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Toyota 4WD systems explained

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  • kmhkmh Posts: 143
    Sorry if I mislead you on the second question.

    What I was wondering was what sort of maintenance considerations should I take into account for my AWD with the LSD? Is there any special needs for maintenance at 60,000 miles because of the AWD and LSD?

    Many thanks!
  • cliffy1cliffy1 Posts: 3,581
    As you noted, the rear LSD does need maintenance. The clutches can and do wear out and will eventually need to be replaced. When they do wear out, you will not notice anything drastic. They will just stop transferring power to the side with the most traction. It literally reverts to an open differential.
  • cliffy 1,

    Thanks again for all the informative postings. My question, which by implication you may have already answered, is whether the sequoia's transmission must be in "L" to engage 4wd low? The reason I ask is that you imply that 4wd low is essentially designed only to extricate the vehicle from a bad situation. My experience with 4wd was with a 76 LC, the ultimate in conventional 4wd as you say, in Colorado, and I rarely ventured off road without 4wd low engaged simply for the ability to handle steep grades. I rarely got stuck, but I was always going uphill. I love the idea of the 3 open miff's saving wear and tear with the tracs computer easing any potential bog downs, but can it work in 4wd low. (trans in "D" ?) Thanks for any info.
  • cliffy1cliffy1 Posts: 3,581
    The low gear range operates in all gears. If you shift the transmission into "L", the center differential is also locked and the TRACS system is disengaged. This means that as long as you don't have the transmission in "L", you have the low gear range plus the TRACS and open differentials working for you.

    Great user name by the way.
  • llofgrenllofgren Posts: 129
    Cliffy1,
    Thanks for your excellent explanations on these systems. You mentioned that the HL center diff maintains (with the transfer case) a 50-50 split between front and rear. If there is Front slip then the viscous coupling thickens and routes more power to the rear.
    Two questions: 1) Can the opposite happen? That is, if there is rear wheel slippage, can power be transfered to the front so that the front wheels are getting more than 50%?
    2)Does the system work when the transmission is placed in Reverse?
    Thanks!
  • cliffy1cliffy1 Posts: 3,581
    The answer to number 1 is yes.

    I'm not positive on the answer to number 2, but I don't see why it wouldn't. The viscous coupling center differential is pretty simple and if one drive shaft begins to spin more than the other, power is transferred. I don't see why direction would impact that.
  • kmhkmh Posts: 143
    Say I'm going along and my front wheels begin to slip in my RX 300. Power is transmitted to the rear tires, correct?

    Everything is going along with power to the front and rear wheels until one of the rear wheels begins to slip... Does the power then move forward to the front wheels?

    At what point does the transfer stop? When there's no longer any wheel slippage?

    I apologize if I sound completely ignorant on this, but I am in a way and merely want to understand what my RX is doing in conditions when the transfer occurs. Thanks!
  • cliffy1cliffy1 Posts: 3,581
    You've got it! The power equalized when traction is equalized.
  • brillmtbbrillmtb Posts: 543
    cliffy,

    Good job on helping folks out on the Toyota systems. I found that the dealers couldnt even explain it and they go to the training.

    I would disagree on something you said in an earlier post. It is not the unibody that would limit the HL. It is suspension design, one aspect of which is the ground clearence you mention.

    Then again, I hope nobody is considering much off roading in the HL anyway. So my point is, the highlander should do fine for its intended purpose just as the whole line of Toyota products seem to do.

    The trick is to figure out what you want to do then find the SUV to match it.
  • Cliffy,
    My wife was driving my '99 SR-5 through Kentucky on I-75 while I slept in the back. It rained hard and fast and she hit a puddle and hydroplaned at about 55 mph. I awoke and saw her turning into the skid but knew she wasn't going to be able to correct it. We ended up rolling over 3 times, landing back on the wheels. Luckily we walked away from it. She doesn't drive my truck very often and isn't familiar with it's handling. Could this have been prevented if this vehicle was equipped with VSC and in 4WD?
  • tonychrystonychrys Posts: 1,310
    Yes. This is one of the reasons I bought a RX300 (VSC and permanent 4WD) for my wife and son to drive everyday. I definitely sleep better knowing the odds are slightly tilted in their favor in case of an unexpected incident.

    Goto LexusSafety.com for good explanations of Toyota active and passive safety systems.
  • cliffy1cliffy1 Posts: 3,581
    Absolutely this would have helped. The VSC is very quick to respond to both oversteer and understeer. I'm not saying it is impossible to roll one, but it would take much more effort. I've played with this system a good bit and am shocked at how well it responds in slippery situations.
  • crapgamecrapgame Posts: 43
    Ok, I am getting there, although slowly. Assuming I purchased a 4WD Sequoia, my wife would be best off leaving it in 4WD all the time in order to avoid the possibility of the rev-limiter kicking in? Won't this place excess wear and tear in the 4WD components, or does the 4WD only kick in when slippage is detected?

    Thanks for your time.
  • cliffy1cliffy1 Posts: 3,581
    You certainly can leave it in 4WD all the time. There really shouldn't be much if any extra wear on the system. Remember, the Land Cruiser used the same system but has no option for 2WD use. If excess wear were an issue, Toyota would not have developed this system for that vehicle.

    On another note, unless you often have loose gravel at the end of your driveway, I'm not sure I would be overly concerned about the rev limiter. If it rains, hit the 4WD button. If you are on a dirt road, hit the 4WD button. Once you are on a major road, turn it off. If this is too much hassle, just leave it on all the time.
  • crapgamecrapgame Posts: 43
    OK, I completely understand you now. Seems relatively simple. There are quite a few gravel roads where I live, and my driveway was gravel until about a month ago. I think in my case, the best bet would be for me to tell her just to leave it in 4wd unless she is on one of the highways.

    Many thanks for your time.
  • drew_drew_ Posts: 3,382
    You can leave it in 4WD mode even on the highway, especially if you have to make a sudden course correction to avoid something. Just set it and forget it then you won't have to worry about when to turn it on and off.


    Drew
    Host
    Vans, SUVs, and Aftermarket & Accessories message boards
  • jcnew4whlrjcnew4whlr Posts: 18
    I was exploring an area near a couple of rivers this weekend, and came to one place on a gravel road with water racing over the road. There were a couple of those tall-wheel 4WD pickups there and I really think with the Sequoia ground clearance I could have gone through. Problem was it was a dead end road so I knew I'd have to come back. Since the river was still rising I turned around.

    So what's my question? With high water and gravel, what's the best setting on a Sequoia? 4H with gearshift in second? 4L with gearshift in second? 4L all the way in Low? (Seemed like overkill.)

    My username says it all---I'm just learning how to do this stuff.
  • cliffy1cliffy1 Posts: 3,581
    My answer would be 4 hi. I would think momentum would be important for this and would only shift to low if I got stuck.
  • llofgrenllofgren Posts: 129
    In the Highlander forum, posts # 494 and 499, describe an RX 300, with an AWD drive system (but no VSC) like the Highlander, getting stuck on a ramp with rollers on the front wheels.....this was a demonstration test.
    I don't understand why the RX 300 got stuck. Shouldn't the viscous coupling in the center diff thicken and kick in to engage the rear wheels?

    Les
  • drew_drew_ Posts: 3,382
    No, because it's an open differential and cannot be mechanically locked. The viscous coupling limited slip differential tries to "glue" both front and rear driveshafts together to prevent loss of traction. However, when there is little traction up front, the differential sends all of the power to the path with the least resistace, which in this case is the front axle. This system works okay if the vehicle has sufficient momentum, but if you're in low speed conditions and high demand for power the VC will fail to deliver the needed power. High torque transfers and continous use make viscous couplings fail often.


    Here is a picture for a stuck AWD RX300. Notice the front wheels spinning but the rear wheels stationary. The ramp simulates a slight uphill snow/ice covered grade.


    Drew
    Host
    Vans, SUVs, and Aftermarket & Accessories message boards
  • llofgrenllofgren Posts: 129
    Thanks for the input! Yes, I did see that pic of the RX 300.
    I still don't quite get it though. Here is the explanation by Cliffy:

    "Highlander and RAV4 by cliffy1 May 16, 2001 (04:01 pm)
    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."

    In the RX 300 situation (picture), the front wheels are slipping, so why doesn't the viscous coupling in the limited slip center diff kick in to power the rear wheels and send the SUV up the ramp? It (the center diff) does kick in under high torque transfer conditons while normally driving to split the power 50/50 front and rear. Starting from a stop on dry pavement is a high torque low speed transfer....why does IT work and the boat ramp thing fails? And what if I am parked with my front wheels on solid ice and my rear wheels on dry pavemnet on a slight upslope? It would seem to me that the limited slip center diff would engage the rear wheels to push the vehicle out. What am I missing here?

    You said that the center diff routes power to the least resistance....but isn't that the direct opposite of what everyone (else) says it does?
    That is, the slipping wheels transfer power to the viscous coupling which then powers the NON slipping (higher resistance) wheels(s).
  • drew_drew_ Posts: 3,382
    Remember the word "limited slip". This means that it has a certain limit, and is by no means a locking differential. As such, up to a certain limit will transfer the torque to the opposite driveshaft as required. However, once that limit is exceeded, the differential will act like a conventional open differential and send power to the path of least resistance. This is why off-roaders insist on mechanically locking centre and rear differentials, not the limited slip variety. This way, the wheels are mechanically bound to spin at the same rate and power won't be lost like it will with a LSD.

    The Lexus fails the roller test (representing snow or ice on the road) because when the front wheels slip on ice the unmanaged center differential allows all torque to get lost at the front wheels (path of least resistance) rather than making any torque available to the rear wheels. Since the RX has an open front differential, even if one front wheel doesn't have traction, all of the power will leak out to that wheel. I remember one owner complaining about his '99 RX getting stuck in a ski resort parking lot, with only one front wheel spinning and the rest stationary. All of the power was being leaked out to that wheel only.

    This is the reason why Lexus finally added stability control and traction control for the newer models. To help to combat this problem.
  • llofgrenllofgren Posts: 129
    I am getting it a little, I think. So why have I never noticed this problem before? I regularly drive my wife's '92 AWD Sub Legacy Wagon and have nave never gotten stuck in it, except once, when ground clearance did me in after 15" of snow. Otherwise it has gotten me through some pretty extreme winter conditions, on remote Minnesota roads.
    What about the Honda RT 4WD setup? Would that do better on the "ramp roller" test since the clutch pack would probably slip less than a viscous coupling?
    How does the Escape/Tribute system allow a continuous AWD system to be locked?
    Lastly, what determines "the limit" to which a limited slip viscous coupling will transfer power to the other axle (as opposed to letting it leak out the path of least resistance)?
    (sorry for all the questions)
    Les
  • cliffy1cliffy1 Posts: 3,581
    Your Subaru, like most other vehicles will never see a roller ramp and that is why you have never experienced something like this. That little demonstration was put on my Mercedes engineers for sales training. Those demonstrations are always set up to make the competition look as bad as possible. Toyota does these all the time as well, sans the pictures on the Internet.

    Drew is correct about the limited slip center, but the chances of ever experiencing something as drastic as rollers on an incline are very remote. I have my doubts that ice could have the same impact. I played with this system last winter and was unable to duplicate the same effect. There was always enough traction to get moving, even on sheet ice.
  • llofgrenllofgren Posts: 129
    Thanks for the reassurance (my HL is "on order"). It supports my subjective sense that this (AWD) system is a good one but it was unsettling to see that roller ramp picture with the RX 300.
    I enjoyed reading your posts from this winter on the Highlander! I saw where the snow had shut down D.C. and you took the two AWD HL's (one with VSC and the other without) to the snow and floored them:)
    I am eagerly awaiting my HL!
    Les
  • eagle63eagle63 Posts: 599
    I've posted this question in another forum on edmunds with mixed answers. How does the rear-diff lock on the tacoma work? People have told me that you must be in 4-lo to engage it, others say it can engage anytime you press the button. Also, is there a speed limitation on the locker? -meaning, do you have to be going under a certain speed for it to work? thanks in advance!
  • cliffy1cliffy1 Posts: 3,581
    Until the PreRunner came out, I thought I knew the answer to this question. The training I went through told us you had to be in "low" before engaging the rear differential lock. I had assumed that this was a mechanical requirement. Since the PreRunner can also have the locker but has no low gear range, I now think this is more a suggestion than a requirement.

    I also know that there is no speed limiter. The warning label in the cab does tell you to keep it under 5MPH but I don't think you are restricted to this.

    Because my store isn't near any dirt roads, I just have not had opportunity to experiment with this.
  • eagle63eagle63 Posts: 599
    thanks for the response. I've talked to a handful of toyota dealers in my city and I got a different answer each time. (one of them didn't even know the difference between the Xtracab and the doublecab!) some told me you had to be in 4lo, others said 4hi or 4lo, and one said anything - it doesn't matter. But you bring up a good point about the prerunner. obviously if the prerunner has it, you can engage it in 2wd.
    hopfully I can test it on my next testdrive. thanks again!
  • vivian3vivian3 Posts: 8
    Thanks for the helpful explanation of 4WD systems. I live in Alabama, and I am looking to buy a Highlander. I am deciding between FWD and AWD. Either way I would get Traction control and VSC. We rarely get snow or ice here. And although I read your site reference to the advantages of 4WD on dry payment, I am a very conservative driver. I have never skidded a FWD or AWD vehicle on dry pavement (although I've done some spectacular skids in cold weather in Canada). Also, I am concerned that the additional weight of the AWD system will increase braking distance and hurt fuel economy. However, we do get heavy rain here. What advantage does a 4WD system give in heavy rain? Will it make the vehicle less likely to hydroplane, or will it just increase traction when making turns and sudden changes? Thanks!
  • cliffy1cliffy1 Posts: 3,581
    AWD will give some advantage in rain, but I don't think hydro planing will be affected. You will get better traction and handling. There really isn't much extra weight on the HL AWD system so I don't think braking distance will be affected much but gas mileage drops a little.

    One other thing to consider is resale. I really don't know the SUV market in AL, but in general, the AWD version will do much better down the road.

    I hope this helps. I don't think you can go seriously wrong either way.
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