T60 works too and is often much easier to come by), and a long-necked funnel is helpful for the transmission fill, while a hand pump for your gear oil bottle is helpful for the rear differential.

    If you want an easy way to test whether the system is working well or not, take your car and a friend (in a different vehicle) to some place with sand, and drive out on it. If your AWD system is not working correctly, you're going to get stuck in short order. ;) If you do get stuck, just strap up to the other vehicle and get pulled out.

    Finally, your car does not have the "VDC" system in it. You don't have any buttons that toggle traction control. The traction control system in that car is purely mechanical: By default, the power is split 50/50 to front and rear axles. Any change in power split, both between axles and within the rear axle, is reactionary based on the speed at which the drive shafts connected to the VC units are spinning. VDC was not added to the manual transmission cars until 2009. Prior to 2008, it was only available in the "3.0" (H6) Legacy/Outback.
  • wwestwwest Posts: 10,706
    Viscous Clutches/couplings are generally quite well sealed, heretically sealed since at time they operate with a great deal of internal pressure. Only a professional shop would be able to change out the fluid.

    Post about 2002 VC's were so severely derated absent TC you had no real "locking". From what I can find out at Subaru your '05 has a standard open differential with a VC between the 2 "output" shafts. If there is a sustained difference in the speed of those 2 shafts, front/rear, then the VC fluid should stiffen dramatically thereby "locking" the 2 shafts together.

    On a 4 wheel dyno the VC in my '00 F/awd RX300 would take about 15 seconds to re-apportion engine torque ~70/30 F/R. The '01 came with TC and VSC thereby making the VC useless and as such it was dropped entirely from the RX330 model.
  • steverstever YooperlandPosts: 39,982
    edited September 2012
    "It's not even a debate," says Jim Vurpillat, global marketing director for General Motors Co.'s Cadillac brand. Luxury-car buyers expect all-wheel drive to be available on any serious luxury sedan, he says, which is why Cadillac is offering the technology on its new compact ATS and large XTS sedans, as well as its midsize CTS model.

    What is debatable is whether all-wheel drive delivers enough safety and performance benefits for the $2,000 to $3,000 extra charge car makers typically add to the price tag, as well as the mileage penalty that the extra weight of all-wheel-drive hardware still exacts in most cars.

    Consumer Reports compared the slick weather performance of a Toyota Corolla to a Land Rover and concluded that the Corolla—with its thin tires and light body—stopped much more quickly in snow than the burly, all-wheel-drive, luxury SUV."

    All-Wheel Drive Goes From Novelty to Necessity (Wall St. Journal)

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