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Toyota 4WD systems explained

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Comments

  • eagle63eagle63 Posts: 599
    "AWD is grip, slip, react, transfer and increase grip. That's why all the sophisticated sensing in the world will not replace having the grip in the first place. "

    -That's actually what I like about part-time 4wd. 50/50 torque split no matter what. no sensors, no guessing. In deep snow it can't be beat.
  • heatwave3heatwave3 Posts: 462
    cliffy: you state "Why do you even bother? You're embarrassing yourself. Where did you ever come up with the idea that the Sequoia comes with a viscous coupling center?"

    Maybe you might want to get off that pedestal of knowing everything. I certainly don't claim to know everything there is to know about 4wd systems but I've owned a variety of different 4wd vehicles and have what I consider a pretty extensive experience of 20 years operating them.

    You've claimed to know alot of information which might be well respected by many people unfamiliar with how these systems work. I've respected your views and the information you have provided, however you have been inaccurate on several occasions regarding some pretty basic info readily available on the net.

    Instead of feeling compelled to respond to me with your sighs and indignation, you might want to consider simply getting your information right instead of speculating or assuming information I share must automatically be wrong because I drive a GM.

    Now to answer your question. Please go back and reread my posts. I've never claimed that the Sequoia has a liq ctr diff. In fact I've stated clearly on various posts that the system has 3 open diffs. (which is one of the weaknesses of Toyota's approach to 4wd IMO).

    I did state that the Land Cruiser which you claimed has the exact same 4wd drivetrain as the Sequoia, in fact, is not the same as the Sequoia. I provided a source (above) which states that the ctr diff on a LC is a viscous liquid locking ctr diff. I also pointed out that the overseas models have a locking rear diff which is also lacking in the Sequoia. No where did I claim the Sequoia has a "viscous ctr coupling".
  • heatwave3heatwave3 Posts: 462
    eagle63: what you fail to understand is that the Sequoia cannot deliver "50/50 torque split no matter what. no sensors".

    You fail to understand the nature of open diffs even with traction control. There was an excellent site provided earlier which can better explain why, but basically your Sequoia will sense slipping in the front tires (for example) and potential move all the torque to the rear wheels. The system IS sensing this slippage and IS moving the torque throughout the open diff system until it finds a wheel that's not slipping. What I would like to know is how the system handles traction if all wheels are slipping as on ice. Open diffs will act as if the vehicle was placed in neutral with the engine accelerating but no power is transferred to the wheels.

    The Sequoia does not and cannot deliver 50/50 distribution of torque to both ends at all times. If you don't believe me ask cliffy since he appears to be the only information source that Sequoia owners will believe.
  • wwestwwest Posts: 10,706
    "Sequoia will sense slipping in the front wheels (for example) and potential move all the torque to the rear wheels"

    NO !!

    The Sequoia system will apply the brakes to the slipping wheel(s) (front, in your example)allowing the engine torque to remain elevated and the torque distribution will therefore remain approximately "equal", at least that is the "target" (equal rotational rate for all four wheels) for the ecu firmware.

    If all four wheels are (persistently) slipping the only solution for that is a tow chain.
  • eagle63eagle63 Posts: 599
    "eagle63: what you fail to understand is that the Sequoia cannot deliver "50/50 torque split no matter what. no sensors".
    You fail to understand the nature of open diffs even with traction control. There was an excellent site provided earlier which can better explain why, but basically your Sequoia will sense slipping in the front tires (for example) and potential move all the torque to the rear wheels."

    -I don't own a Sequoia and couldn't care less what kind of 4WD system it has. I'm talking about plain vanilla part-time 4wd.
  • heatwave3heatwave3 Posts: 462
    eagle63: Now I getcha. I thought you were referring to the Sequoia's 4wd system when you made your comments about sensors and 4wd systems.

    Yes, there is the advantage of simplicity and "forced torque" through a part-time 4wd system. The major downside however is the inability to use the system on dry pavement. You give up the advantage of the extra traction in turns and the simplicity of not having to activate 4wd should the roads suddenly turn nasty.

    That's why so many manufacturer's are trying different designs to come up with the maximum traction of 4wd combined with useability of use on all road types.
  • heatwave3heatwave3 Posts: 462
    wwest: you don't appear to understand your own drivetrain and its advantages and liabilities. Think through you post once again

    "The Sequoia system will apply the brakes to the slipping wheel(s) (front, in your example)allowing the engine torque to remain elevated and the torque distribution will therefore remain approximately "equal", at least that is the "target" (equal rotational rate for all four wheels) for the ecu firmware."

    Your own post tells you that what your saying is inaccurate. When the TT4 system determines that a front wheel for example is slipping, it automatically brakes that wheel to slow or stop its rotation. By doing so, it automatically drives the torque away from that wheel. In fact it can brake that wheel until all torque previously going to that corner has been re-distributed elsewhere. It does not, in fact, "maintain" the torque at that wheel as you state. It does exactly the opposite.

    It will continue to brake wheels until it finds a wheel or wheels that can actually use the torque without slipping. It does not do anything to maintain a 50/50 split of torque.

    If the front wheels are slipping on ice for example there will be no torque delivered to the front wheels. There is no target torque distribution the ECU is trying to maintain as the system does not measurement of torque. It fact it doesn't care what the torque is. It only measures slippage or impending slippage and then attempts to prevent it from occurring by grabbing the brake on that wheel.

    If all 4wheels are slipping, 3 open diffs will act as if the vehicle is in neutral, with the engine disengaged from the tires. In contrast, an awd vehicle will rotate the tires, possibly slipping but moving forward nevertheless, as it builds momentum the slippage will be reduced and the vehicle will move forward.
  • pschreckpschreck Posts: 524
    How can a vehicle gain momentum if all four wheels are slipping? If there is any traction to gain momentum with, then the wheels aren't truly slipping. And if a AWD vehicle has enough traction to move forward why wouldn't the full-time 4WD vehicle have enough traction to move forward?
  • wwestwwest Posts: 10,706
    There is no need for an LSD on the GS300, nor ANY lexus with TRAC. For a RWD vehicle as long as both rear wheels have a reasonably close or equal, level of traction, the open diff'l will deliver equal torque to both rear wheels.

    TRAC only comes into play if one rear wheel begins to slip and then TRAC will instantly apply, release, apply, release, etc, the brake to that wheel, using the on/off cycling of the brake to modulate the frictional coefficient of that wheel such that it's rotational rate roughly equals that of the opposite wheel.

    If this condition persists for more than a few hundred milliseconds and the operator does not react quickly and lift the throttle then the TRAC ecu will itself dethrottle the engine.

    The Sequoia, ML, and I think the new LC, AWD systems work the same way except TRAC will use the brakes on any wheel or wheel to alleviate wheelspin.

    Three open diff'ls with TRAC to insure true AWD operation on most surfaces, dry, slippery, snow, ice, etc.

    The Sequoia only locks the center diff'l in L4 and with the transmission shifter in "low", otherwise it uses TRAC to apportion torque in AWD mode if slippery conditions are encountered.
  • tidestertidester Posts: 10,110
    If there is any traction to gain momentum with, then the wheels aren't truly slipping.

    Not true! Just because the wheels are slipping doesn't mean friction has been reduced to zero.

    tidester
    Host
    SUVs
  • pschreckpschreck Posts: 524
    I believe that's what I was trying to say. My point being that if the AWD vehicle can find some traction then why can't the full-time 4WD find traction? If a tire sticks to the ground even for a fraction of a revolution, you should move. Isn't that a function of the tire as long as the wheel is turning?
  • tidestertidester Posts: 10,110
    Got ya! I thought you meant something a little different.

    tidester
    Host
    SUVs
  • heatwave3heatwave3 Posts: 462
    pschreck: There are unusual circumstances where it is beneficial to turn traction control off. The reason all traction control system can be turned off is because under icey or snowy conditions, from a standstill, it can be advantageous to spin the wheels to generate some forward momentum that a traction control system would prevent from occurring.

    In fact, I suspect your owner's manual will probably reference these circumstances and provide the recommendation to turn of the traction system in order to "rock" the vehicle out of a stuck situation or in an effort to spin your wheels to move slightly forward until greater traction is achieved. These are not common situations but they do occur.

    An open diff (w/o the traction system engaged) would aggravate the situation since the ability to spin the tire will be exaggerated as all the power of the engine is direct to one wheel through the open diff. With an LSD, both tires would spin providing a greater chance for some forward momentum to be realized.

    With the traction system engaged and an open diff, it would be impossible to force the tires to rotate if they are slipping. I see this as a potential problem in a wet, boggy or icey traction situation (of course at that point you could then engage a system like the Toyota's into the part time mode which would force the power to the tires by locking the ctr diff.)

    wwest: your description above is somewhat accurate, however it misses on several points. While the GS does not NEED the LSD because of the traction system, it would be a better handling and faster accelerating car with the LSD combined with the traction system. It would also provide greater traction and flexibility in icy conditions for the reasons I just described above. That's why the highest performance vehicles combine LSD diffs and traction control.

    Secondly, you are wrong on the following statement "AWD systems work the same way except TRAC will use the brakes on any wheel or wheel to alleviate wheelspin."

    AWD systems "ensure" that torque is delivered to a wheel. TRAC only prevents the available torque at a wheel from causing it to spin. These are two entirely different principles. The trak system on a Sequoia is focussed on preventing slippage w/o caring about whether power is actually delivered to the wheel. All AWD systems primarily are focussed on first ensuring power is delivered and then secondarily, through the liq visc ctr diff and LSD, transferring it to wheels that aren't slipping.

    That's why its my belief that a Sequoia with all its wheels on ice might very well just sit there, with no tire rotation whatsoever even though the engine was accelerating.

    For the record, I did hear back from Toyota today in answer to the question I asked "When the Sequoia is operating in full-time 4wd what is the distribution of power from front to rear in % of torque on dry road surfaces? Your assistance would be greatly appreciated."

    Their answer was
    "We apologize for our delayed response to your email.

    When the 2001 Sequoia is operating in full-time four-wheel drive mode, the amount of power distributed varies, as it is determined by the traction. However, the maximum percentage of power distributed among the front and the rear in part-time four-wheel drive mode, with the center differential locked is 50/50."

    From this reply it would appear that the max torque distribution is not 50/50 when in full time 4wd and only when in part-time with the ctr diff locked.

    I have sent a follow-up question for better understanding the Sequoia's system
    "Regarding the response provided to me below on the Sequoia 4wd system below, I would like some further explanation to better understand its operation. Specifically, I would like to know what the minimum % of torque (if any) that is designated to the front wheels when placed in full-time 4wd mode and when the system is being operated on dry roadways? Or does the system direct all the power to the rear wheels and only transfer power to the front wheels when its needed due to slippery conditions?

    Thank you in advance for your reply to this email to help me better understand how the “full-time 4wd” system functions."

    I will share the reply when its been received.
  • wwestwwest Posts: 10,706
    heatwave3

    Can you tell me of any vehicle with all four wheels slipping on ice that would move forward better then any other?

    Even 2WD?

    If all four wheels are slipping on an icy surface you need MORE TRACTION!

    Tire chains or...

    A tow chain!

    And either of these will work just as well with a 2WD.
  • heatwave3heatwave3 Posts: 462
    wwest: to answer you question "Can you tell me of any vehicle with all four wheels slipping on ice that would move forward better then any other?"

    Yes an awd vehicle with studded tires:)

    No seriously. I would say an AWD system with mechanical diff that distribute power and have minimum torque levels to each corner, combined with a traction system that prevents wheel slippage and possibly an electronic engine controller that reduces engine output in order to better match power to traction. I might even argue that on completely flat slick ice, I would want to turn off the traction control just at the start of going from a standstill to moving and rely on the engine management and and drivetrain delivering power to all four wheels. Once I was rolling I would definitely then want the traction control activated.
  • oswalt1oswalt1 Posts: 2
    I've a 2002 Sequoia and have been following the discussion in hopes of a definitive answer on whether I can leave it in 4wd all the time. I like how it handles better, etc. in this mode so this appeals to me. I did take a second to call Toyota, and after a brief consultation with a technical specialist, their rep told me that it should not be left in 4wd all the time, but rather this should only be used when needed. So, logic and all you guys say it can be left in, I'd like to leave it in (but don't want to risk damaging the drivetrain of my new $45K ride), but Toyota says don't. If I don't, then I have to put up with 4WD that doesn't engage immediately when I need it - that little flashing light will be annoying then because I'm probably stuck or close to being stuck). Clearly, the old style lever in my Durrango had its advantages. Is there any way to definitively resolve whether I can leave in in 4hi all the time?
  • cliffy1cliffy1 Posts: 3,581
    Here is an interesting link for some of you. This comes courtesy of Piasan via pscheck (I think). http://isuzu-suvs.com/ Scroll to the bottom and click in the Sequoia info. You will need Acrobat Reader to view it.
  • cliffy1cliffy1 Posts: 3,581
    I gotta tell you, I've never had a tremendous amount of faith in the guys at the consumer service 800 number. You may want to call them back and ask the following questions in this order:

    1. Do both the Land Cruiser and Sequoia operate off the Active-Trac system?

    2. Is the Land Cruiser always in 4WD?

    3. Is there any damage to the drive system of the LC by being left in 4WD all the time?

    4. If the systems operate in the same manner when engaged, why would it not be safe to use the Sequoia in 4WD all the time as well?

    My guess is that you'll hear a pause, followed by "ummm" followed by, "Well, I'm not sure. I'll have to get back to you on this." At which point, the "technical" guy will actually call somebody at engineering and find out that you can. He will then try to figure out a face saving way to inform you of this.
  • wwestwwest Posts: 10,706
    My copy of YOUR owners manual says the 4WD drive mode can be used at all time but suggests better gas mileage would be obtained in 2WD mode.

    On reading the manual it appears that to be safe you should never put the transmission shifter in "low" position. The manual is unclear about whether or not the center diff'l would get locked in this mode, so the safe thing to do is avoid the possibility.

    The manual is quite clear that in 4lo and with the shifter in the "low" position and the transmission is actually in low gear the center diff'l will be/get locked.
  • cliffy1cliffy1 Posts: 3,581
    I posted this about 18 months ago in the Sequoia topic but I think it bear repeating again here. Back when the Sequoia was being introduced, salesmen were taken to "ride and drive" events around the country to introduce us to the product. There were various demonstration but one really stood out.

    A large tarp was set up in a parking lot with water flowing over it and dishwashing soap applied to it. It was on a slight incline. Being that we were there on a 90 degree day, this was as close an approximation of ice as we could come up with.

    First, a Ford Explorer was brought on to the corner. The steering wheel was turned all the way to the right and the driver mashed the throttle to full. The wheels spun a bit and the thing drifted and slid but did manager to get off the tarp with a decent amount of control.

    Next, a Chevy Tahoe was brought to the line. We were asked to step back. The rear wheel spun in place for a moment and then it lurched forward and began to spin. It didn't make it off the tarp and ended up facing the place that it started. Had it actually made it to dry pavement, it may have flipped as the slide was VERY violent.

    Finally a Sequoia was brought to the line. At first, it was left in 2 wheel drive. The rear wheels slipped for a brief moment and then stopped. I could hear the rev limiter kick in. It made a very slow but controlled turn off the tarp.

    The Sequoia was brought back for another pass, but this time in 4WD. It took off like a shot, under complete control. It turned the corner without so much as a hint of a fishtail.

    It was an impressive demonstration.
  • cliffy1cliffy1 Posts: 3,581
    Thanks. I was going to look that up myself, but you saved me the trouble.
  • wwestwwest Posts: 10,706
    Ain't VSC (w/AWD*TRAC***) wonderful...

    *** (AWD*TRAC) = "virtual" AWD !

    or:

    Three open diff'ls for hard/dry surfaces and TRAC to implement (VIRTUAL {UNREAL!!}) locking diff'ls for LTS, low traction surfaces.
  • cliffy1cliffy1 Posts: 3,581
    You and I disagree from time to time, but I'm with you on this. My only hope is that Toyota will add the Active-Trac system to the Tundra in time for my next lease. Selling aside (and since I don't sell any more, that is easy to say) I think this system ranks right up there with sliced bread in terms of being pretty cool.
  • pschreckpschreck Posts: 524
    Have you seen the Sequoia transfer case pages? What a system. Multi-Mode Full time 4WD (a step above plain vanilla AWD), VSC, Active TRAC, EBD, locking center differential,etc. YOW!
  • heatwave3heatwave3 Posts: 462
    pschreck and wwest: Sounds like you guys have a strong need for reassurance on your drivetrains. Do you all feel better now that you've patted each other on the back with your "yows" and virtual celebrations?

    I certainly wouldn't want to dampen anyone's party with real world comparisons to proven systems used by performance manufacturer's from around the world, so I'll just watch as you all convince yourselves that the Sequoia system is actually a cut above an Abrahms tank when measuring traction.

    In the interim, I posed a few questions to Toyota along the line of cliffy's thinking above and upon receiving a response I'll pass it on to everyone.
  • heatwave3heatwave3 Posts: 462
    Cliffy: the demonstration on a soapy tarp you shared, sounds like an interesting comparison however I wonder if it was a truly fair comparison. You described the performance of the Sequoia in 4wd as "The Sequoia was brought back for another pass, but this time in 4WD. It took off like a shot, under complete control. It turned the corner without so much as a hint of a fishtail."

    It sounds like the 4wd system was engaged with the center diff locked by the way it accelerated. In fact it sounds like the Sequoia might have been in 4wd lo. If so this type of performance could have been achieved with almost any 4wd system on the market if it were engaged in its part-time 4wd mode ensuring torque and power was delivered to every wheel.

    From your description the Explorer and Tahoe were in their full-time 4wd mode and not 4wd hi which engages the center locking diff.

    Do you have any direct knowledge of what 4wd mode the Sequoia was actually in?
  • cliffy1cliffy1 Posts: 3,581
    You're grasping at straws here. At the speed it left the mat, there is no way it was in low and because the center can only be locked in low, it wasn't locked. Is that the best you can come up with to explain the poor performance of the others? Isn't it just possible that you have been wrong and the Sequoia really is superior?
  • heatwave3heatwave3 Posts: 462
    cliffy: its absolutely possible the Sequoia is better. In fact, its my view that the Sequoia as a vehicle and its 4wd system IS better than both of the vehicles it was compared to in the demo you described. I just happen to think the comparison might have been a shade less than fair because it was a dramatization.

    Making it more dramatic was in Toyota's (and the dealership running the drama) best interest and it certainly wasn't a scientific comparison. My only point was that both the Explorer and the Tahoe were not engaged in the best 4wd mode for the trial being run. The results might have still showed the Sequoia to be better, however I doubt it would have been as dramatic as you described if the other vehicles were given the opportunity to be engaged in their highest traction mode.

    Also 4wd lo will have the perception of acclerating far faster through the first 5-10mph than a vehicle engaged in 4wd hi due to the low gear ratio. The fact that the Sequoia "took off like a shot" sounds to me like it was engaged in 4wd lo since its doubtful that it exceedded 10 mph in this demo.
  • pschreckpschreck Posts: 524
    Took off like a shot in 4WD low? I don't think so. Try taking "off like a shot "sometime in 4WD low. Not gonna happen. And to lock the center diff on a Sequoia the transmission also has to be in low. Trust me, I own this truck, and it ain't going anywhere fast in 4WD low and locked.
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