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Toyota 4WD systems explained

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Comments

  • 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
  • Stever@EdmundsStever@Edmunds YooperlandPosts: 38,950
    Please come back here to see your replies (having people email you defeats the purpose of having a forum!). You can Track This Discussion and keep up to date on replies easily that way.

    There is a box to check when you track a discussion that says "Send me an Email Alert when new messages are posted."

    And welcome!
  • 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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