Toyota 4WD systems explained

sebring95

I'm moving here to discuss the 4X4 systems further from the Tahoe comparison. I'm really looking at either the JGC, Tahoe, or '03 4runner but since the Sequoia and 4runner have the same system it's relative. According to cliffy's original description of the Sequoia's system, when you shift into 4-low it locks the center diff and turns off the traction control (that'd make it a part-time system at that point). According to Car and Driver, they agreed that shifting to 4lo locked the center differ but DID NOT turn off the traction control. Therefore the traction control simply brought the vehicle to a stop because it couldn't spin the tires. I had a similar run-in with a RX300 trying to climb a steep snowy driveway. It basically stopped dead and slid back down. A Jeep Grand Cherokee makes this driveway look like a cake walk without all these computers and such running the show. That's what I meant by a simple 4X4 system. A part-time system will climb a lightly muddy hill even with crappy tires. These computers are controlling everything and basically you can't use the trucks beyond pavement and marginal weather. I've had my Jeep and Tacoma buried in serious mud and dug my way through and the Tacom had open-diffs. The point you'd go through anything deeper would require more ground clearance before anything else. Obviously ltd slips, lockers, etc. make things easier, but sometimes you've got to be able to spin. Traction control is better called spin control. Now if you could really control traction that's be great! Like a set of M/T's that turn into highway all-seasons with the touch of a button.

wwest

I have owned an 85 Jeep and a 92, not JGC, but with both full-time and part-time. All my experiences were that when the part-time didn't work, spinning the wheels was useless, it was simply time to get out the tire chains and IMPROVE THE TRACTION.

Now, I can remember spinning the tires and rocking the car back and forth to get out of a "singular" incident, like climbing your driveway, but the majority of incidents wherein more traction is needed are not "singular".

And I like to think if I ever own one of these wherein I want to disable the traction control I would remember to simply pull the ABS pump fuse.

cliffy1

The center differential is only locked on the Sequoia when you shift the transfer case into low AND shift the transmission into low as well. Also, I read the article you mentioned and I don't recall them claiming the wheels were brought to a stop, nor do I recall a mention of the car sliding backwards.

cliffy1

The others are correct. You can engage the 4WD system safely on your 2002 Runner any time you want, even on dry pavement. If you have even the slightest question about traction conditions, turn it on and let it do its thing.

pschreck

Actually, to lock the center diff, you have to engage 4WD Low and put the transmission into Low. Perhaps that's where C&D made their mistake.

I'm not sure what you mean by "it couldn't spin the tires". I say that because the brakes don't lock down the wheels when all four lose traction.

I can tell you that I drove the Sequoia up some pretty steep hills in 5-6 inches of fresh snow with no problem. Same result when the snow had been packed down by other vehicles. I'm hoping we get a serious winter this year so I can really give her a workout.

Keep in mind that the RX300 system is not the same as ActiveTRAC. wwest can describe that system in great detail.

sebring95

they didn't mention the Sequoia sliding backwards, that was a personal observation. My in-laws and I test drove the RX300 after a big snow to see how it did on their driveway and it couldn't make it. Basically stopped half-way up and slid back down.

I'd have to review the Sequoia test again, but I remember they said they could get the differential to lock-in going to 4lo but that didn't turn the traction control off. They felt that was why they got stuck. That and the tires. The other SUV's made it and I've looked closely at the Tahoe and it doesn't have much in the way of tires either. That's an easy fix on anything, but then you lose some of the handling/ride that goes along with those highway tires so you have to wonder if it's not all relative. Either way, I won't buy something if I can't turn the traction control off and I'd rather not have to pull the ABS fuse to do it. My ultimate vehicle would have an off switch for ABS and an off switch for traction control.

wwest

With my 92 Jeep I always pulled the ABS fuse except during the winter months. ABS is there to help you steer the car, maintain directional control, often, most often on high traction surfaces, to the detriment of stopping more quickly.

Now that the technology is clearly available I don't understand why the activation of ABS isn't delayed if the deceleration rate is above a certain "threshold", and/or there is no threat of loss of directional control.

pschreck

Are you sure that they locked the center differential? Or are they saying that they got it into 4WD Low?

axo

I have a '99 4Runner lmtd 4x4. I am experiencing a bumping motion after stopping the car. Has anyone else experience this? Is there a fix for this?

Thanks

sebring95

I'm out of town and my mag is at home, but if I remember correctly what they said was something like this: "putting the Sequoia into 4lo disengages the auto 4X4 system and locks the center differential. However, the traction control continues to work and between the caked up mud on the tires and not being able to spin our way up, the Sequoia couldn't climb the hill." Again I'm guessing on the wording, but what I got out of it was yes you can lock the differential, but the traction control continued to operate. The Sequoia was the only one unable to complete the obstacle, and had the most sophisticated (on paper) 4X4 system. I remember some other off-road tests where there were issues with the stability control getting the best of the Seqouia as well. If the 4runner has this system, I question whether I want one as I've had no problem getting through severe off-roading with much less sophisticated systems. Might be pretty good on-road, but any simple full-time system can handle snowy roads.

cliffy1

Well, if that is what the article said, they are wrong. Shifting into 4WD low does NOT lock the center on the Sequoia. The A-Trac system functions as normal in this mode. It isn't until you also shift the transmission into low that the center locks on the Sequoia. It is different in the Runner because you have a separate differential lock.

Don't assume magazine writers know what they are doing all the time.

eric4wd

Cliffy, thanks for your valuable input on this board. I've learned a lot on different 4WD systems and know just about everything now on my brand new 2002 Black Sequoia Limited!! Actually, I'm going to have it next week.. very anxious! Quick question on the 4wd low mode.. I live up a steep hill (150 feet long with 25 degree pitch) and wondered if it is advisable to use this mode when going downhill on slippery ice? I thought moving slowly in low gear will prevent me from using the brakes, hence no slinding sideways involved.. but would you consider this an extreme situation preventing potential damage to the system? I used to put sand before going down even with my current Volvo CrossCountry AWD but that was a pain in the a** when under an emergency situation. Thanks!

cliffy1

A 25 degree grade must be a lot of fun when it ices over. I'll bet it makes for a great sledding hill. Your plan to use 4WD low for engine braking is a good one, although you may not need to go that far. Shifting the transmission into Low, rather than drive might do the trick. If not, use the transfer case as you described.

elwaywon2

I've got a 2001 4Runner and last winter I found the traction control feature very irritating. I live in arctic North Dakota where there is patchy ice 9 months out of the year.

Here is what ticks me off: I'll be in 4x4 at an intersection waiting to turn. Traffic opens up and when I go a wheel spins momentarily on an icy spot. No big deal expect that traction control kicks in, beeps and engine power is removed from the wheels for several seconds. Now the truck is halfway out in the intersection with no power when I need it the most. After a couple seconds the power returns etc.

Can I just disengage this feature altogether? It seems worthless to me--I don't like the computer ever removing power--it seems dangerous.

Thanks

cliffy1

If you are in 4WD and the rev limiter kicks in, you have a defect. What you describe should never happen. It will if you are in 2WD, but never in 4WD.

wwest

Downhill/ice/4WD.

Really BAD news!

You would be much better off using the RWD only mode with the tranx in LOW. Use the engine braking to "drag" the rear wheels and keep them behind you and reserve the front wheels' traction coefficient for "steerage".

4WD rev limiter defect, engine dethrottling.

If you try to "spin out" on a slippery surface and a disparate wheel rotational rate develops, and you do not "modulate", moderate, the throttle quickly, almost the instant you feel/realize the TRAC is operating, then the system will step in and do it for you.

There is a "delay" between the time a wheel slips, TRAC activates, and the dethrottling, but it's not a lot. The greater the need for dethrottling (the faster you're wheels are "spinning") the sooner the system will step in and do it for you, in some cases that might be virtually instantly.

khuynh

When does ATRAC stop doing its thing?? Above what miles per hour (MPH)?? In my brother's Discovery SII, the system goes into a "partial" mode after 30 mph and shuts off after 62 mph...i guess you won't need it at that high of speed due to momentum. Does the ATRAC do the same or similar thing??? I think at above 60 mph, VSC is the only thing needed to save your life. Thanks.

hermesnt

Actually, the rev limiting is the VSC working. The engine limiting is a function of the VSC and NOT TRAC. Yes, engine limiting is found in 4WD, but with an open center diff. If you lock the center diff - then VSC is disabled and off you go. That is my experience with the truck after a year and a half and reading the manual

Cliffy - can you edit posts, because your initial posts indicate the TRAC handles engine limiting etc. I think that is incorrect for 'Runners as once the center diff is locked and VSC is off you do not have any engine limiting. I;ve testing this in deep sand

cliffy1

The A-Trac system doesn't have a cut off speed that I am aware of.

Edmunds only allows editing of posts within something like 20 minutes of the post. That is too bad because there are a couple of edits that I would like to handle. Because I did a cut and paste from a Word document, I missed a few %$#& errors. Beyond that, your descriptions is right on, but I don't get the feeling this is what happened to the guy with the problem. The VSC engine limit will not cut you off for several seconds as he described. It is very brief and only intended to get you pointed in the right direction.

wwest

I would have to go look to be sure but I think the ABS/VSC/TRAC/ABD/BA control functions are all integrated into a single microprocessor, or certainly if it's multiple processors they are very tightly coupled or closely linked.

Think about it, if the TRAC isn't being successful in applying the brakes to limit the wheelspin something (or somebody) had better, within milliseconds, dethrottle the engine damn quick.

Anti-lock braking/vehicle stability system/traction control system/automatic braking force distribution/braking assist.

Did I get all that right?

cliffy1

The system on the Runner/Land Cruiser/Sequoia differes from the Highlander/RX300 that you are describing. The question in this case was on a Runner.

wwest

I wasn't describing anything, verbatim, except maybe the ML320. The functions I mentioned and described are inseparable from a real time programmer's viewpoint, so I can't imagine "parsing" them out to separate ECU's.

VSC stands for vehicle stability control, it needs to be able to selectively apply brakes depending on its sensor readings, obviously it will at times also need to dethrottle the engine.

Does anyone out there have the 4runner shop manual electrical section to verify this?

Oh, the only "published" differences, electronically (ECU, firmware), I know of between the HL/RX system and the Sequoia's is that the Sequoia's TRAC "firmware" can use the application of braking to allocate torque front to rear or rear to front whereas the HL/RX relies on the flaccid VC across the center open diff'l for that purpose.

From a hardware viewpoint the Sequoia has a low range and a center diff'l that can be locked and very likely a much more robust ABS hydraulic pump, but other than the F/R torque allocation above issue I would bet that the firmware sources are virtually the same.

somakandan

I posted this info in the other discussion thread, as well.
I am also posting it here for any feedback from the experts on 4WD system of Sequoia.

In my 2002 Sequoia (10K miles), when I drive, I have my center differential lock light blinking. This started to happen just last weekend. Its still blinking when I drive. In the past 5 months of use this never happened. The operation was normal - that is, the light blinks for few seconds during the shifting(2WD -> 4WD) process, and then it goes away - expected behavoir.
Now, it blinks ALWAYS regardless of whether I am in 2WD or 4WD. It starts to blink as soon as I turn on the ignition and stays blinking until I turn off the ignition.

I am going to take it to the dealer to check it out.

Any ideas of what this could be?
Anybody seen/heard about this before?

wwest

Your 4wd system has failed, stuck in 2wd all the time or in 4wd all the time. At least that's what it seems to say in your owners manual.

somakandan

It seems like that, but, what it surprise me is that when I engage in 4WD mode, I could feel (quite positively) that all 4 wheels are powered - I am saying this from my past experience, though.

I am doubting that the sensor that senses the 2WD->4WD shift operation might be faulty.

The toyota dealer (in Columbus-OHIO - Toyota Direct) is clueless what might be. He looked at it for a day and was clueless. He said that he might need until next week (around 4-5 days) to further diagnose. He provided me a rental car, though as part of the regular 36,000 mi mfr warranty - the std warranty that comes with all sequoias.

Do you folks out there, have some other ideas?

wwest

The way I read your owners manual the indication you are having is to be considered quite serious. Therefore I'm a little surprised that the dealer let you drive away.

Even in "normal", or full-time, 4WD mode your Sequoia is still primarily "mechanically" RWD, all the "dog-clutch" does is engage the front driveline output from the center "open" diff'l. It is then left to the TRAC system to modulate braking to achieve "virtual" 4WD.

Even in my Jeeps I was never able to "feel" the full-time AWD mode vs RWD except in a really tight turn.

starnrg

Hello i am a owner of a 2002 4runner, I also do some hardcore wheeling in my rig. First off when your doing hardcore wheeling make sure you have the center diff button pushed in (locked) and when you do have it locked the VSC is turned off this is a good thing you dont want it turning on when your on a steep hill. The traction control(active trac is not turned off when the center diff is locked ,(cliffy1 stated that it was). This is also good so if a wheel gets offground and will brake it and slow it down and try it to grip at a speed where it can get traction. Everytime i go offraod i have the center diff locked. When it rains i always pop it into 4 wheel drive on the fly with the the center diff unlocked (not pushed in) so then the vsc has a less chance of going off (when i am in 2wd it always goes off when i am in rain and then when it goes off I get the flashing skid control symbol flashing with a buzzzing sound . Sometimes they go dont go off together , like if i get harsh harsh wheel slippage maybe 2 wheels slipping at onnce i will get both the symbol flashing and the buzzing sound. if it just a minor slippage i only get the buzzing sound and i also get a beeping sound which i think is the computer telling me it working to hard and is using its full potential, this has to do all with the vsc. I dont think vsc is bad at all 1 time in 2wd i was losing my rear end around a corner when it was wet i though i was going to go off the edge but it kicked in and i stayed on the road So if your going hardcore wheeling and doing some trail runs keep that center diff locked (pushed in) So then it will turn that VSC crap off. Everything works the same when in 4low as 4high, I dont really use 4low to much unless i am going up very steep incline or downhill or when i am rockclimbing i need the torque

starnrg

all i got to say is when in snow and driving on the road you want the vsc on so you dont go in a ditch or something right and keep it in 4 high, keep the 4low for rockclimbing up very steep inclines and when going down. or if your pulling someone who is stuck. you dont need 4low in mud just keep it in high and let active track do its job and keep the center diff locked (pushed in) so that vsc wont go off then. and remember it also helps to have some mud terrain tires.

cliffy1

Thanks for the real world review. Its good to hear from somebody who actually uses this thing in the way it was intended.

idahodoug

Starnrg -

Something doesn't fit in what you posted. You're a 'hardcore' wheeler yet you "don't really use 4lo too much"? And you suggest not using 4low in mud?

There is no such thing as hardcore wheeling in high range - sounds like you're messing around a bit with that sweet truck, but consider your audience here.

Doing any offroading beyond a rough dirt road in high range can put enormous strain on the drivetrain. The loads of wheeling on the transmission are handled much easier in low range. I also recommend low range for mud which again puts enormous strain on the drivetrain. Putting the vehicle in low range anytime it is working hard is better for the truck and also provides easier control for the driver.

The mud tires were a good recommendation as well.

IdahoDoug

pschreck

The Car and Driver comparison report has reared it's ugly head over on the "Toyota Sequoia vs Cadillac Escalade vs Lincoln Navigator vs GMC Yukon Denali vs Chevy Tahoe" board. It would appear that C&D didn't know how to use the Sequoia system.

The article is posted on the C&D website. Anybody read it? What do you think?

bldaz

532 from idahodoug says he doesnt understand why you wouldnt use 4 wheel low in a mud situation. There are lots of times crawling(compound low)is not used, MUD, SAND, anytime you need speed more than slow maneuvering. Four wheel low in mud will only slow you down and for sure get you four wheel STUCK>goodluck:)

sebring95

I normally do not run 4lo in mud. The wheels need to spin to clean out the mud, which is why the Sequoia got stuck regardless. I've used 4lo when crossing deep water holes with sticky mud on the bottom. That type of mud is very tough to get through and I've been in situations where I had to rock it out to climb the bank. Very hard on the clutch if you're in high. The water tends to clean the tires out decently so you don't need to spin as much for traction, but you need alot of power particularly up steep banks.

idahodoug

bldaz,

If you're running fast through mud or on sand, then you must be talking about a significantly less strenuous situation than I am thinking of. If the sand is consolidated enough you're not laboring and if the mud is only a few inches deep then there is no need for low range. However, as I alluded to earlier - this is not really "fourwheeling" but simply driving fast through conditions that don't really challenge the vehicle or require its 4WD traction, and I could probably follow you in my Subaru. It's all a matter of semantics perhaps. Your "wheeling" is my gravel fire road with a few mud puddles. Consider using low range any time your vehicle labors, particularly if you head into terrain - it will lengthen the life of your drivetrain.

Wheeling in mud calls for wheelspin as Sebring pointed out, and getting out of a water crossing is another good use for low range as you will need the torque multiplication if the bank is steep. However, you can get plenty of wheelspin in low range in mud and you can get it more easily in low range due to the gearing. As a general recommendation, entering any muddy area or water crossing you should be in low range as nearly every recent 4WD vehicle has its center differential open in high range and automatically locked in low range. Under these conditions it is best to already have these advantages on your side should the hole prove deeper than you thought, a tire come off the rim, an unseen obstacle bounce the truck off your line, etc.

IdahoDoug

intmed99

Here is my take on it: (please correct any of it)

The 4Runner uses an ABS-based traction control, called ActiveTrac. As described before, you can actually have only ONE wheel traction to move forward. ActiveTrac is ALWAYS on, even in 2WD mode where it works on the rear axle.

Here are the different features of the 4Runner's 4wd system: (VSC is vehicle stability control)

(Note: read below for more info on HOW to engage the different modes)

1. 2WD mode: Rear wheel drive; switch to 4WD up to 62 mph.

2. 4WD HI: 4WD on any surface (full-time!), however, VSC is STILL active...thus, if you get stuck bad or climb a hill & slip, engine power will be cut because of VSC's inherent functions.

3. 4WD LO: 4WD with torque multiplication (2.57 times)...again, VSC is active. (read below on how to go to 4-LO)

4. 4WD HI with center diff LOCK: same as #2 but VSC is OFF, thus allowing you to spin your tires to get out.

5. 4WD LO with center diff LOCK: same as #3, but with VSC off.

To go from 2WD to 4WD HI, you push the 4WD button on the side of the transfer case lever, up to speeds of 62 mph. After pushing it, you are in 4HI and can go up to top speed on dry land with no damage.

To go from 4HI to 4LO, you need to stop the 4Runner...shift to NEUTRAL, then switch/push the transfer case lever to 4LO. Then, you put the transmission back into DRIVE.

To LOCK the center differential (thus, TRUE 50/50 power split between front and rear axle), you need to push the button to the left of the steering wheel on dash. This should be done only on loose terrain (because of 50/50 split) to prevent binding of the center differential. Again, this will disable the VSC, allowing you to power through mud (which you cannot do effectively if VSC is ON).

In reality, the MAIN reason most people LOCK the center diff in the 4Runner is to turn OFF VSC. Very few off-roaders (esp. beginners) will ever see the need of the 50/50% split. An example is if BOTH your front (or rear) wheels or 3 wheels are in mud (no traction)...in which case, locking the center diff is a good thing because it allows 50% of power to be routed to the AXLE that still has the wheel(s) with traction. Without the center diff LOCK, 100% power will be routed to the axle that has both wheels slipping (thus, you are stuck). However, this is rarely encountered in off-roading.

Hope this helps, Thai.

wwest

No actual experience, only logic and the Sequoia owner's manual.

"2." VSC will only cut the engine power back if the driver isn't exercising enough finesse or judicious use of the throttle (actual experience on my GS300). ActiveTrac isn't likely to be used except moderately (if at all) to parse engine torque between the front wheels. Broken fingers and thumbs would likely be the inadvertent result.

"MAIN reason....turn off VSC"

TOTALLY wrong!

On snow or ice, or any slippery surface, it is totally unpredictable as to which wheel/tire, at any given instant, will or will not have traction. Center locking diff'l are made for these circumstances.

Engaging the center diff'l lock can be used to disable VSC, but that certainly would NEVER be considered the primary ("MAIN") purpose/use of the diff'l lock.

cliffy1

You've got it. I respectfully disagree with wwest on the center lock issue and agree with your position. After driving these one ice and snow, I KNOW it functions better in the unlocked mode.

wwest

"Driving" vs getting unstuck, or in some cases just getting the darn thing to MOVE.

Once the vehicle is moving forward with some reasonable level of momentum then I have no doubt that an unlocked diff'l will work perfectly fine, especially with ActiveTrac.

It's during that initial effort to get the thing up and moving to gain some momentum that I found I most often needed to use the locked mode.

intmed99

Well, i have no opinion on snow driving...i live in Texas. However, in off-roading, rain, etc. (except snow), i think most people do NOT need to lock center diff all that often.

VSC is ONLY there to prevent oversteer/understeer. ATRAC is what provides TORQUE to the wheel with traction (any wheel, not just the front) via brakes. VSC is more of a braking wheel(s) to correct oversteer/understeer...it is NOT use to provide traction (in an off-roading sense).

Cliffy, thanks, but some of the info came from you correcting me about center diff/LO range/etc. a while back. Love my 4Runner!

wwest

Please remember that I never speak of off-road, as a sporting event of itself, experiences. I buy AWD vehicles strictly for their on-road usefulness in the wintertime here in the pacific northwest.

I said that the only time I found diff'l locking was really needed was to initially get the vehicle up and going on low traction surfaces. I should add others, such as in ski area parking lots or stop and go traffic, on low traction surfaces.

And intmed99, you're right, those conditions don't come along very often even here, but when they do....

VSC/TRAC/ABS/ActiveTrac, automatic brake pressure distribution, brake assist, (are there others?)any or all of the above, are ALL integrated within ONE ECU(microprocessor)(at least by T/L) and ONE real time firmware program, and all share the same ABS hardware. So basically they are ONE, inseparable entity.

Your definition of VSC is right on.

Front wheel torque allocation. Ever see LSD on the front axle? (can we leave the hummer out of this?)

The reason you don't is because the feedback to the steering wheel would be as bad, or maybe worse, than the steering wheel feedback you might encounter with the diff'l locked on dry pavement in a tight accelerating turn.

Even the VSC "torque allocation" at the front must be quite moderate, modulated, even more so the lower the speed is when over-steering is detected and the outside front wheel brake is applied.

So most VSC/Trac "braking type" L/R or R/L torque allocation systems only really operate at the rear (Damn good {better actually} substitute for failure prone mechanical LSD), if a single front wheel slips, especially with FWD or AWD with FWD bias (RX, HL, & Rav${?}) the engine is immediately dethrottled.

Are hummer owners with LSD in front CAUTIONED to never have their fingers and/or thumbs inside the steering wheel? Or is the hummer steering wheel "braced" or "torqued" against the feedback when the LSD activates?

"ATRAC is what provides torque to the wheel with traction" ...by applying the brakes to the opposite REAR wheel that has lost traction.

intmed99

I am not sure, but i believe the Hummer uses the same ABS-based traction control as the Discovery, 4Runner, Land Cruiser, etc. They did away with LSD/lockers a few years ago. So, no, the Hummer does NOT have LSD anymore. It does have awesome torque multiplication!

As for the "thumb inside steering wheel" idea, this is recommended on ALL vehicles when off-roading, even those with no LSD/lockers! It has nothing to do with having an LSD or not...it has to do with safety when the front wheel hit a rock/pit...the shock can transmit from the wheel to the steering wheel, causing it to violently turn, thus may cause injury to the unsuspecting driver.

As for ATRAC mainly working on the rear, i am not too sure about that. My brother's Discovery's 4-ETC works quite effectively in the front! Same goes for my other brother's LX470. I actually saw both of these vehicles work their thing in the mud.

There are plenty of people that put LSD on the front axle. The main limiting factor in doing that is the STRENGTH of the axle itself, not how it affects steering. Yes, it will affect steering somewhat, however, you can easily select a less-active LSD (that is, less aggressive). Many put locker in the rear and LSD in the front.

wwest

I think of anyone intentionally taking any SUV off-roading much the same as someone going deepsea diving. Without the proper preparation and training they're simply getting in over their head.

How many times do you think someone has been surprised to learn via brake application and instant ABS activation that they're operating on thin ice, a low traction surface?

Now think of the completely unsuspecting soccer Mom (or Dad) that gets the steering wheel suddenly yanked from their hands on encountering "hard" front LSD (virtual or real) activation.

The latter simply isn't going to happen because no automotive manufacturer wishing to remain profitably in the business is going to design a car that "surprises" someone in this manner.

But let's you and I think this through a bit, maybe even "design" some VSC/Trac/ATrac software/firmware. I think we can ignore the AWD implementation of the HL & RX, and X5 since they are clearly not designed for the off-roading market.

Let's pick the 4runner for instance. So as not to surprise that soccer person we implement VSC and Trac in a very straight-forward, standard, manner in RWD mode. The front wheels aren't driven so there is no worry about "LSD" activity at the front and after a few seconds (hundreds of milliseconds?) of continuous torque allocation via differential rear braking we MUST dethrottle the engine if the driver doesn't.

Agreed?

4WD mode.

Now we already know that the VSC system will implement moderate, "modulated" braking of the outside front wheel in an over-steering circumstance. So implementing a virtual LSD in the same manner, "moderate" differential braking of the front wheels, is a no-brainer. Of course just as we did previously with the rear traction, if the condition persists we have no choice but to dethrottle the engine.

4WD, locked diff'l mode. Now we're "free" to do whatever we wish with front LSD activity. Things have obviously gotten serious and we can take it for granted that we're operating on a (continuous, spotty, intermittent {off-road boulders})low traction surface (otherwise the operator is asking for trouble). HARD LSD activity is not as likely to be "reflected" as HARD feedback to the steering wheel for the same reason we can trust that there will be no driveline "windup" and thus little, or no, feedback to the steering wheel.

FWD, or AWD w/front torque bias, Toyota HL and Lexus RX.

We are free to implement rear LSD as "hard" as we wish, with loss of traction at the front wheel(s) the rear wheels almost never get more than about 5 to 10% of the engine torque (throughout its torque range) and only about 25 to 30% in extreme circumstances. And those extreme circumstances are only likely to be encountered on a 4 wheel dyno, NEVER on an actual travel surface.

But what do we do about front LSD implementation?

All I can really give you for an answer is that my 01 AWD RX300 with VSC/Trac/etc, has NO front LSD implementation. Seemingly there was not even a dethrottling activity.

Since VSC operates at the front using the brakes it would seem obvious that at least a "soft" virtual LSD could be implemented. But the kicker, difference, is on these vehicles the front wheels, not the rear, are the primary motive force.

Hope that helps everyone.

intmed99

First off, you lost me on your first few paragraphs. All i was saying is that the "thumb blah blah" is what you learn when off-roading...i have NEVER seen it in any other context! Unsuspecting soccer mom?? Did the '99 RX300 with LSD have that in their manual??

Why would you want LSD on front AND also have VSC for a family car (RX300)?? I believe, for cost benefit, Lexus did away with that LSD feature. There was simply no need for it. I am sure that Lexus did not sell many RX300 with LSD in the first place.

If you are right, why did Toyota do away with the electronic locker on the rear axle when they incorporated the ATRAC/VSC in 2001??? E-locker, in theory, should NOT cause much problem for ATRAC, since ATRAC senses difference in speed/slippage. With E-locker, there is no difference, thus, ATRAC should not care. So, back to the original question, why did Toyota do away with the E-locker when they incorporated ATRAC?? BECAUSE it is NOT needed (from a cost $$$ view)...ATRAC does a very good job. A few 4Runner owners are actually trying to retrofit the locker onto a 2001+ 4Runner.

Thanks.

wwest

I have never wanted nor championed front LSD, real or virtual. I have only tried to put into words my thoughts about why it doesn't exist and shouldn't exist, period. But most especially not in a FWD or AWD vehicle with FWD torque bias.

The 99 RX300 only had REAR LSD as an option. Rear LSD will not cause feedback to the steering wheel, thus no "warning" was necessary.

Hey, I'm on your side here, over on the GS thread at CL there seems to be TONS of boy-racers putting mechanical LSDs in their VSC/Trac equipped GS's. Unless they can figure a way to disable the GS's Trac the LSD will never wear out!

Again we seem to be talking at cross purposes. I am campaigning for Lexus to scrap the VC in the RX300 in favor of implementing the Sequoia's virtual AWD system, a clone of the ML's but who really cares

aorta

What is the definition of :

1. Virtual LSD
2. Virtual AWD

given : LSD = limited slip differential & AWD = all wheel drive

wwest

First we had actual mechanical LSDs, Limited slip differentials. And now we have virtual LSDs, devices that do not actually exist but yet serve as functional equivalents. Lexus Trac and Porsche's PSM have LSDs implemented in "firmware", using the rear brakes in a differential manner as a substitute for a mechanical LSD.

Virtual AWD, uses three open differentials which work fine to evenly distribute torque as long as all four tires have roughly equivalent roadbed traction. Once any wheel loses traction then the torque available at all wheels is equal to the amount needed to freely spin the tractionless tire. Except the virtual aspect then steps in and applies moderate braking to the tractionless wheel thus increasing the level of torque available to all four wheels.

"Virtual" substitution for center diff'l lock and front and rear LSDs.

Also see our company home page regarding "virtualization of legacy computer peripherals"

strobedata.com

sebring95

So if you are going to off-road or be in other situations where you don't want ABS you don't have nothing but open diffs. I have and know several others who pull the ABS system when wheeling. I think you guys are talking about on-road stuff, although I've done some neat tricks in gravel with ABS too. Really shouldn't work under 25mph in my book. From that point to dead stop you aren't going to lose control by skidding. I've gone right into a couple slippery intersections because ABS decided I really didn't want to stop as fast as I was telling it to. Would you rather skid 5' or roll 10'?

wwest

Yes, these features make use of the ABS' hydraulic pump. And I would imagine not just a few people pull the pump fuse to disable ABS when off-roading. That will probably be less common as/if more vehicles like the Sequoia, ML, X5, etc, arrive on the scene.

And I have long contented that ABS contributes to as many accidents as it prevents, and recent studies and reports are now bearing that out.

On my 92 Jeep I would always pull the ABS fuse except during the winter months.

Now that VSC and PSM systems are becoming more common I fully expect that some manufacturer, sooner than later, will figure out that ABS could ALWAYS remain passive unless the "VSC" system indicates impending loss of directional control.

Also, there seems to be an accelerometer, AND a yaw sensor, in my 01 RX. A longitudinal accelerometer could be used to detect braking effectness in "real time", actually detect whether or not ABS should come online or remain passive for each and every braking application.

farfegnugen

"...that VSC and PSM systems are becoming more common I fully expect that some manufacturer, sooner than later, will figure out that ABS could ALWAYS remain passive unless the "VSC" system indicates impending loss of directional control.

I've been holding back for a while, but have to say, I would suggest that you participate in an advanced driving course! :-p
Here's a good one, if you're willing to drive up to Vancouver.:
http://www.bmwccbc.org/schools/control.html

In all of the ones that I've attended, ABS was regarded as one of the most significant advances in braking. The problem is that most people don't know how to use it in a panic situation. In all of the panic stops exercises that we did, all of the ABS cars had far longer stopping distances than those with ABS. The reason was that threshold braking and modulation had to be utilised, which is basically what ABS does; pumping the brakes is an incorrect method, especially now that all cars have power assisted brakes.

It is nonsense that a car will stop faster with locked brakes than with ABS activated. When a tire stops rotating, it loses 30% of its traction versus one that is at its tractive limit. ABS keeps the tires at the tractive limit, albeit there are wide variations in the systems out there. Some are obviously far more advanced than others; the maximum traction point of a tire is when it is revolving at 15% slower than it would be if it were freely rolling over the surface. The more advanced anti-lock braking systems are better at balancing the brakes to this sweet spot. Additionally, without ABS, when one is on a split-mu surface, the tires on the side of the vehicle with less traction will lock up first, causing the vehicle to spin.

Watch this video clip:
http://www.theautochannel.com/media/netshow/misc/stomp-stay-steer.asx

ABS is not as good on loose surfaces such as gravel or sand, simply because locked front wheels will create a buildup in front of the wheels, which will help to slow the vehicle down. The MB M-class, for example, has a special ABS mode in low range which allows the front wheels to lock up cyclically which ABS remains active for the rear wheels. In low range, the ABS algorithm is also different from the normal ABS mode, allowing the system to remain useful in off-road situations.

intmed99

Just look at all the car magazines. Whenever they test a vehicle with ABS and one without, you will always see one thing...the one with ABS stops significantly shorter (on dry land).

In addition, not just MB, but more and more SUVs nowadays are putting in different ABS modes for on-road and off-road driving. Toyota Land Cruiser has had one since 1999. I believe the '02 4Runner has one too. Nissan has one in it's Pathfinder.

BTW, MB's 4wd system is POS! You cannot turn off the stability system's "cut-engine-power" function. In the 4Runner, all you need to do is LOCK the center diff! Same goes for the Land Cruiser. So, please no more comparing our system with that of Mercedes...it is not fair for Mercedes!