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Toyota 4WD systems explained

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Comments

  • hdfatboyhdfatboy Posts: 324
    This discussion is not about fairness. It was about information and sources. I appreciate you sharing the Toyota product brochure for the 2005 Sequoia as that is the first resource of any type that depicts the Sequoia in that year having a VF4AM transfer case. Obviously the other sources were incorrect.

    The Toyota info you shared does clear the matter up regarding the type of transfer case used in the pre-08 Sequoias. Curious that so many other sources would either be incorrect or would not have referenced that transfer case in the earlier model years.

    Based on your access to the Toyota site can you clarify which model years the Sequoia, LC, LX470 and the 4Runner each transitioned to a Torsen center LSD from an open center differential?
  • wwestwwest Posts: 10,706
    "..It would make quite a racket after it fails...."

    No, it wouldn't/doesn't. The spring pre-loaded clutch type, just as would a regular clutch in a stick shift, simply ceases to work once the friction surface has worn off. The VC type might "lock" in failure mode but since it is designed for "failsafe" operation that is rather doubtful.

    So a driver probably wouldn't notice the failure until the moment of need arises.
  • wwestwwest Posts: 10,706
    "..Once you are moving you wouldn't want ALSD (A-LSD) you would want TRAC on to keep stability.."

    Someone, please, correct this if you have information otherwise but I think I have read documentation indicating that A-LSD is a sub-mode of TRAC. Whereas TRAC's default, always on, mode will instantly apply braking to a slipping wheel and simultaneously dethrottle the engine, if you put the system in A-LSD mode it will allow a certain level of wheelspin/slip just as one might sometimes want to get unstuck.

    I think, also, A-LSD capability is only to be made available on RWD or R/AWD vehicles.

    For many years now owners have complained about this aspect of TRAC, no ability to use wheelspin, say for rocking the vehicle back and forth to get unstuck, out of a mudhole, and now Toyota is offering A-LSD as a possible solution. Previously the only solution was to somehow disable VSC/TRAC altogether.

    "..want TRAC on to keep stability."

    Methinks you meant to say VSC.
  • wwestwwest Posts: 10,706
    "That being said, it has disadvantages to a mechanical LSD in that slip has to occur before it goes into action...."

    Assuming a VC, Viscous Clutch, implemented LSD is considered "mechanical" your statement is wrong, a VC must experience a period of slippage before it will "tighten" the coupling coefficient.

    "A mechanical Torsen system.."

    Doesn't this add to the confusion? I have never considered the Torsen system to be in the LSD "realm", more of a torque control/distribution system rather than an LSD.
  • hdfatboyhdfatboy Posts: 324
    I was referring more to a Torsen type which would not operate seemlessly after failure.
  • hdfatboyhdfatboy Posts: 324
    Almost all references to the Torsen differential, including the manufacturer's refer to it as a LSD.

    You description of the viscous liquid diff is accurate and one of the reasons its not the best mechanical design for transfer of torque under severe conditions. It can be ineffective when operating in sand for example in transferring torque from the rear to the front of an AWD vehicle. It excels however at providing a very smooth torque transition when operating on slippery road surfaces.
  • wwestwwest Posts: 10,706
    "There is no reference anywhere I've searched that the Sequoia had a lockable mechanical center LSD prior to 2008."

    I thought the Sequoia ALWAYS had a lockable mechainical center diff'l for 4WD/4X4 mode...??

    And yes, while that was otherwise an "open" diff''l (or transfer case equivalent) I thought they used TRAC braking in AWD mode to effectively have a center LSD..?

    And isn't that basically the same system used quite effectively on the RX330/350, the HL and the Sienna AWD systems today, the VC version of those systems having been dropped prior to the '04 model year.
  • wwestwwest Posts: 10,706
    The way I read all of this is that Toyota has adopted the Torsen "C" center diff'l design so the engine torque to the front would/could be limited to not more that 40%. The "old" system, TRAC braking, could clearly result in 100% of the engine torque to the front driveline therefore quickly converting a reasonably beniegn handing rear torque biased AWD vehicle into a wildly uncontrollable, SCARY, FWD, F/AWD vehicle.
  • Torsen center diff? Atrac on the front and rear diffs? I'm going from an 06 4runner which has an amazing 4wd system. I don't offroad much, but I want something thats capable in the rough weather. I knwo it wont match my runner, but I don't want to get stuck in the snow. Please advise gurus.
  • hdfatboyhdfatboy Posts: 324
    "I thought the Sequoia ALWAYS had a lockable mechainical center diff'l for 4WD/4X4 mode...?? "

    I thought I knew the answer until 2Toyotas shared information from the "Pay for" toyota website. While the Sequoia has always had a Lockable ctr differential, I thought the addition of the Torsen LSD (in place of an open diff) was new for 2008. He shared information describing a Torsen Ctr Diff in the 2005 model. Therefore I do not know when Toyota transitioned from an open diff to the Torsen LSD ctr diff. for the Sequoia.

    I was hopeful that 2Toyotas could look at his website to determine what model year the upgrade actually took place. (As well as when the upgrade to the Torsen occurred for the LC, LX470 and 4Runner.)

    Most external information sources suggest the Torsen was added to the drivetrain for these vehicles in 2008 with the 4Runner getting it earlier. These sources must be wrong based on his review of the Toyota site.
  • 2toyotas2toyotas Posts: 104
    The 4Runner was redesigned in 2003 and began using the VF4AM transfer on the V6 and the VF4BM on the V8. Both have the torsen center diff, with the V6 having a 2WD option.

    The Sequoia used the VF3AM transfer from it's introduction in 2001 until 2004. It has an open center diff. In 2005 the Sequoia started using the VF4AM transfer already in use on the V6 4Runner, which has the torsen. For 2008 the Sequoia is now using the JF3A transfer which also has the torsen.

    The Land Cruiser/LX470 used the HF2A transfer which has an open center diff. from 1998 until 2007. For 2008 the Land Cruiser/LX570 will use the JF2A which has a torsen center diff. One interesting thing is the Land Cruiser/LX used the HF2AV center diff until 1997 which had a Viscous center diff. They dropped it in 1998 for an open center diff.

    ATRAC began in the Land Cruiser/LX in 2000, and the 4Runner and Sequoia in 2001.

    The torsen center diff is definitely an improvement over an open center diff. I had an 03 Sequoia and then traded for an 05, and the big difference is on inclines, with the open diff ATRAC would brake up to 3 wheels at a time and 1 wheel would get power., it was not enough to get the Sequoia up inclines, you would need to lock the center diff. With the torsen both axles always get power, the front up to 53% and the rear up to 71%. ATRAC works side to side on both axles, while the torsen puts it front to back, so it climbs inclines with ease, without the need to lock the center diff.

    In my experience with this system, on snow and ice unlocked with the torsen working, ATRAC brakes spinning wheels and also cuts power when needed, and keeps the truck in full control. In sand or mud it is better to lock the center diff, this way power is split 50/50, and ATRAC only brakes the spinning wheel, but does not cut power, so it keeps you moving.
  • 2toyotas2toyotas Posts: 104
    Wwest you are right on. My point was if you are going down a highway at 50 mph you wouldn't want ALSD on you would want the default TRAC on. What I meant by TRAC for stability was ALSD does not really cut power, so if you were driving along in ALSD mode and hit ice on one rear tire your back end could slide out, but in default mode TRAC, it would also cut power to keep you straight, hence stability.
  • hdfatboyhdfatboy Posts: 324
    Very helpful summary. Thank you for the timeline info.

    Just to summarize...the Open to Torsen conversion for the center diff of these models took place in the following years:

    4Runner - 2003
    Sequoia - 2005
    LC/LX470 - 2008

    Is there any other Toyota made 4wd vehicle with a torsen ctr diff?

    You also shared that the Sequoia and LC/LX changed transfer cases in 2008. The Sequoia from VF4AM to a JF3A. The LC/LX from an HF2A to JF2A.

    Do you know what the primary difference is between the previous transfer case and the Sequoia's new JF3A? Also do you know what the difference is between the JF2A and the JF3A? Is the primary difference in the transfer cases between the LC/LX and the Sequoia the new "Hill Start Control" and the new "Crawl Control" which are in the LX/LC but not in the Sequoia?
  • nedzelnedzel Posts: 787
    The 2008 Highlander will do fine on trips to the mall in the snow. I always advise snow tires (which I have on my 2003 4WD 4Runner), since AWD/4WD systems help you go but don't help you stop. Whether or not you get stuck in the snow is all up to you -- don't drive in snow that is deeper than the ground clearance, use some caution, brake and steer gently and you'll be fine. I never got my 1987 Acura Integra stuck in the snow and it was far less capable than an AWD Highlander.

    The 2008 Highlander is significantly larger and heavier than the previous version, so don't expect to get significantly better mileage than your V6 4Runner.
  • wwestwwest Posts: 10,706
    NO torsen in the 4runner after 2003, apparently one year only...!

    Cannot find any documentation that the torsen was used after 2003 but lots of stuff indicating TRAC was used in its place. Same for the Lexus GX & LX series.

    Strange, very.

    Lot of component pictures/diagrams/breakouts of the VF4AM transfer across the years and models but only the 2003 4runner shows a torsen.
  • nedzelnedzel Posts: 787
    WWest: that is not correct. Go to: http://www.toyota.com/4runner/specs.html

    Look down at drivetrain: "Multi-Mode 4-wheel drive with Torsen® [1] limited-slip center differential with locking feature" for the V6 and "Full-time 4-wheel drive with Torsen® [1] limited-slip center differential with locking feature"

    The 4Runner drivetrain hasn't changed since 2003.
  • wwestwwest Posts: 10,706
    Yes, I know what all the advertising says, hardcopy and on the internet.

    Clearly, I may be wrong but the information at techinfo.toyota.com is pretty detailed, detailed well enough that a good mechanic could use it to overhaul the transfer system.

    The word "torsen" is not used beyond 2003 in any of the documentation and I have no idea why that would be unless it isn't used or there is some legality involved.

    Additionally if you read the document ion for traction control it appears that a torsen diff'l would be of no use since the TRAC system would quickly brake any wheel (and dethrottle the engine simultaneously) that exhibits spin or slip.

    As somewhat of a parallel, I bought a 2001 AWD RX300 because it had HID VSC/TRAC and a viscous clutch across the center diff'l to provide torque to the rear driveline if front slippage occurred.

    I quickly discovered that the VC was probably useless since the TRAC system activated at the first sign, seemingly the very instant, wheelspin developed.

    Apparently Lexus discovered the same thing as the VC was discontinued for the RX330 series as was announced in NCF, New Car Features for the new 2004 RX330.

    But guess what....??

    Lexus continued to advertise, hardcopy sales brochures, on the internet, and in various PR pieces, that the RX330 had a VC, Viscous Clutch when the shop/repair manual indicated otherwise.

    Lexus finally admitted, in writing (email), that the RX330 did not use a VC and apologized for the mis-information.

    And now here I am arguing with Lexus as to whether or not the new(er) RX350 uses the VC. The advertising says yes but the shop/repair manuals say not and the TRAC use indicates it might be useless even if it is installed.

    Sorta of like the issue of a torsen with TRAC in the 4runner.
  • wwestwwest Posts: 10,706
    Read the material, however sparse, on TC, Traction Control, on the provided link.

    If you have a torsen diff'l in the 4runner wouldn't you need to disable TC in order to put it to use...??
  • greengreen Posts: 15
    A torsen center diff, compared to an open center diff, enhances brake TC by multiplying the effect of a slipping, braked wheel by more than 1:1 across the axle diff to the other wheel. The amount depends on the bias ratio of the torsen.
  • wwestwwest Posts: 10,706
    Yes, but doesn't that seem counter-productive...??

    The front wheels are already slipping (abet at a rate limited by TC) an obvious indication of a slippery roadbed surface, and the Torsen will MULTIPLY the torque at the rear wheels (torque sustained via TC braking), or vice versa.

    I would think one would want LESS torque at the opposite wheels, not MORE.
  • wwestwwest Posts: 10,706
    The linked YouTube video gives certain evidence that the Highlander AWD system is quite clearly heavily biased to the front, as is probably the RX3X0 and the Sienna.

    But how...??

    http://youtube.com/watch?v=t09ExAUgtyE&feature=related
  • hdfatboyhdfatboy Posts: 324
    All 3 of these "4wd" vehicles (Highlander AWD, RX330 and Sienna AWD) are built off a variation of the Toyota Camry drivetrain which is FWD. All 3 of these vehicles have open center and rear differentials (unless someone has any new information on the '08's that they now have a LSD ctr diff)

    In reality these are FWD vehicles that transfer power when there is slippage using the brakes from the front wheels that are slipping to the rear one's that aren't. Generally these systems cannot transfer torque over a certain speed such as 35mph. Once you're over this speed, the brakes can no longer be applied to transfer torque to wheels that are not slipping and you essentially have a FWD vehicle.

    This type of system will generally use up brake pads more than a vehicle with a center LSD or Viscous liquid ctr diff. While Toyota refers to the system as "AWD", it would not meet the definition of AWD by most other manufacturer's that use this term such as Subaru, Audi, MB, Volvo or GM, however it is nice marketing even if its not a very effective 4wd drivetrain.

    Unless Toyota has added a Viscous Liquid center differential or a limted slip differential to these 3 vehicles, they would generally not meet the definition of AWD used by all other manufacturers of AWD vehicles.

    Since there's no operator controls it wouldn't meet the definition of a typical 4wd system either. Best way to think about this system is that its a FWD vehicle with traction control that transfers power to the rear wheels when the front wheels slip, but only at lower speeds and only a small amount of the available torque as shown in the video. Better than FWD alone but not nearly as good as a real AWD system or a 4wd system.
  • wwestwwest Posts: 10,706
    But I would think that TRAC could brake the front wheels enough to "force" enough torque to the rear to get the car in motion even with the obstacle blocking the rera wheels.

    Or is it just a matter of poor or inadequate TRAC design?
  • hdfatboyhdfatboy Posts: 324
    Using brakes to transfer torque is a very delicate balancing act. Braking too much will stop your momentum before the rear wheels have picked up the torque to keep the vehicle going. Therefore the software has its limitations and will only reduce the front wheel torque to a certain amount and only under a certain speed.

    The video shows the significant limitations of using braking to transfer torque across an open differential. As the video shows, a system such as Toyota's that is solely based on braking to transfer torque is a poor substitute for a mechanical LSD or viscous liquid CTR diff.. That's why the RX330/350 can't climb a slippery incline even when the front wheels slip and the rear has traction.

    If the vehicle was moving at speed, the system wouldn't even attempt to transfer torque in order to avoid causing an unstable driving situation, not to mention how much additional wear and tear would be place on the front brakes if the system attempted to use front wheel braking to transfer torque at higher speeds such as a highway ramp or even a high speed curve in a major highway.

    This type of system is simply Toyota's way of keeping drivetrain costs down and giving the marketing department a "claim" that most consumer's will never understand or see the difference with real AWD and 4wd drivetrains. Their approach allows for higher prices with lower costs and a better margin on their "AWD" vehicles. Smart buyers will know the difference and see this drivetrain as nothing more than a glorified traction system for limited road situations.

    Toyota loyalists will undoubtedly have a different perspective however the videos tell the real world story. Here's another example showing the difference between the Tundra's rear open diff with "electronic torque transfer via the brakes" vs the mechanical auto-locking rear differential in a Chevy pickup. A rear LSD would perform somewhere between the 2wd Tundra and the 2wd Chevy in this video. http://www.youtube.com/watch?v=Z8PpZF77tgk The video was filmed at the Eaton proving grounds which makes the Eaton auto-locking differential used in the 2wd Silverado pickup.
  • greengreen Posts: 15
    You may or may not want more depending on conditions, but the torsen-C center will require less frequent TC engagement and less brake pressure when it does engage, compared to an open center.
  • I have an 07 4R V6 4x4 and want to say that the last two paragraphs of your post include absolutely the clearest and simplest description of my 4 wheel drive system I have read and that includes the owners manual!
    Thanks, DQ
  • 2toyotas2toyotas Posts: 104
    I did a little research on this whole topic. First the Tundra, if you go on Eaton's site they compare the GM's to a Ford F150 and a Dodge Ram. On the rollers, both the Ford and the Dodge slide back, with a mechanical limited slip rear differential, in 2WD and in 4WD. They can not make it up the rollers. The Tundra in ALSD mode did make it. They also state on their site that most trucks in 4WD can not get through the twisted ditch. It is funny that they only tested the Tundra in 2WD in ALSD mode. ALSD mode is mainly when you need some wheel spin in sand or loose conditions. They never showed it in 2WD normal mode, which is with TRAC on. Same thing on the ramp. TRAC would have slowed the spinning wheel and cut power to send power to the other wheel, I don't know if it would have worked, but it was never mentioned. One other thing is the Eaton locking rear diff. works in 2WD and 4WD in high and low range, but only up to 20 mph, after that it acts like an open diff. If it were me I would take the Tundra with a Traction system that always is on, and put it in 4WD when needed, you now have ATRAC working on the front and rear axles, and they didn't dare show that, because it would have ate up that whole test track!!

    As far as the Highlander, I would think it would do a better job than that, it looked like the TRAC system was disabled, but i can not prove that. I do agree, I would like to see the Viscous Coupling back in the center diff. My first Sequoia was an 03 which had a lockable open center diff. On steep inclines ATRAC would have to kick in to much and it would stop momentum. I would have to lock it, and it would be fine. I then bought an 05 which had the Torsen center diff, which corrected that problem.
  • HELP, HELP !! - I keep seeing "unlock center diff" - but how do you do that???? I am stuck in C Diff Lock, and will have to have my car towed - there does not seem to be any toggle/switch to turn it off. THANK YOU !
  • nedzelnedzel Posts: 787
    Have you read the owner's manual?

    In the 4Runner, the center diff is locked and unlocked by a push-button on the dash, low and to the left of the steering wheel. In the 2008 Sequoia, I believe it is to the right of the steering wheel. Look at this picture: http://www.autoblog.com/photos/2008-toyota-sequioa-limited-4x4/686920/

    See the button in the middle that has a little drawing that shows 4 wheels and an x in the middle? That locks and unlocks the center diff. You might have to put the car in neutral to unlock the center diff.
  • mike805mike805 Posts: 33
    Your article by Toyota on the Torsen ctr diff. is very good but requires a lot of study to understand. This would answer a lot of question if people took the time to understand this article. I'll have to give some thought to just how the gear angles provide the friction for limited slip and don't just wear out.
    I had thought mechanical limited slip was generally done with friction disks as was the case in my '60s GTO and Corvette. In that case I'm not sure but what using the brakes isn't a better solution. It is sure easier to replace brake pads and in those cases the limited slip did wear out.
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