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Toyota 4WD systems explained

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Comments

  • 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.
  • brillmtbbrillmtb Posts: 543
    I believe that there are different types of Limited slip differentials and I would be very suprised if you could demonstrate a difference in tire wear on normal driving around corners.
  • brillmtbbrillmtb Posts: 543
    I think most of what you have said is correct. I found a review of the Sequoia (the new system for the sakes of this site) where, for the first time, the reviewers did not like the system in serious off roading. One guy went as far to say that he wished he had wire cutters by the end of the test to disable the electronic control of the system.

    While I think this is an extreme statement and the system probably works well in standard snow/ice settings where it did not preform well in the review was in sand.

    It seems that when all wheels had limited traction and the driver was tring to carry momentum through a loose sand section to avoid getting stuck (anyone who had done 4wding knows this situation well) the Toyota system in the Sequoia first started breaking the wheels then powering down. This apparently resulted in decreased momentum and getting stuck.

    It seems like this could be a potential reality and not some made up story by a die hard chevy guy but I would love to test it out if I can find a friend with a Sequoia.

    I think the more control you have over a system the better. I have the Montero and it has settings for 2wd, 4wdhi, 4wdhi-lock, 4wdlow with a limited slip rear. You just have such good control. I personally have seen the difference between the AWD-like setting (4wd hi) and 4wd hi lock with the lock setting giving me better positive control over the vehicle in mud where I would have ended up in the river if there was any hesitation in the system at all. So I know first hand what you mean when you say that putting power to all 4 wheels with a 50-50 fixed slit is often times better than letting the vehicle chose.

    Also, dosent the Subaru, and most all AWD, limit the power available to the front wheels. Some are only up to 30% I think.
  • pschreckpschreck Posts: 524
    I'd like to read that review, where can I find it? It's unlikely that I'll ever be in the sand as I hate the sand and beach in general.

    I agree with your statement about having control over your system. Just one reason that I wouldn't want a truck that only offers AWD. Of course if I were to do any SERIOUS offroading I'd want something smaller than a F/S SUV anyhow.

    As to the Montero, it is just too small to suit our needs at this point in time. Then there's that rollover thing.
  • cliffy1cliffy1 Posts: 3,581
    It took me some time, but I went back through the archives of the Sequoia topic to find a post from a Sequoia owner who used it off road. Below is a cut and paste of a post from Slickrock who seems to have abandoned the Town Hall.

    While looking for this, I was reminded of how long Heatwave has been looking for things to complain about.

    #1760 of 5543 Sequoia in Sand and Sandstone by slickrock Mar 29, 2001 (09:59 am)
    Thanks Cliffy, I thought nobody would ask...

    I spent a fair amount time off-road last week in Moab, Utah, and learned quite a bit about how the Sequoia performs in actual off-road conditions. The trails were rated up to 3 1/2 (at 4 you risk vehicle damage), and included sand, slickrock (sandstone), streams, and slopes. I have some pictures, but haven't figured out exactly how to share them here.

    First let me say that my baseline is an older ('84) stick-shift Landcruiser, so while the Sequoia was different, it was familiar in many ways. Here are some observations and possible points for later discussion.

    The Sequoia can do trails rated up to 2 1/2 in high range. Beyond that you need low range. I will limit the rest of the discussion here to low range performance.

    In low range, the Sequoia has plenty of engine and braking power for the steepest hills (~35 degrees) that I tried. It also had no power problems climbing ledges, but traction was sometimes an issue when there was loose dirt mixes in with the rock.

    I did some experimentation in Low/Low (locked center differential) vs Low/Second (VCS/Tracs). What I found was that you had to turn VCS off, otherwise it kicked in (and messed with the throttle and braking) when you didn't want it to, because the wheels will slip on sand and dirt.

    On the other hand, I ran into at least one case climbing a dirty rock ledge where L/L wouldn't make it up(diagonally opposite wheels were slipping), but L/S (and Active-Trac) got me up. It was not smooth or quiet, but it outperformed the locked center differential.

    However L/S was deadly going down hill (on steep hills). There was definitely insufficient engine braking. But you don't need (or want) Active-Trac going down hill. So my rules for off-road driving-mode selection are actually pretty simple:

    1. In 4WD Low Range off-road, turn off VCS (push the button) and use L/D.
    2. In Low Range going up a difficult hill, use L/S (Active-Trac).
    3. In Low Range going down a difficult hill, use L/L (Max. Engine Braking).

    The second thing I noticed is that size matters. The Sequoia is long and wide. The width came into play in maneuvering around large (>1 ft.) rocks on the sides of the trails and narrow trails. There were also some very tight turns that required a bit of jockying. But it's better than a Hummer, and to tell the truth, the tightest turns were in the City Market parking lot.

    The length is another story. The Sequoia is a looooong truck. It has plenty of ground clearance (I may have hit the skidplates once or twice), and so the breakover angle was not a big issue. I thought the running boards would take hits, but they are high enough and tucked in well enough that they were not a problem. But I would want to remove them (8 bolts each) before trying a 4-rated trail.

    I only touched the underside of the front bumper once, so I am satisfied with the approach angle. But the departure angle (and that long tail) leaves a lot to be desired. I hit the trailer hitch receiver many times (that's to be expected -- I consider it part of the skid plate system). But I also hit the underside of the giant one-piece plastic rear bumper a few times (which is 2" higher, but who said the rock was perfectly level).

    The PLASTIC bumper is definitely not part of the skid plate system (or at least not for very long). Hey Toyota people who supposedly read this board, when sandstone meets plastic, guess what always wins!! This was a design mistake.

    So crossing gullys (and any other concave surface) became an interesting challenge, and I would have to say that the limiting factor to the Sequoia's off-road performance is the integrity of the giant one-piece rear plastic bumper cover.

    Do 2001 4-Runners and Land Cruisers also have these plastic bumber covers??

    I should also mention tires. I did these trails with some trepidation given the stock passenger car Bridgestones. In the future, I will replace them with something starting with LT and having a C or D load rating. Maybe Michelin LTX A/T 265/75R16's. That would be add 1/2" to the height, and be much safer off-road. The odometer would take a 3.4% hit (that improves the warranty), but the speedometer would finally be right. I also think the ABS/VCS/Tracs ECU's could wouldn't notice the minor difference.

    Beyond the 1/2" tire lift, I do need a solution to protecting the left and right underside of the rear bumper cover. Some sort of real skid plate or sacrificial add-on. Any suggestions would be welcome. I would be reluctant to consider lifts or air shocks, because I don't want to mess too much with the suspension or ride. Maybe TRD or Toyota off-road will offer something someday. In time there will be Sequoias in the junkyard with good rear bumper covers. Maybe I can make something suitable out of one of them.

    Finally, I must say that it was a pure pleasure to cruise the western freeways at 75-80 mph. I never felt fatigued even after a full day of driving, and the vehicle generally performed flawlesly on the highway. On the other hand, it only got 15 - 16 mpg at 75 mph (I suppose it would have done better at 55 mph).
  • wwestwwest Posts: 10,706
    May I suggest new definitions for SUV categories ???

    Some people need/want SUVs for SPORTS off-road.

    Others simply want an SUV for reliable "on-road" wintertime, or any low traction surface, point A to point B, travel.

    It occurred to me on reading the above posts that maybe what is needed is a third SUV category that the manufacturers and the public can use to define their vehicle's capabilities.

    Categories.

    1. SUVs like the RX300, Highlander, and MDX that are primarily minivans with four doors, high seating and reasonably large interior volumes but little or no ability to travel in wintertime relaibly on low traction surfaces.

    2. SUVs such as the Sequoia, X5, and ML that can travel on wintertime low traction surfaces reliably but cannot and/or should NOT be used for true SPORTING style off-road.

    3. SUVs that fall into the Jeep category, can be used in true off-road SPORTING events, mud racing, etc.
  • cliffy1cliffy1 Posts: 3,581
    How about Sport wagon, Sport utility vehicles and utility vehicles? I actually disagree with your assessment on the Sequoia, but I get your point.
  • pschreckpschreck Posts: 524
    Does your Porsche have any LSDs in it, or does it get around them by using the AWD and a great set of tires for traction purposes? Just wondering.
  • wwestwwest Posts: 10,706
    My 2001 911/996 has PSM and a VC mounted in the front in line with the drive line to the front diff'l. The PSM system supplants the need for a mechanical LSD by applying moderate braking to either rear wheel that loses roadbed traction.

    I suspect, but don't really know, that there is no PSM/LSD activity at the front wheels since the clear majority of torque is always to the rear wheels.

    I have never had the PSM actuate the traction control mode yet (both rear wheels slipping) but I understand it to be much like the GS300, apply braking first and then moderate the throttle if the driver doesn't.
  • cliffy1cliffy1 Posts: 3,581
    So, according to what Wwest tells us, it looks like Porsche did things on the cheap like Toyota by eliminating the LSD (aka magic differential) on its model. :)
  • jynewfjynewf Posts: 26
    The Land Rover Discovery and Range Rover also use a braking-type system. In the case of the 99-2001 Discovery, the braking/traction control system is the only torque distributing system. There is no center locking, VC or LS diff, either in HI or LO mode, in the 99-2001 Disco.

    Also, the Mercedes M-Class also uses such a system, and Audi may have also used a braking type system in some of its cars in the past, along with a torsen center diff.

    Notwithstanding Heatwave3's earlier comments, Toyota certainly is not going it alone in implementing such a system in its vehicles.
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