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Toyota 4WD systems explained

cliffy1cliffy1 Posts: 3,581
edited February 8 in Toyota
I frequently get asked questions about various 4 wheel drive system on Toyota products. In the past, I replied on an individual basis. Finally, I wrote a fairly comprehensive piece on it but felt it was too long to post here. I have broken this up into four parts. The first is a list of terms and definitions. Next is an explanation of the Active-Track system on the Sequoia, Land Cruiser and 4Runner. Next is conventional 4WD systems like the Tundra and Tacoma have. Finally is a discussion of the AWD of the Highlander and RAV4.

I hope this is helpful.
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Comments

  • wwestwwest Posts: 10,706
    Absent a rear LSD the 99 & 00 RX300 have three OPEN differentials. Just like a RWD car with no LSD, if any ONE "driven" wheel loses traction then ALL engine power will be routed to the tire with no traction and it will simply spin freely and nowhere will you go.

    So in a 99 0r 00 RX300, absent LSD or some inertia, any one of the four wheels having lost traction and you don't GO! Add LSD and now you have a vehicle that only gets stuck if either front wheel loses traction or both rear wheels.

    All RX300s have a viscous, fluid, clutch between the center differential output and the rear driveline. You can think of this as a rubber band drive belt with the exception that the more tension you place on the rubber band the harder and harder it gets to stretch.

    As you can see, if all four wheels have good traction then the rear wheels will most readily turn at the same rate as the directly driven front wheels. But, are they turning at an equal rate due to traction with the roadbed or due to "rubber band drive tension". Since in this example, and only for this example, all four wheels have traction it really doesn't matter how or what is driving the rear wheels. Lexus can honestly claim that in this circumstance each wheel is getting 25% (50% to the front and 50% toi the rear) of the engine torque and no one can ever prove otherwise

    Viscous Clutch: I'va always been puzzled by the "fact" that science has come up with a fluid that increases its viscosity as it is heated. I have now come to believe that what is actually happening is that the fluid's "effective" viscosity increases, not its actual viscosity.

    A viscous clutch is composed of two sets of interspersed clutch plates, one set attached to the "input" shaft and the other to the "output" shaft, which are immersed in a fluid in a sealed container. Almost all fluids will expand their volume with increases in temperature, and the silicon fluid in a viscous clutch is specually formulated to increase its volume dramatically with even slight increases in temperature.
  • cliffy1cliffy1 Posts: 3,581
    Transfer Case: A device behind the transmission that sends power to the rear only or to the front and rear in an exact, 50-50 split

    Center Differential: A devise that allows the power to be split between the front and rear in different amounts.

    Front or Rear Differential: A devise that splits power to the right or left side of the vehicle. It looks like a ball when viewed from the rear.

    Open Differential: A differential that is very rugged and reliable that splits power 50-50 but if one wheel begins to slip, power is routed only to that wheel.

    Limited Slip Differential (LSD): A differential that splits power 50-50 but if one wheel begins to slip, power is routed to the other wheel. This is accomplished in one of two ways: Either by a series of clutches or multiple plates encased in a heavy oil. The clutch type needs rebuilt somewhere between 60K and 100K miles. When the clutches fail, you will have no indication of it unless you become stuck.

    Rear Locking Differential: A push button mechanism that sends a worm gear into the rear differential to physically lock the axle together. NEVER use this on dry pavement.

    Center Differential lock: On certain vehicles, you can lock the center differential, which creates the effect of a transfer case. NEVER use this on dry pavement.

    Traction control (TRACS): A system that uses your ABS sensors to detect wheel slip. TRACS operates in one of three different ways. It can either apply brakes to the wheel that is slipping, it can reduce the engine RPMs to slow down the wheels or it can do both. In any case, by reducing wheel spin, forward movement is accomplished, albeit slow and controlled.

    Vehicle Skid Control (VSC): A lateral skid sensor that detects understeer or oversteer. If you begin to skid sideways, one, two or three corners of the vehicle have brakes applied and throttle is adjusted to get you moving in the direction the steering wheel is turned.

    Two Speed Transfer Case: Gives one the ability to gear the vehicle down for maximum pulling power. Works best with a transfer case or locked center differential.

    Part Time 4WD: This generally refers to a system that allows the use of the vehicle in 2 wheel drive. Prior to the Active Trac system, a part time system meant a transfer case type system.

    AWD, All-Trac, Full time 4WD: These terms refer to a 4WD system that is always engaged. It implies the use of a center differential rather than a transfer case. The A-Trac system defies this as a simple definition now.
  • cliffy1cliffy1 Posts: 3,581
    Beginning with the 2000 model year Land Cruiser, Toyota began using a new 4WD system called ActiveTrac. This same system was incorporated into all 2001 4Runners and the new 2001 Sequoias. The same basic system has also been used in the Mercedes M Class as well as post ‘99 Humvees.

    These systems operate in essentially the same way with a few exceptions. When engaged, you have three open differentials working for you (front, rear and center). Open differentials are extremely reliable and require very little maintenance. If you have equal traction at all 4 wheels, power is evenly divided between them all. If one wheel begins to slip, the open differentials begin to send all available power to that one wheel. Normally, this would be very bad. This is when a traction control system (TRACS) takes over. TRACS, applies brakes selectively to a slipping wheel. This braking action literally fools the differentials into sending power everywhere except the slipping wheel.

    When you are in 2WD (in the Sequoia and Runner), you still have traction control working for you. Obviously, this only will send power left to right but this is better than nothing. There is one thing to be careful of in this condition. When you are in 2-wheel drive, there is a second part of the TRACS that can be hazardous if you are not paying attention. This is the engine speed limiter. This combines the braking action of TRACS with a rev limiter. Your engine speed will be cut back to 1500 to 2400 RPM. This allows for controlled forward movement but it will be slow. The danger with this is if it engages when you are trying to pull into fast moving traffic. This rev limiter only operates in the 2WD mode, so if you know you have any reduced traction, make sure you are in 4WD.

    On all three vehicles, you have the option of locking the center differential. It is rare that anybody would ever need to do this. On the Land Cruiser and 4Runner, this is accomplished by pushing a button on your dash. On the Sequoia, you shift into 4 wheel low and shift the transmission into “L”. This turns off the TRACS computer and the VSC system. The vehicle is now in a conventional 4WD mode. All 4x4 Toyota trucks have operated in this condition. You should not ever use this mode on dry pavement as you will damage the drive system and tires.

    The other part of this system is the VSC or vehicle skid control. VSC will selectively apply brakes and throttle to prevent understeer or oversteer. It works in both 2 and 4 wheel drive. This is a rather amazing system and does an incredible job of giving the driver control of the vehicle. Understeer is responsible for a large number of SUV rollovers and oversteer is very common on icy surfaces. The Sequoia will allow you to turn off the VSC but only when you are in 4WD. The only reason to turn this off is if you are off road and want to be able to slide sideways. On the Runner and Land Cruiser, the VSC and TRACS are disabled when you lock the center differential.

    The Land Cruiser is always in the 4WD mode. The 4Runner and Sequoia can be used in either 4WD or 2WD. It is safe to leave either in the 4WD mode at all times. You will loose a bit of fuel economy, but will handle better. Unexpected loose gravel and slippery surfaces will not be a problem.

    To engage the 4WD system on the Runner and Sequoia, press the button. The green and amber lights will flash on you dash. While it is flashing, the system has not fully engaged and you should avoid sharp corners at this time. If you are accelerating up a hill, these lights will continue to flash. If this happens, take your foot off the gas for a moment and tap the brake. This gives the differential a chance to engage fully into the 4WD mode. The same procedure applies to disengaging the system.

    To get into 4WD low, you must first be in 4WD. Stop the truck and place the transmission in neutral. Now, move the floor shifter forward to the low range. This takes a firm hand. This mode is only to be used to remove yourself from a very difficult situation. Once you are unstuck, shift back into the high gear range.
  • cliffy1cliffy1 Posts: 3,581
    These are typical part time systems. Under good road conditions, you are in 2WD with the rear axle getting all the power. Power is again split between the right and left wheels. An open differential will route all power to one wheel if it can turn faster than the other. If this happens, engage the 4WD system. This sends exactly half the power to the front axle where another open differential splits power. Between the front and rear axle, you will normally be able to gain forward traction but because of the open differentials, there is a possibility that you wont. Open differentials are vastly more reliable and longer lasting than limited slip differentials, which is why Toyota has stuck with them.

    With this part time system, you can engage it up to 62 MPH (50 MPH if you don’t have a push button system) but it really isn’t appropriate to drive it at this speed. Because the front and rear axles are turning at exactly the same speed, you can damage the system on dry pavement. This system is only appropriate for more severe conditions.

    The advantages to this type of 4WD are simplicity and speed of engagement. You are not relying on brake sensors for your 4WD system and it should be more rugged. Also, unlike the Sequoia and 4Runner, the system engages the moment you shift into 4WD. The other models take several seconds and feet to engage.
  • cliffy1cliffy1 Posts: 3,581
    These utilize a limited slip center differential and open front and rear differentials. It is a viscous coupling center differential. If one of the front wheels begins to spin faster than the rear, the heavy liquid in the center begins to firm up which routes more power to the rear. Once torque is equalized, the 50-50 power split is resumed. This system is always engaged and requires no driver input.

    It is possible to become stuck with this system. This is because of the open front and rear differentials. If both right tires were on ice, all power would be routed to these wheels. This is a fairly unlikely occurrence on a light duty vehicle like these. On the Highlander, you can get VSC, which includes traction control. If the right wheel begins to slip, brakes are applied to this wheel and power is sent to the left. On the 4WD model, there is no rev limiter associated with the traction control.
  • loma1loma1 Posts: 32
    On the 4Runner can you use 4Lo without locking the center differential? If you can, you still have traction control I assume.
  • cliffy1cliffy1 Posts: 3,581
    That is correct. On the Runner, you can run with the full TRACS/VSC system in 4 Low or lock up the center and have a conventional 4WD system.
  • leomortleomort Posts: 451
    on 4Runners. Can you use the 4 wheel drive in hi mode when driving on snowy highways? I know you're not suppose to on Dry roads. How much benefit do you get from the locking diffy when off-roading? Don't see many 4Runners with this option. Looking at possibly getting a used 4Runner.

    Leo
  • drew_drew_ Posts: 3,382
    Off-roading in a TRACS-equipped '01 4Runnner:
    tbird45 "Toyota 4Runner" Jun 4, 2001 8:03pm


    Drew
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  • leomortleomort Posts: 451
    I thought Toyota got rid of the locking rear diffy on the 2001's? I thought I also read somewhere that this option can't be had with an automatic transmission?

    Leo
  • cliffy1cliffy1 Posts: 3,581
    My first question for you is, what model year is your Runner? If you have a 2001, there is a center differential lock, and that places it into a conventional part time 4WD mode. This should not be used on dry pavement. With the 2001, it is safe to use 4WD high (unlocked center differential) on dry pavement.

    If you are talking about an older one, they had a conventional part time system. Read back to the section here that discusses the Tundra and Tacoma systems. That should answer your questions.
  • To summarize, do you mean to say the the HL with 4wd and VSC is the least likely to get stuck in moderate snow conditions? Also, is the limited slip dif a clutch or plate type? What is the likelyhood of its needing service?
    Thanks for all your informative posts.
    JM
  • leomortleomort Posts: 451
    if I can only use the 4x4 off road, then it really isn't worth getting if it can't be used on pavement in snowy or bad road conditions during winter.

    Leo
  • leomortleomort Posts: 451
    it appears that I can use the 4x4 during adverse weather condtions on pavement, as long as it's not dry. Just how "bad" is the key.

    Leo
  • cliffy1cliffy1 Posts: 3,581
    On the HL, the least likely version is the 4WD with the VSC. The limited slip differential in the CENTER is a viscous coupling, while the optional rear LSD is a clutch type. The down side to clutch types is that they wear out. When they do, you may not even know it. They will just behave like a normal open type.
  • teeceebteeceeb Posts: 4
    Cliffy,

    On the awd highlander there is a sticker on the center differential that states 85w-90w , would it be ok to use mobile 1 syn 75w-90w gear oil? you mention that when the front wheels spin faster than the rear, the heavy liquid firms up. so should a 75-140 be better?

    thanks
    tc
  • cliffy1cliffy1 Posts: 3,581
    I'm the wrong guy to answer that question. I am a sales guy and while I have a good level of knowledge of how things work, I leave the actual mechanics to the mechanics.
  • rootheadroothead Posts: 3
    Hi - I just found this board and have a question regarding how to use the different aspects of my 2000 TL's full-time 4WD system. Here is the situation that I got into and I wonder what I should have done differently. Any advice you could provide would be greatly appreciated

    A friend took his 2000 Montero out into a field to to take his son to a well-stocked pond to do some fishing. Here in Central Ohio, we have gotten 8-9 inches of rain in the last few weeks. All went well for him until he got stock in a low area that quickly turned into a mud hole. He called me and I came over with my tow chain. As I went off the pavement, I continued to stay in 4WD-H mode and easily travel 1 mile back into the field over rolling hills and muddy trails. I place the TL into 4WD-L (after stopping the Cruiser and placing the sifter into "N", I moved the 4WD sifter into the "L" position.) and pulled his Montero out. I then followed him back towards the road. Here is where the problem arose - we needed to go down through a gully that was mud on both sides (down into the gully as well as back up on to "normal ground)and the low point of the gully was 10-12 inches of thick mud. Believe it or not, the Montero went down and up and over to the "dry" side with no problems. The TL and I (still in 4WD-L mode)followed him at a safe distance. All went well until I started to spin the right front wheel. I sat there 1/2 way up the side and started to slide into some trees. I stopped and backed down into the gully until I reached the flat muddy area at the bottom and then tried again - still no luck. Finally, I backed down and turned off the VSC via the dash button, placed the transmission into "N", shifted the 4WD into "H" and hit the gas - up and over I went. My questions are:
    (1) What should I have differently so as to avoid the mess that I got into;
    (2) under what conditions do I use the 4WD-L mode and when should I ever have to turn off the VSC system?

    The TLC owners mannual is useless from this perspective.

    Thanks for your time and assistance.
  • cliffy1cliffy1 Posts: 3,581
    Interesting set of circumstances. The fact that you were able to get out of the mud hole without a tow speaks volumes about the LC capabilities. To avoid getting stuck in the future, I would suggest keeping it in the high gear range except when pulling somebody else out or when maximum torque is needed. Momentum is everything when dealing with mud.

    As to when to turn off the VSC (by locking the center differential), this is only a good idea when you need to be able to slip laterally. Driving in sand is a good example of when this works best. I would suspect that you would have been able to get out of the mud with the differential unlocked and the VSC functional. I wasn't there, but it sounds like you handled it well.
  • drew_drew_ Posts: 3,382
    Don't forget that 4WD Low multiples the torque to the wheels by over 2 times. This means that the tractive limit of the tires is reached faster and hence you can literally dig yourself into a hole by spinning the wheels with so much more torque. This is one reason why 4WD Low shouldn't be used in sand or snow (unless it's really thick and deep snow).


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  • 2nupe2nupe Posts: 1
    I recently purchased a 1998 Toyota Landcruiser. I know very little about locking differentials, etc. I live in Maryland where we don't get much snow. Under what circumstances (a couple of inches of snow, ice, wet pavement) should I change the setting from 4H to 4L and when should I lock the differential (I also have the rear differential option)? Also, how do you change settings (on the fly or at a stop) and how do you lock the differential (on the fly or at a stop)?

    I have three instruments, the 4H N 4L shifter, the locking differential knob on the left of the steering wheel, and the differential button in the main controls near the air conditioning buttons. Which controls which?
  • cliffy1cliffy1 Posts: 3,581
    Unless you are going off road, there are never too many situations in MD that would require you to either lock the differentials (either center or rear) or shift into the low range. To engage low, you first need to stop and place the transmission in neutral. Use this only if you are pulling somebody else out of a ditch or are in a ditch yourself.

    As for the center and rear locker, again unless you are doing some fairly serious off roading, don't mess with them. If you are, send me an e-mail at sclifford@kjtoyota.com and I'll go into it further.
  • On my new highlander do?
  • tonychrystonychrys Posts: 1,310
    The Snow Button will force the auto-transmission to start in 2nd gear after you come to stop (like at a traffic light) rather than 1st. The reason is that by starting in 2nd gear you are applying less torque to the wheels and less likely to spin them on a snowy surface.
  • cageymcageym Posts: 6
    I live in the suburban Chicago area where, frankly, my need for an all wheel drive vehicle seems not too great to me. But as I talk with friends about shopping for a new car (about 100% sure it will be a HL), everyone tells me that I am crazy not to get AWD. They maintain the AWD will handle better, etc. I counter with reservations about fuel consumption and additional expense, and they scoff at me. We all seem to agree about the value of the skid control, at least in theory. Anyway, I'd appreciate some discussion of the pros and cons of 2WD vs AWD in these instances. I mean, I will not be off-roading in this car and the snow has never been much of a problem for me in my little FWD compact....

    Thanks for your indulgence!
  • drew_drew_ Posts: 3,382
    You may want to read the messages in the 4WD and AWD systems explained discussion topic too. You're already spending a fair amount for the Highlander. Why not spend a bit more to get the safety and security of AWD? Getting an SUV without 4WD or AWD, in my humble opinion, is like buying a minivan with only 5 seats.


    Here's something that may help you to decide.

    http://www.4x4abc.com/4WD101/tractionturn2.html

    http://www.4x4abc.com/4WD101/need.html


    Drew
    Host
    Vans, SUVs, and Aftermarket & Accessories message boards
  • chadhburkechadhburke Posts: 27
    Thanks for the previous posts on various terminology and an explanation of the AWD system on the Highlander. I've got a HL with the skid control option. If I understand it right, the HL has open differentials on the front and rear, in which case it is possible to have both left or both right wheels spinning under the right (or wrong) circumstances. Now, it sounds like at that point the VSC feature would kick in by applying the brakes to the spinning wheels. At that point, power would be diverted to the other wheels.

    Assuming what I've said is right, then it seems that it would take some work to get the HL stuck. I know very little about driving in "off-road" conditions, and would like to know if I am likely to get stuck in say mud of different depths, snow, etc. Also, is it ok to drive my HL in off road conditions so long as I don't encounter serious ground clearance obstacles? (i.e. are the suspension and other components "tough" enough to handle rough but not abusive terrain?)

    Thanks
  • cliffy1cliffy1 Posts: 3,581
    By jove, I think you've got it! The real limiting factor in off roading for you is ground clearance and the fact that the HL is a unibody. The unibody means that the body and frame are a single unit and you can flex it by going too serious off road. The 4Runner has a frame to take up this stress but rides more harshly on road.
  • kmhkmh Posts: 143
    I own a '99 RX300 AWD which has a rear limited slip differential. I haven't found a clear answer as to what the power ratio or split would be should there be on this model. Is it 50/50? 70/30?

    My '99 is coming up on 60,000 miles - what sort of maintenance should I look into for the AWD or LSD?
  • cliffy1cliffy1 Posts: 3,581
    Yours is a 50-50 power split. Your second question is a bit confusing though. You ask whether you need an AWD or LSD. The LSD is only something you can get on the AWD model. Were it me, I'd skip the LSD and get the VSC instead, which I think is standard on the RX anyway.
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