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Toyota 4WD systems explained

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  • I have a 2000 4Runner, but I have the Limited so maybe that's why I have the AWD button on the side of the 4wd shifter. Anyway, I don't have any button on the dash, that's all controlled by the 4WD shifter. As for what happened, the only explanation I can come up with is that you were in AWD mode, but you waited too long to shift into true 4WD and the center diff didn't engage. When you shift into H4, a green dot in the 4WD indicator starts to flash - it turns to steady when the center diff fully engages. My gut feel is that it wasn't engaged - if it was, then at least one of the rear wheels should have turned. I still don't know what the hell the AWD is actually doing (look back on this thread - nobody has an explanation) but I have a hunch that a light tap on the brake pedal to slow the spinning wheel _might_ allow the other wheel on the same axle to get some power. Oh, and L4 wouldn't have any advantage here over H4 except possibly allowing you better modulation of the accelerator to keep from "breaking out" a wheel with a tendency to spin.
  • I don't know about the 2000 but on my 2006 the center differential lock only engages in L4.
  • Hello. I have been a silent follower to these edmunds forums for some time now & decided to fianlly particiapte. My wife & I just purchased a 2007 V6 Highlander with the so-called AWD. I have a question with the 2007 Highlander AWD system. There seems to be a bit of controversy (at times combative) on how this system works. And so I was wondering if we could clear the air to this subject matter. Forgive me if this discussion is redundant.

    2toyotas made mention that for 07, the viscous coupling was put back into the 07 model year Highlander. (This is interesting since the 07 model year is going to be a short model year with the introduction of the 2008 redesigned Highlander.) I have read that VC was used prior to 04, and a different set-up was used for models years 2004-2006. I would like to know the pros & cons to each system and a general understanding on how each work. If Toyota did go back to the VC for 2007, what would be the likely reason?? Could it be to reduce transmission hesitation with the use of the VC?? I have read a plethora of transmission hesitation with toyota front wheel drive biased vehicles. Having said that, I have not seen anyone complain of transmission hesitation prior to 04 model year Highlander? I say this realizing that DBW was introduced and has been blamed for this... But maybe a pro for VC is less transmission hesitation in the Highlander??
    Look forward to your responses.
  • wwestwwest Posts: 10,706
    Without the VC the 04 and later models (for the RX series anyway) relied only on "traction" braking to maintain and apportion engine drive torque to wheels with traction. Since rapid and continuous brake application can result in overheating the rotors fairly quickly the engine is dethrottled simultaneously with wheelspin/slip.

    Overall not a very good way to get "unstuck".

    I don't know of anyone who knows why the VC was dropped for a period of time but having it back in the "loop" cannot be a bad thing. My own guess as to why the VC was dropped is that going into the 04 model year Toyota knew that they had a problem with the ATF overheating and tossed the VC as a possible source of that extra heat.

    Insofar as the hesitation issue is concerned Ford has just announced that the new Ford Edge is using a variable displacement ATF pump, IMMHO in an effort to solve the very same problem for their FWD vehicle series.

    I would suspect that Toyota has gone to the same measures and now the new 07 models seem to be exhibiting a 3-4 shift engine flare problem.
  • Thank you for the reply. Couple more questions. Where is the VC located in the Highlander? Does it really add to ATF fluid temperature??

    I would like to know more about a VC. It is my understanding that a VC consists of plates with a special fluid inside. When the fluid is heated, it creates friction between the plates and transfers power to the rear wheels. But I wonder how long the front wheels need to slip in order to heat the fluid enough to transfer power...

    Do we know for sure that the VC was introduced back into the 2007 Highlanders? I spoke with a Toyota Service manager today when I was waiting for my vehicle - he said he thought all highlander model years used a VC... I felt he was uncertain with his response. Any thoughts if the VC would help with the hesitation that is plaguing Toyota?

    I believe the 3-4 shift flare is exclusive to toyota's new 6 speed auto that, at this time, is only used in the new Camry & ES 350; I suspect this new transmission uses clutch packs like Honda transmissions to shift faster & be more responsive...
  • wwestwwest Posts: 10,706
    The VC is a part of, mounted within, the PTO, Power Take Off, to drive the rear wheels. It operates in gear oil and the PTO has metal to metal contact with the center/rear diff'l case which uses ATF.

    How long it takes for the viscous fluid to heat to the point of providing a satisfactory level of coupling is a function of the fluid formulation and the level of over-driving to the rear axle. Some systems even have a measured level of compressable gas inserted within the hermetically sealed VC case to delay the onset of coupling.

    On my 2001 AWD RX300 it was a matter of seconds before the VC was coupling ~25% of the engine torque to the rear driveline and it never rose beyond that level. Hopefully the newer VC will have improved upon that.

    The hesitation time line:

    Late in the last century it was realized (FINALLY!)that not only could engine compression braking interfere adversely with the anti-lock braking system's ability to release the brakes and thereby allow directional control to be maintained, engine compression braking (on the front wheels) alone often resulted in loss of directional control should the roadbed surface happen to be highly slippery.

    So for many FWD and front biased AWD vehicles the shift pattern was changed such that the transaxle would always upshift on a FULL lift-throttle event and only downshift into the appropriate gear ratio once the throttle was re-opened or the vehicle came to a full and complete stop.

    Much the same thing an experienced human driver would do on a slippery roadbed with a FWD vehicle equipped with a stick shift.

    The problem that grew out of that was that oftentimes the transaxle was now in the wrong gear ratio if the driver suddenly/immediately went to accelerate "hurriedly" shortly after a full lift-throttle event. It was not possible for some transaxles to fully and completely accomplish a downshift in these instances absent some serious level of clutch slippage.

    So a lot of 99 RX300s have premature transaxle failures at 70-80,000 miles.

    I believe that as a quick fix Toyota went to a higher fixed capacity ATF pump as of the 2001 RX300 MY. That, of course, resulted in too much pressure/flow capacity overall resulting in the 2001 RX300 series needing the ATF drained and flushed every 15,000 miles or as a minimum each time the ATF began to look and smell burned.

    As of 2004 the RX series went to DBW (and most likely back to the lower capacity ATF pump) to solve the problem once and for all. The idea was to "protect the drive train", delay the rise in engine torque during that second quick shift sequence until the ATF pump, with the engine at idle, could provide enough pressure/flow to complete the new shift sequence.

    We all know the rest of THAT story.

    I can't speak for Toyota but Ford probably adopted the variable capacity ATF pump for the new Edge to combat this very same problem.

    The "arrival" of the 3-4 engine flare problem would indicate that Toyota has at least changed something in another effort to solve the upshift sequence change made in ~99.
  • 2toyotas2toyotas Posts: 104
    Sorry but the 07 Highlander does not have a VC. It will after the 08 redesign. The Lexus RX350 which is new for 07 now has the VC back in the center differential. I beleive the system will be better with the VC back. It will keep power to both axles while TRAC sends power left and right. Without the VC if both front wheels were on ice and the left rear wheel was on ice TRAC would brake both front wheels and the left rear wheel and all power would go to the right rear wheel. With a VC in the center differential, it would keep the front wheels spinning together and brake only the left rear wheel, and power would go to the right rear wheel. I think without the front wheels being braked it will help the vehicle keep momentum.
  • wwestwwest Posts: 10,706
    It may be that the reason the VC was dropped was because the traction control system, at least in my 01, activated so quickly and in such a way that made the VC useless.

    Remember that the VC cannot be "active" full time due to the need to allow the rear wheels to rotate at a different rate in a turn, tight turn, and not incur driveline "windup" or undue tread wear rates.

    In my 01 traction control almost instantly starts braking the front wheels if they start to spin and just as quickly dethrottles the engine. So with my 01 version of traction control there is no opportunity for the VC coupling coefficient to rise absent disabling traction control.

    So maybe someone figured out how to rewrite the traction system firmware so the VC has some use....
  • I live here in Colorado and with all the snow lately, the 4WD doesn't engage when you push the button. This has been an intermittent problem and was wondering if anyone has had this problem or have any suggestions as to what is going on. Thanks
  • tommyg12tommyg12 Posts: 158
    Just curious....When my FJ (5speed AT/part time 4wd) is in 2wd motion, does the front drive shaft spin, and if so is the front axle spinning also? This is my first part-time 4wd vehicle and just trying to understand if the transfer case is not engaged what is spinning (besides front wheels obviously).
  • Tommyg, I was having dings in my windshield (Camry) fixed last week and a guy with an FJ was there getting his second windshield in six months (since new). He said the windshield is at such an angle (almost 90 deg) that rocks don't glance off, but cause lots of damage. I know it probably is a great vehicle but beware.
  • I have a 2005 4runner SE and I was wondering when you should turn off the VSC. If the road conditions are icy should I be driving in 4WD with the VSC off or on? I am confused. Please somebody explain.

    Thanks,

    John
  • wwestwwest Posts: 10,706
    Check your owners manual to be sure but I would bet that in 4WD, locked center diff'l, ABS and TC, traction control, are automatically turned off. There is good reason to suspect VSC is also off.
  • 2toyotas2toyotas Posts: 104
    ABS and Traction Control never turn off. Only VSC when you lock the center Differential. When the center diff is unlocked Trac will brake the spinning wheels and control engine output. When the center diff is locked it does not control engine output, it will just brake the spinning wheels. Unlocked will keep the vehicle under better control most of the time. Although I live west of Philadelphia, and during our last snowfall, which was a very heavy and icy snow, I went through an unplowed lot and unlocked center diff was very NANNYish. It would barely keep the truck moving, and almost stopped the truck. Very slow forward progress.
  • wwestwwest Posts: 10,706
    Sorry, TC can NEVER "just brake" the spinning wheels. If an inexperienced driver were to continue to lead foot it with TC continuously braking to prevent slippage the rotors would quickly overheat and warp, not to meantion the possibility of transmission or even driveline damage.

    When was the last time you stood on the brakes while lead-footing the gas pedal?

    ABS MUST be disabled if the center differential is locked. Otherwise ABS would have to work in synchronization at the front and the rear, releasing brake fluid pressure to both at exactly the same time. The front tires will always have more traction during braking, due to inertial effects, that means that they MUST roll slower than the rear and that is not possible with a locked center diff'l.

    There are some makes that instantly unlock the center diff'l when ABS activates but this cannot be done reliably with a dog clutch, sliding spline, to lock the center diff'l.

    Can somebody say, express this, better than I?

    Anybody?
  • 2toyotas2toyotas Posts: 104
    Wwest we are in 2007 not 1980, I have 3 toyota vehicles. A Sequoia, a 4Runner, and a Tundra and ABS and Trac works on all with the center diff locked. The only time ABS and Trac doesn't work is if there is a rear diff lock which only the FJ and Tacoma have. Also with the TRD package on the FJ you can lock the rear diff and Trac will operate on the front. Not sure about ABS. Please don't give out the wrong info if you are not sure, some people might not know any better and think you really know how they work.
  • I have a 1996 Toyota 4Runner Limited. I had it on a trip requiring 4WD in Mexico and had some difficulties and confusion getting into and out of 4WD. I put the car in park and found the 4WD shifter to go in and out of 4WD smoothly some times but not others. When I reached a paved road to go home I shifted the shifter out of 4WD but I am not sure about whether it shifted out at that time. After an eight hour drive I put it in reverse and heard a thud. Is that when it left 4WD. My questions that were not answered clearly when I went to a dealer are: what is the procedure for getting in and out of 4WD? is there a reason the shifter does not always move easily? What was the thud? I have never used an on line forum. Will I get a reply to this in my e-mail? Thank you. johnbakunin@ccusd.org
  • steverstever Viva Las CrucesPosts: 41,952
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  • wwestwwest Posts: 10,706
    Because of the nature of the BEAST, the center locking diff'l sliding spline dog clutch, it is not at all unusual to have problems getting into and/or out of the locked state. Most modern systems have a way of indicating that the diff'l is not in lock when it is supposed to be and vice versa. Most commonly the indicator lite flashes until the dog clutch is engaged or not.

    The most common way I found was to drive dead slow while putting it into, or out of, lock or even dead slow in reverse. I know that many manuals indicate that you can shift on the fly but there are MANY exceptions, one of the more common ones is to have slightly different tire wear rates F/R.
  • wwestwwest Posts: 10,706
    I am traveling at the moment, HI, and for some reason I cannot access my techinfo.toyota.com account to review your owners manual.

    But I wonder if you're not confusing the AWD mode of your Sequoia vs a true 4WD. Sequoia's AWD mode uses a "soft" locking of the center diff'l so all braking related functions can remain operational. In point of fact it is TC, Traction Control, that is providing this "soft" locking feature, "virtual" center and rear LSD as it were. It may even provide a low level, much lower than at the rear, of virtual LSD functionality at the front diff'l.
  • Thank you for the response. I am still not clear. My car has an automatic transmission. I have two shifters. One is for shifting from drive to reverse and to neutral. The other takes me in and out of four wheel drive. I think that you are telling me that the first shifter should be in drive or reverse and that while driving dead slow I should shift the second shifter to 4WD. Am I understanding this? Thank you again.
    -John Bakunin
  • wwestwwest Posts: 10,706
    First, follow, try the instructions as directed in your owners manual. If that does not work then yes, drive dead slow in drive first, and then try dead slow in reverse until the 4WD clutch locks into 4WD, or unlocks, whichever may be the case.
  • pjc1pjc1 Posts: 72
    Couple questions as this is my first vehicle with this feature:

    I understand that you should not use this feature on dry pavement however the manual suggests just avoiding dry pavement. I guess my question is if you lock the differential on dry pavement or while it is locked you hit dry pavement will you harm the system? I would seem odd to me that inadvertant dry pavement use would harm the system.

    If traveling in snow or slush and you need more traction does locking the center differential provide more traction?

    The manual is unclear on this too, but can you lock and unlock it while moving in H at any speed?

    Thanks for the help...

    P
  • pjc1pjc1 Posts: 72
    PS I have a 2006 Land Cruiser
  • greengreen Posts: 15
    Assuming it works like a 4th generation, full-time 4WD 4Runner - and I'm confident that's the case - I offer the following.

    If you lock the center diff on dry pavement, you can stress the driveline, depending on how much turning you do, and possibly harm it. When my 05 4runner was new, I inadvertantly drove 30 freeway miles with the center locked and didn't realize it until I tried to exit the gas station when turning my wheels and feeling a binding. 30k miles later and TONS of hard, off-road use and apparently no damage was done. As far as stress on the system, any time the road surface is slippery to allow wheels to slip slightly and unbind the driveline, the vehicle can be driven safely in center locked mode. If you had to drive it locked on dry pavement, keeping turning to a minimum would be best.

    When locked, the center diff will give better traction by defeating VSC which can cut engine power when wheels slip. So snow and ice in normal driving, you would leave the center unlocked so as to benefit from all VSC can do at speed. In unusual cirmumstances - in a ditch, a very deep snow white-out, low traction off-road, etc., locking the diff will make sure your engine power doesn't get cut by VSC but still enables traction control to limit wheel slip, side to side, on the front and back axles.

    You can engage the center lock while in motion. Personally, I've never done it going more than 10mph or so but the manual seems to indicate, by omission, that there's no limit on speed.
  • pjc1pjc1 Posts: 72
    Green... Thank you.
  • nedzelnedzel Posts: 787
    I agree with green, in normal driving, even on snowy roads, leave the center diff unlocked. If you are going offroad, then lock the center diff.
  • my001my001 Posts: 17
    RAV4 and Highlander

    These utilize a limited slip center differential and open front and rear
    differentials. It is a viscous coupling center differential. If one of the
    front wheels begins to spin faster than the rear, the heavy liquid in the
    center begins to firm up which routes more power to the rear. Once torque is
    equalized, the 50-50 power split is resumed. This system is always engaged and
    requires no driver input.

    It is possible to become stuck with this system. This is because of the open
    front and rear differentials. If both right tires were on ice, all power would
    be routed to these wheels. This is a fairly unlikely occurrence on a light duty
    vehicle like these. On the Highlander, you can get VSC, which includes traction
    control. If the right wheel begins to slip, brakes are applied to this wheel
    and power is sent to the left. On the 4WD model, there is no rev limiter
    associated with the traction control.
  • harboharbo Posts: 136
    So does the 2WD always engage the rev limiter which causes loss of power, and sinking in mud, sand and snow? Guess what happens next. Your stuck, and on your belly digging.
    Got to be a shut off on that kind of a brainless system.
  • wwestwwest Posts: 10,706
    Sorry, but the VC, viscous coupling, was dropped across the entire Toyota/Lexus FWD based product line as of the 04 model year. It has now been adopted again for the RX350 models but insofar as I can learn not for the others.

    Absent the VC these vehicles rely exclusively on TC braking for maintaining a high torque level and thereby distributing a reasonable level of torque to the wheel(s) with traction.

    And the torque split has never been 50/50 except with equal traction at all four wheels, when you care not anyway. The rear driveline is overdriven by a 2.98/3.12 (2.98/3.48 '04 and after) factor so the front will always receive the majority of engine torque.

    And even my 2001 AWD RX300 has engine dethrottling if I persist on revving the engine in slippery conditions. The engine MUST be dethrottled to prevent brake rotor warpage from overheating due to continuous TC braking.

    TC, Traction Control, will delay the onset of engine dethrottling for a few hundred milliseconds on RWD or AWD vehicles whereas FWD vehicles, due to their hazardous nature, will have the engine INSTANTLY dethrottled upon driven wheel slip/spin.

    And as a general rule TC will NEVER be used to automatically implement a "virtual" LSD on a FWD vehicle due to the possibility of yanking the stearing wheel right out of an unwary driver's hands. So TC will apply braking to BOTH front wheels and dethrottle the engine even though only one wheel is slipping/spinning.

    The exception is the new RAV4 "auto" LSD function/mode which should not be used before reading the CAUTION note in the owners manual.
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