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Toyota 4WD systems explained

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Comments

  • brillmtbbrillmtb Posts: 543
    I just bet that Toyota is taking all this in and will respond with a switch to make these traction control features under the control of the driver.
  • loma1loma1 Posts: 32
    I tried power braking in two wheel drive to see what would happen. I kept the brake pedal depressed while pressing down on the accelerator and without any wheel slippage at all the rev limiter cut power.
  • cfeldmancfeldman Posts: 14
    I was wondering if I will be at a disadvantage going from AWD(Subaru) to a Part time 4WD (4 Runner) as far as driving in rain and overall handling.
    The way I understand it from Subaru is that nothing beats the AWD.
    My question is why not keep the 4 runner in 4WD all the time and activate the traction control all the time? Would this not give you the best road handling equal to AWD??
    Thanks
  • heatwave3heatwave3 Posts: 462
    cfeldman: on you questions above. It really depends. There are several different Subaru 4wd systems. All of the manual Subarus have a system witha 50/50 torque split front to rear. The WRX has a system providing 45%fr/55%rr. The other Suabaru systems, for the most part, place most of the torque if not all of it to the front wheels until slippage occurs.

    Most manufacturers of awd (Porsche, Audi, Volvo, BMW, GM, Ford) deliver a balance of torque split between the front and rear that's typically relective of the handling characteristics and power of the vehicle.

    Your 4Runner, I believe, when operating in auto 4wd is really not quite awd since the only time power exists at the front wheels is when the rear wheels slip and power is transferred to the front end.

    Most AWD systems deliver "forced" power to the front end which typically will provide added handling and traction benefits even while operating on perfectly dry pavement. The auto 4wd systems really only provide a benefit when slippage occurs. These auto systems are certainly better than 2wd but they do not provide the same benefits as AWD.

    I hope that helps.
  • jynewfjynewf Posts: 26
    Heatwave3 is incorrect in his explanation on how the 4wd Hi mode of the 4runner works on 2001+ models.

    The auto 4wd mode links the front and rear driveshafts of the 4runner together using an open center differential. Power is supplied to all four wheels. The open differentials at the front, center and rear compensate for the differential wheel speeds on dry surfaces, so you can leave it in 4wd Hi all the time.

    When slippage occurs at one or more wheels, the TRACS system brakes the wheel that is slipping, preventing power from being lost at the slipping wheel.

    Of course, when you slip it into 4wd HI/LO, then the center differential locks, so that the front and rear axles rotate at the same speed.

    Such a system has its plusses and minuses versus other systems (i.e. viscous coupled diffs or part time systems with locking diffs), which I won't go into here.

    Therefore, Heatwave3 is incorrect in saying that power is not supplied to the front wheels unless the rear wheels slip. With the drive shafts coupled, the center diff distributes power through the path of least resistance, i.e. through both the front and rear drive shafts equally. The point of the TRACS system is to prevent all power from being routed to a spinning wheel, irrespective of whether the wheel is on the front or rear axle.

    I know for a fact that my 2000 4runner (with a 4wd system that is somewhat similar to 2001 except that it does not have a TRACS braking system), when in 4wd all mode, supplies power to all four wheels, including the front wheels, all the time.
  • jynewfjynewf Posts: 26
    Cfeld:

    Yes, on 2001+ 4runners, you can leave the 4runner in its allmode 4wd all the time, which gives you a system that is similar to, but not equivalent, with a typical AWD system that employs viscous coupled differential.

    The all mode 4wd system in the 4runner supplies power to all four wheels all the time. However, when slippage occurs, the braking action of the TRACS potentially retards progress of the vehicle, especially in situations where all four wheels are slipping. This is the achilles heel of brake actuated 4wd systems, and is a problem with 2001+ 4runners, as well as Sequoias, and the M-Class Mercedes.

    For some (but not all) AWD systems with viscous coupled center diffs (VC's), a certain amount of the torque can be forced toward one axle or another. In other cases, some AWD systems simply leave the VC flaccid until a major amount of slippage occurs, so that most of the time only two wheels are driving the vehicle. This is the case with the Honda CRV, Ford Escape, Lexus RX300, where essential the vehicle acts like a front wheel drive car, until the front wheels slip, in which case a certain percentage (usually 20-50%) of the power is routed to the front wheels as the VC heats up and engages.

    If it is the case that Subarus "pre-tense" the center VC so that power is always being supplied to both the front and rear, then the major difference in feel between the Subaru and the 2001+ 4runner is that a certain amount of slippage needs to occur on the 4runner to brake back a slipping wheel, whereas the Subaru will still keep plowing ahead.
  • heatwave3heatwave3 Posts: 462
    jynewf: I think your second post above is accurate however I do not think your post #427 is an accurate depiction of my views. The 4runner system, even post-2001, is different from an awd system for the very reasons you described in your second post and I did in my post.

    Yes, the Toyota system can be run on all road conditions. No, it is not equivalent to awd. The system deployed in the 4runner with an open center and rear differentials will not generate "force" at a particular corner of the vehicle as is the case with awd that incorporates LSDs at the center and rear of the drivetrain.

    An open center differential will direct the engine's power towards the wheels with the least resistance. Therefore in a hard corner with an open ctr diff, the power will be directed away from the corners experiencing the greatest resistance (such as the front outside wheel of the cornering). This transfer of power away from resistance will occur through the open diff until such point as the tires with the least resistance but the most power overcome the friction of the road and slip.

    At this point, and only at this point, will the trac system brake the slipping wheel and redirect that power to a tire that is not slipping.

    This is a significantly different mechanical effect than an AWD system that is maintaining power at a particular wheel even though resistance is increasing. The dry and wet road effect is substantial.

    On a dry road the vehicle with AWD can "pull" with the front wheels because of minimum designated torque while the rear wheels continue to "push" the vehicle. It results in greater distribution of power irrespective of which tires are meeting greater resistance.

    The Toyota system, while superior to 2wd and possibly even auto 4wd systems, will still direct all power away from the tires meeting greater resistance (typically the front tires, particularly in a turn) and will continue to do so until the rear wheels slip. It is conceivable that if the rear wheels are maintaining their traction, 100% of the engine's power could be directed to the rear tires.

    With a viscous liquid center diff, the factory design can be set to commit a minimum % of engine power to the front wheels under all circumstances which is an advantage over the brake actuated, open diff 4wd systems. This is why all performance vehicles use some mechnical design to designate a minimum amount of engine power to the front wheels under all circumstances through some type of restrictive center and rear diffs, whether it be Porsche, Volvo, GM, Audi or Ford.

    Admittedly, the addition of a Traction control system to a "true" awd setup is an extra layer of security, however it is not a replacement for a limited slip center diff and a limited slip rear diff as exists in almost all AWD systems.

    These additional remarks are consistent with my earlier post, however they should clarify any concerns you might have had about my post being "incorrect". I do not believe it was.
  • jynewfjynewf Posts: 26
    Let me clarify my response as to what I feel is "incorrect" about your post.

    In particular, I didn't think the following statement was correct:

    "Your 4Runner, I believe, when operating in auto 4wd is really not quite awd since the only time power exists at the front wheels is when the rear wheels slip and power is transferred to the front end."

    As to your statement about the "auto 4wd" being "not quite awd", that I wholeheartedly agree with.

    However the statement that "the only time power exists at the front wheels is when the rear wheels slip and power is transferred to the front end," is in my opinion incorrect, since it implies that no torque is sent to the front wheels until wheelslip at the rear occurs.

    I think your most recent post helps clarify this:

    As you point out in your most recent post, an open center differential will direct the engine's power towards the wheels with the least resistance. Agreed. Thus in straight line driving, torque will be apportioned *roughly* equally between the front and rear driveshafts, which I think you will agree with, not simply to the rear wheels.

    I also agree with your statement that in turns, the front wheels travel a further distance, so that greater resistance is felt at the front drive shaft, diverting torque to the rear driveshaft to equalize the torque. In this instance, the split will not be roughly equal, but will be whatever is necessary to equalize resistance in the open diff. However, again, in most situations, some degree of torque will be apportioned to the front wheels, not just during slippage of the rear wheels.

    I also agree with you that in drastic situations, where resistance is met in the front, and the rear have traction but less resistance, then up to 100% of the power will be diverted to the rears. Two comments: (1) The converse is also true, i.e. if resistance is met in the rear, and the front wheels have traction but less resistance, then up to 100% of the power will be diverted to the front; and (2) It's hard for me to think of a situation where this might occur. I would think that in such a case the the wheel feeling less resistance would quickly slip, thus actuating the TRACS system.

    Please don't get me wrong, I'm not a big fan of open-diff, brake-actuated systems. In fact, I purchased my 2000 4runner in Nov. of 2000 precisely because I did NOT want the TRACs system that was in the 2001. Instead, I wanted the part-time system with the locking rear diff, given that I offroad my vehicle.

    My main concern with your earlier (as opposed to later) post was that it seemed to imply that the 4runner was mainly a rear-wheel drive vehicle, with power only going to the front upon slippage.
  • jynewfjynewf Posts: 26
    By the way, have a good Memorial Day weekend ...
  • heatwave3heatwave3 Posts: 462
    jynewf: We are definitely on the same page. Having driven both types of systems, I do want to share with you that's its not just during drastic sitations that the difference between these two 4wd systems will be felt or benefit the driver.

    In the Sequoia system, because of the open ctr diff, the front wheels (or for that matter across the rear wheels) will never have the ability to provide more power than the available resistance. Sounds like you'd never want that to occur but from a performance sense, it is quite an interesting and benefical feeling when an awd system actually gives you a sense of "pulling" the front end through a turn or as you accelerate out of a turn or when traveling across a wet or snowy roadway.

    This may be a nuance that is neither appreciated by the average suv owner or even sought. But having had the opportunity to experience it, I would want it in my next vehicle.

    Have a great Memorial Day Weekend!
  • pschreckpschreck Posts: 524
    The experience is the same in our Sequoia as it is in our Outback. We own both vehicles so I experience the "nuance" weekly.

    Both systems pull you through the turns, and in south central PA we certainly have the turns as well as the hills.

    Have a great Memorial Day Weekend!
  • heatwave3heatwave3 Posts: 462
    pschreck: Actually the sensation you have in your Outback may be very much like that in your Sequoia. It all depends. And if it is the same, its specifically because neither vehicle is capable of providing power to the front wheels in proportions greater than the resistance experienced by the front tires. It would be impossible for the front wheels of an automatic Outback or a Sequoia to "pull" the front wheels of either vehicle through a turn.


    Let me explain:


    If your Outback is a Manual, it will have a standard viscous liquid center differential which will give the Outback an entirely different feel on the road than your Sequoia. If on the other hand, you have the more common Automatic Outback, you'll have an open center differential with an electronic system similar to that in your Sequoia.


    Once again, because these systems have an open center diff they will be incapable of "pulling" the frond end through a turn or for that matter through anything unless the rear slips, forcing power away from the tires with the least resistance and least traction (most likely rear) to the tires with greater traction (front).


    In fact, unless you got that Outback with the optional viscous liquid rear differential, you in reality have a one wheel drive vehicle until that one tire slips. Prior to that one tire slipping, the open drivetrain will force equal distribution of the power equally to all 4 wheels so long as the resistance is equal. As resistance increases on the front wheels (for example during a turn) power will be redirected to the tire(s) that have the least resistance (most likely one of the rear tires).


    If you have the optional visc liq rear diff on a manual Outback you have a well layed out awd vehicle for dry and wet road performance (that's why the hi-perf WRX comes with this type of awd system). This type of system would be comparable to the Denali, Escalade, Porsche, Volvo and other higher performance vehicles.


    If, otoh, you have an auto Outback without the optional lsd in the rear, you have a system that would be considered awd in name alone. In fact, that's why it would feel the same as a Sequoia.


    Both of these vehicles, with open ctr and rear diffs, will never have more power going the front wheels than the resistance will allow. As the resistance on the front wheels increases the open diffs will direct the power to one of the rear wheels. They will continue to direct power away from the front wheels as resistance increases until the front tires reach 0% or until one of the rear wheels slip, whichever comes first. If slippage occurs first, the electronics will re-direct power through the open drivetrain back to the front wheels. In a turn (whether its dry or wet), that's the last thing you really want.


    Both systems are ok for transferring power, once slippage has occurred but they are not nearly as surefooted as an awd system that puts power at a corner of the vehicle and keeps it there even as resistance increases.


    Hopefully, that wasn't too confusing. Here's a site that can provide you some of the details of the different awd setups in the Outback depending on which model and which options you purchased <http://www.subaru.net/usproduct/2000/overviewspecs.htm>

  • wwestwwest Posts: 10,706
    Tracs:

    When the vehicle is turning the computer "knows" the level at which the left/right rotational rate should be disparate (speed and steering wheel position) and will not activate TRAC unless the disparate rate exceeds the calculated rate.

    With strictly open diff'ls any wheel that is "slipping" is not contributing its share of "force", torque, to the roadbed, and that limits the level of torque which the other wheels can deliver to the roadbed.

    But TRAC applies brakes to any wheel(s) that "slip" allowing the engine to sustain a higher level of torque to deliver to the non-slipping wheels, at least until the computer decides that the driver is an idiot and hasn't a clue as to how to modulate the throttle appropreate to the roadbed conditions.

    If all wheels spin, slip, at the same basic rate then TRAC is brain-dead and doesn't interact at all because it doesn't have a "reference" to use to judge slippage with. It simply doesn't have any way of "knowing" that the vehicle isn't moving forward comesurate with all four the wheels' rotational rate.
  • heatwave3heatwave3 Posts: 462
    wwest: I think I'd have to see the events you just described in action to believe it. On your description, the computer would enable all of the engine's power to be delivered to all four wheels through the open diffs even though there was 0 traction at all four corners. I just don't think so and believe the Tracs system would instead brake the system tight.

    Are you suggesting that all manufacturer's of hi-performance off-road and on-road vehicles that have lsd center and locking rear diffs are just wasting their money? That Porsche, Hummer, Volvo, Audi, Subaru, GM and Ford would rather waste their money by adding hardware that consumer's care little about and adds weight to the vehicle instead of just reverse engineering their ABS system into a Traction control system? And that Hummer, Porsche and others that have both mechnical diffs that limit slip in addition to traction control are REALLY wasting their money and the engineers simply don't have a clue while Toyota has made a major discovery and are keeping it a secret from the rest of the world?

    Look.... traction control is a wonderful electronic benefit to the vehicles that have it. I would like to have it on my awd Denali XL. I would never want it in replacement of a locking rear differential and limited slip center differential if it meant I had to have an all open drivetrain. Physics tells you the lag will always diminish the capabilities when compared with a mechanical drivetrain that prevents slippage from occur in the first place.
  • cliffy1cliffy1 Posts: 3,581
    OK, so you don't believe it. You have made your position known, even though you don't own a Toyota and this is a Toyota topic. Until you actually drive one of these, as I have and reported the results several times, perhaps you need to find a new topic to haunt.

    Oh yes, one more thing, LS differentials take more time to transfer power than an open differential. I don't know what physics book to send you to in order to prove this, but it is true. It is also true that LSD rear ends have a series of clutches that *will* wear out and create an open differential. How soon this happens depends on the use. Whether or not the driver is aware of this depends on how attuned he/she is to the vehicles performance when it was new.
  • cliffy1cliffy1 Posts: 3,581
    Ummm... who are you talking to? I'm not a salesman.

    I also noticed that you just admitted that you are attempting to scare customers. Very interesting. Like I said, come back when you have actually driven one in the conditions you seem to fear.
  • heatwave3heatwave3 Posts: 462
    Cliffy: you state "Oh yes, one more thing, LS differentials take more time to transfer power than an open differential. I don't know what physics book to send you to in order to prove this, but it is true. It is also true that LSD rear ends have a series of clutches that *will* wear out and create an open differential. How soon this happens depends on the use. Whether or not the driver is aware of this depends on how attuned he/she is to the vehicles performance when it was new."

    For such a smart guy, you sometimes miss some important differences. An open diff system does not "transfer" power per se (in the absence of a traction control system experiencing slippage). It evenly distributes power according to the resistance opposed at a particular wheel. It performs this task no faster, nor slower than a LSD. Both will "distribute" the power with equal speed within the parameters of their design.

    "Transfer" of power is an entirely different concept. Transfering power in a 4wd system specifically means to distribute the power differently than would otherwise be the case under normal circumstances.

    In a Sequoia, normal means to distribute power to each of the corners inversely proportional to the resistance that power meets. The higher the resistance, the less power the open drivetrain will direct to that wheel. That's what enables the system to operate without binding during turns or to experience unusual wear.

    That principle is also what PREVENTS the Sequoia system from delivering more power to a corner to overcome resistance but not traction in the absence of slippage at another corner of the vehicle. Its a passive and cheap way of distributing power and does nothing in the way of "transferring" power.

    A locking or LSD setup both distributes power and transfers it according to pre-established design parameters. Such as the Denali's 38%/62% power distribution or the Subaru WRX's 45%/55% distribution or the Porsce's 50/50.

    The Sequoia has no distribution ratio because it just passively sends the power to the wheels with the least resistance regardless of available traction. The system passively continues this re-distribution (in the opposite direction of where most drivers under slippery conditions would need that power), until the trac system kicks in, brakes a wheel or wheels due to slippage and redistributes the power to a wheel or wheels experiencing less resistance than the one(s) under braking power.

    Then, and only then, does the Sequoia's system "transfer" power back through the open drivetrain. Under most circumstances, the system with a locking or limited slip diff would have avoided the slippage in the first place and consequently would have less likely had a need to transfer power. You will never "feel" a locking diff or LSD distributing its power. You will "always" feel the lag of a brake actuated traction control system "transferring" power as it senses slippage.

    Anyone seriously looking for traction in both off-road and on-road performance will incorporate differentials other than open designs into their drivetrain. Just ask, Porsche, Volvo, GM, Subaru, Audi and BMW and Ford?

    With respect to the clutch packs in LSDs I have never experienced or read about failures that have occurred. I have owned LSDs in 4 BMWS, 5 GM vehicles, one Nissan and one Jeep product and several of these with over a 100K miles. None of them ever experienced a rear of center diff problem.

    I'm sure they certainly fail, however its my guess that the failure is no more or less than an open diff setup. If you have some data to suggest otherwise, I would be interested in reviewing it.
  • cliffy1cliffy1 Posts: 3,581
    So, what you are saying is that LSD is actually a clairvoyant system. I'll now refer to these are magical differential (MD for short) since, as you say, they can predict where traction will be lost and transfer power without the driver feeling it.


    A quick google search turned up many references to how and when MD's wear out.


    http://www.off-road.com/dodge/ramcharger/prodrev/Auburn.htm

    http://www.cyberflunk.com/m5/tech/drivetrain/mf173.html


    http://www.truckworld.com/How-To-Tech/Lockers-Differentials/4x4Lockers-Diffs.html


    http://www.kormanfastbmw.com/rdiffere.htm

  • cliffy1cliffy1 Posts: 3,581
    From www.jeepfan.com somes the following quotes:


    "High levels of "clutch preload" will result in good torque transfer but some chattering of the clutches during cornering may occur. Lower levels of preload results in minimal chatter but reduced levels of torque transfer to the wheel with traction. Because LSD's restrict true differential action, tire wear is accelerated. Changes in vehicle handling may also occur, particularly in short wheelbase vehicles. Wear rates on limited slip differentials are generally higher than on other types due to the reliance on friction to reduce wheel slippage. Also, special lubricants may be required to minimize rough and noisy operation."


    On locking differentials:


    "While automatic locking differential provide excellent performance off road, vehicle handling, particularly on highway, is sacrificed. Unlocking during cornering can be sudden, resulting in a rapid change of direction, particularly in short wheel based vehicles. During sharp cornering an audible racheting sound usually occurs as differential action takes place and a loud banging noise may be heard when the unit locks up again. Tire wear usually increases"


    This can be found at http://www.offroaders.com/jeepfan/difftech.htm

  • pschreckpschreck Posts: 524
    heatwave: Wrong forum. That was posted elsewhere. But thanks for helping to spread the gospel.

    You stated: "all wheels are slipping, than the traction control will shut all four wheels down."

    How do you know that? Do you own a vehicle with this type of system? Where did you read this? Is it speculation? Show me your data.

    heatwave, you don't discuss anything, you tell people what you THINK and present it as fact.

    Fact is that I have driven this truck in the snow and ice. IT WORKS, PERIOD! I don't need you to tell me that you THINK it doesn't, I KNOW it does.

    When you own a Sequoia, then MAYBE I'll take you seriously.

    Btw, did you ever think that maybe the Sequoia weighs more than the Tahoe/Yukon because Toyota uses thicker metal on the body and elsewhere?

    Oh yes, I just engaged my 4WD Low while standing still. Keep in mind that I always run in 4WD High. Once again you are wrong. I OWN THE TRUCK! I can test these things. You can only speculate.
  • pschreckpschreck Posts: 524
    Welcome back. Hope all is well.
  • pschreckpschreck Posts: 524
    Hey man. Good to hear from you.
  • wwestwwest Posts: 10,706
    Except confusion reigns!

    AWD/4WD with three OPEN diff'ls:

    As long as all four wheels have approximately equal roadbed adhesion then each wheel will get approximately 25% of the available drive torque.

    If a wheel or wheels begins to slip then ALL of the available torque will be instantly routed to the slipping wheel or wheels and if the driver doesn't quickly lift the throttle the engine will over rev and destruct.

    But on the other hand if I add Sequoia's VSC/TRAC system the computer will apply braking, but only to the wheels that are slipping, and the engine output level can continue to deliver substantial power to the wheels with traction.

    If all four wheels have simultaneously lost traction the only software method that can be used to actuate TRAC (or dethrottle) is to determine that the vehicle (seemingly) is accelerating faster than it weight/hp ratio would allow.

    LSDs, VCs, and all other implementations of AWD/4WD torque transfer facilitation will soon be relics of the past, the Sequoia system will reign supreme.
  • heatwave3heatwave3 Posts: 462
    pschreck: You ask the following "Why would you think that the Active TRAC system would apply the brake to a wheel that isn't turning? Once the wheel stops turning the brake is going to be released. The truck isn't going to just sit there with the brakes applied to all four wheels. Why would you think that all four wheels would be braked at the same time if the reason for braking the slipping wheel is to transfer power to the other side?"

    A brake activated traction control system with a completely open differential drivetrain has numerous advantages and weaknesses. The real benefit is that it can be run all the time with limited wear and no impact on turning radius or binding.

    One of the weaknesses is that if the open differential cannot find a tire with traction and all wheels are slipping, than the traction control will shut all four wheels down. Additionally, it is physically impossible for the system to deliver more power to a wheel than that wheel has resistance. Therefore it is incapable of overcoming resistance at a corner unless other corners are slipping.

    That's why almost every vehicle I can think of including my Toyota Avalon comes with a switch that can deactivate the system when stuck in snow or on ice in order to be able to rock the vehicle by spinning the wheels to move the vehicle to a location with better traction. In fact, my Avalon owner's manual specifically gives the instruction to deactive the traction control system in these circumstances. The fact that Toyota wouldn't have provided for this circumstance by providing an override switch on the Sequoia is quite telling.

    In the Sequoia, if you were to have a switch that deactivated the Tracs System with an all open center diff and open rear diff, do you have any idea what would occur if the system were deactivated and you were stuck on snow or ice? Only one wheel out of the four would rotate. That's why most if not all serious vehicles looking for traction either on or off-road will provide a limited slip center or locking rear differential or BOTH, to ensure more than one wheel gets power regardless of traction conditions.

    Unfortunately Toyota has taken a cheap approach by eliminated the hardware and selling the traction control system as a substitute. Its not, although given the likely use of a Sequoia it is unlikely to matter to most of its owners, since they will never go off-roading.

    That having been said, I would be very uncomfortable taking the Sequoia onto a beach or on any type of icey road surface when compared with a vehicle having a locking rear diff and at least a limited slip center diff. Unfortunately, there is no option for a locking rear diff in the Sequoia and locking center diff only occurs in 4wd lo which is hardly a useable feature for normal driving.

    I stated

    "1) if you are in 2wd and stuck on ice or snow at all four corners, your not going to be able to engage the 4wd system until the drivetrain has been able to rotate enough to engage the system. The rev limiter and traction system will conspire to prevent this."

    and you replied "Who, in the name of all that is right with the world, would be driving a 4WD vehicle in such conditions while in 2WD? Especially one that can be driven in 4WD in ANY conditions."

    Actually, there was a poster on the Toyota 4wd forum that described that very situation in a 4runner system which is identical to the Sequoia's

    "Just wanted to know if something like this has happened to anyone else and if I'm retarded or what but this past winter I parked my SR5 (read: not active trak but 'normal' Toyo 4WD) after a day of skiing in 2WD (roads were dry) and it snowed about two feet that night. Next morning, I tried to pull up the oh-so-slight incline of my buddy's driveway and VSC/Traction kicks in, retarding power to where I make no progress because of the wheelspin. Can't 'rock' it loose and MORE IMPORTANTLY a push of the button to 4WD only causes dash lights to blink and it won't engage in 4WD (it seems to take a long time - like 250 feet of pavement when moving under normal conditions anyway) - so effectively I'm stuck. Pressing diff lock button does NOT cancel VSC/Traction and though I moved the lever around into 4LO (trans in Neutral, of course) it still was no use as 4WD would not engage. Bottom line: I had to have two friends push me up and out of the driveway, until I could go the usual block or so under 2WD power and 4WD finally set in.

    SO WHAT GOOD is the "new" 4WD system if you can't use it to get un-stuck (read: you have to be IN it to use it)? My crappy old Explorer would at least sit and spin wheels until the 4WD locked in...

    Very disappointed that VSC and Trac cannot be switched off at the dash - I could've rocked it loose if that were the case. What a stupid oversight on a truck made for heavy-duty offroad use.

    That said - am I missing something here? It's my wife's car so I'm happy to have VSC and Traction control for her but I feel that Toyo neutered this truck in a bad way."

    I don't think the situation this owner described is so unique as to never occur to Sequoia owner. And is actually a very vivid description of why being able to turn the Trac control off is a "good" thing and eually important why having a mechnical system to limited the slippage in the first place might have enable this driving to get out of his predicament even while in 2wd. Instead he had 1 wheel drive,a trac system that shut him down and an inability to engage a different drivetrain setup.

    I state:

    "2) if you all are in 4wd (not low gear), there is no way to engage the 4wd lo gearing until momentum has been achieved. If you are in deep snow or on ice at all four corners, the traction system will prevent the wheels from turning, which will prevent the engagement of the locking center diff and lo gearing."

    Actually, you are wrong with the following view
    "...In fact, you must have the transmission in neutral to shift into 4WD Low. How can you get momentum when you are in neutral? When I have shifted into 4WD Low I have been at a standstill."

    You may have been at a standstill but if the drivetrain was unable to rotate due to the trac system (for example in snow or ice) the 4wd lo is not going to engage, as illustrated in the owner's predicament described above.

    I stated:

    "3) If you are engaged in 4wd lo, the center diff is locked but you still have an open rear (and front) diff, essentially providing the vehicle with 1 wheel drive in the rear and 1 wheel drive in the front, in lo gear, until the traction control system engages. On ice or deep snow, the system could very well engage preventing any wheel from rotating."

    And you replied "Wrong. See above."

    As you can see from the example above, my description is both accurate on paper and the way the system will work in the hands of owners of such a system.
  • wwestwwest Posts: 10,706
    You have a basic miss-understanding going here.

    Your Avalon is FWD and its TRAC system will therefore apply the brakes to both front wheels when either is slipping, otherwise you might get your thumbs broken or knuckles busted from differential braking on the front wheels.

    RWD TRAC systems and AWD/4WD TRAC systems will individually brake either slipping rear wheel without incurring this effect.
  • cliffy1cliffy1 Posts: 3,581
    I must admit that I didn't read all of your last post because it is unreadably long, but I did note that you never addressed the multiple links I posted demonstrating the weakness of MD's and their vulnerability to wear.

    I also think I see where you are missing a critical point. The TRACS system in the A-Trac system does not apply the brakes down hard. It is the same, rapid pumping system employed by the ABS system. The wheels are NOT locked down but rather forward momentum is briefly hindered to allow the open differential to "distribute" power elsewhere or equalize it.

    I told a story a while back that bear repeating now. I had a customer (back when I was selling) who was looking at the Land Cruiser. We test drove it on a morning after an ice storm. We went to a church parking lot that was completely glazed over. From a standing start, I had him punch the throttle and turn the wheel hard to the left. We heard the ABS system clicking and groaning and had a warning bell go off. The Cruiser didn't slip at all but rather, made a very rapid and controlled U-turn. All four wheels were sitting on a VERY slippery surface which had much less friction than the sand you seem to fear.

    As I said, let me know when you have real world experience proving your points.
  • cliffy1cliffy1 Posts: 3,581
    The Avalon is an interesting situation. Wwest is very correct that there is a huge difference between a FWD and 4WD in how the TRACS is designed to operate. The TRACS is supposed to be employed in snow and ice and that is the exact wrong time to turn off the system. Rather than burying your front tire deeper in the snow, the TRACS will slow the engine, while applying the ABS to allow you to creep forward.

    The time to turn off TRACS on a FWD is when you are drag racing or perhaps at a traffic light when the pavement is wet. It is NOT time to turn it off when you are stuck in a snow bank or in an icy parking lot.
  • wwestwwest Posts: 10,706
    I believe the actual procedure is to apply the front brakes and if the operator hasn't or isn't properly modulating, moderating, the throttle appropreate to the roadbed conditions then after a few hundred milliseconds it will step in and do it for him/her.

    At least that's how my 2000 GS300 RWD does it.
  • pschreckpschreck Posts: 524
    I read the links that you posted. It would seem that a LSD can potentially harm handling. I'm NOT saying that it will hurt but rather has the potential to hurt handling if set up improperly.

    It would also to appear that they start to wear with the first mile put on them.
  • cliffy1cliffy1 Posts: 3,581
    And they contribute to increased tire wear... and when they do wear out, the owner may not realize it because it just functions like a regular open differential.
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