Edmunds dealer partner, Bayway Leasing, is now offering transparent lease deals via these forums. Click here to see the latest vehicles!
Popular New Cars
Popular Used Sedans
Popular Used SUVs
Popular Used Pickup Trucks
Popular Used Hatchbacks
Popular Used Minivans
Popular Used Coupes
Popular Used Wagons
Comments
2toyotas made mention that for 07, the viscous coupling was put back into the 07 model year Highlander. (This is interesting since the 07 model year is going to be a short model year with the introduction of the 2008 redesigned Highlander.) I have read that VC was used prior to 04, and a different set-up was used for models years 2004-2006. I would like to know the pros & cons to each system and a general understanding on how each work. If Toyota did go back to the VC for 2007, what would be the likely reason?? Could it be to reduce transmission hesitation with the use of the VC?? I have read a plethora of transmission hesitation with toyota front wheel drive biased vehicles. Having said that, I have not seen anyone complain of transmission hesitation prior to 04 model year Highlander? I say this realizing that DBW was introduced and has been blamed for this... But maybe a pro for VC is less transmission hesitation in the Highlander??
Look forward to your responses.
Overall not a very good way to get "unstuck".
I don't know of anyone who knows why the VC was dropped for a period of time but having it back in the "loop" cannot be a bad thing. My own guess as to why the VC was dropped is that going into the 04 model year Toyota knew that they had a problem with the ATF overheating and tossed the VC as a possible source of that extra heat.
Insofar as the hesitation issue is concerned Ford has just announced that the new Ford Edge is using a variable displacement ATF pump, IMMHO in an effort to solve the very same problem for their FWD vehicle series.
I would suspect that Toyota has gone to the same measures and now the new 07 models seem to be exhibiting a 3-4 shift engine flare problem.
I would like to know more about a VC. It is my understanding that a VC consists of plates with a special fluid inside. When the fluid is heated, it creates friction between the plates and transfers power to the rear wheels. But I wonder how long the front wheels need to slip in order to heat the fluid enough to transfer power...
Do we know for sure that the VC was introduced back into the 2007 Highlanders? I spoke with a Toyota Service manager today when I was waiting for my vehicle - he said he thought all highlander model years used a VC... I felt he was uncertain with his response. Any thoughts if the VC would help with the hesitation that is plaguing Toyota?
I believe the 3-4 shift flare is exclusive to toyota's new 6 speed auto that, at this time, is only used in the new Camry & ES 350; I suspect this new transmission uses clutch packs like Honda transmissions to shift faster & be more responsive...
How long it takes for the viscous fluid to heat to the point of providing a satisfactory level of coupling is a function of the fluid formulation and the level of over-driving to the rear axle. Some systems even have a measured level of compressable gas inserted within the hermetically sealed VC case to delay the onset of coupling.
On my 2001 AWD RX300 it was a matter of seconds before the VC was coupling ~25% of the engine torque to the rear driveline and it never rose beyond that level. Hopefully the newer VC will have improved upon that.
The hesitation time line:
Late in the last century it was realized (FINALLY!)that not only could engine compression braking interfere adversely with the anti-lock braking system's ability to release the brakes and thereby allow directional control to be maintained, engine compression braking (on the front wheels) alone often resulted in loss of directional control should the roadbed surface happen to be highly slippery.
So for many FWD and front biased AWD vehicles the shift pattern was changed such that the transaxle would always upshift on a FULL lift-throttle event and only downshift into the appropriate gear ratio once the throttle was re-opened or the vehicle came to a full and complete stop.
Much the same thing an experienced human driver would do on a slippery roadbed with a FWD vehicle equipped with a stick shift.
The problem that grew out of that was that oftentimes the transaxle was now in the wrong gear ratio if the driver suddenly/immediately went to accelerate "hurriedly" shortly after a full lift-throttle event. It was not possible for some transaxles to fully and completely accomplish a downshift in these instances absent some serious level of clutch slippage.
So a lot of 99 RX300s have premature transaxle failures at 70-80,000 miles.
I believe that as a quick fix Toyota went to a higher fixed capacity ATF pump as of the 2001 RX300 MY. That, of course, resulted in too much pressure/flow capacity overall resulting in the 2001 RX300 series needing the ATF drained and flushed every 15,000 miles or as a minimum each time the ATF began to look and smell burned.
As of 2004 the RX series went to DBW (and most likely back to the lower capacity ATF pump) to solve the problem once and for all. The idea was to "protect the drive train", delay the rise in engine torque during that second quick shift sequence until the ATF pump, with the engine at idle, could provide enough pressure/flow to complete the new shift sequence.
We all know the rest of THAT story.
I can't speak for Toyota but Ford probably adopted the variable capacity ATF pump for the new Edge to combat this very same problem.
The "arrival" of the 3-4 engine flare problem would indicate that Toyota has at least changed something in another effort to solve the upshift sequence change made in ~99.
Remember that the VC cannot be "active" full time due to the need to allow the rear wheels to rotate at a different rate in a turn, tight turn, and not incur driveline "windup" or undue tread wear rates.
In my 01 traction control almost instantly starts braking the front wheels if they start to spin and just as quickly dethrottles the engine. So with my 01 version of traction control there is no opportunity for the VC coupling coefficient to rise absent disabling traction control.
So maybe someone figured out how to rewrite the traction system firmware so the VC has some use....
Thanks,
John
When was the last time you stood on the brakes while lead-footing the gas pedal?
ABS MUST be disabled if the center differential is locked. Otherwise ABS would have to work in synchronization at the front and the rear, releasing brake fluid pressure to both at exactly the same time. The front tires will always have more traction during braking, due to inertial effects, that means that they MUST roll slower than the rear and that is not possible with a locked center diff'l.
There are some makes that instantly unlock the center diff'l when ABS activates but this cannot be done reliably with a dog clutch, sliding spline, to lock the center diff'l.
Can somebody say, express this, better than I?
Anybody?
There is a box to check when you track a discussion that says "Send me an Email Alert when new messages are posted."
And welcome!
The most common way I found was to drive dead slow while putting it into, or out of, lock or even dead slow in reverse. I know that many manuals indicate that you can shift on the fly but there are MANY exceptions, one of the more common ones is to have slightly different tire wear rates F/R.
But I wonder if you're not confusing the AWD mode of your Sequoia vs a true 4WD. Sequoia's AWD mode uses a "soft" locking of the center diff'l so all braking related functions can remain operational. In point of fact it is TC, Traction Control, that is providing this "soft" locking feature, "virtual" center and rear LSD as it were. It may even provide a low level, much lower than at the rear, of virtual LSD functionality at the front diff'l.
-John Bakunin
I understand that you should not use this feature on dry pavement however the manual suggests just avoiding dry pavement. I guess my question is if you lock the differential on dry pavement or while it is locked you hit dry pavement will you harm the system? I would seem odd to me that inadvertant dry pavement use would harm the system.
If traveling in snow or slush and you need more traction does locking the center differential provide more traction?
The manual is unclear on this too, but can you lock and unlock it while moving in H at any speed?
Thanks for the help...
P
If you lock the center diff on dry pavement, you can stress the driveline, depending on how much turning you do, and possibly harm it. When my 05 4runner was new, I inadvertantly drove 30 freeway miles with the center locked and didn't realize it until I tried to exit the gas station when turning my wheels and feeling a binding. 30k miles later and TONS of hard, off-road use and apparently no damage was done. As far as stress on the system, any time the road surface is slippery to allow wheels to slip slightly and unbind the driveline, the vehicle can be driven safely in center locked mode. If you had to drive it locked on dry pavement, keeping turning to a minimum would be best.
When locked, the center diff will give better traction by defeating VSC which can cut engine power when wheels slip. So snow and ice in normal driving, you would leave the center unlocked so as to benefit from all VSC can do at speed. In unusual cirmumstances - in a ditch, a very deep snow white-out, low traction off-road, etc., locking the diff will make sure your engine power doesn't get cut by VSC but still enables traction control to limit wheel slip, side to side, on the front and back axles.
You can engage the center lock while in motion. Personally, I've never done it going more than 10mph or so but the manual seems to indicate, by omission, that there's no limit on speed.
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.
Got to be a shut off on that kind of a brainless system.
Absent the VC these vehicles rely exclusively on TC braking for maintaining a high torque level and thereby distributing a reasonable level of torque to the wheel(s) with traction.
And the torque split has never been 50/50 except with equal traction at all four wheels, when you care not anyway. The rear driveline is overdriven by a 2.98/3.12 (2.98/3.48 '04 and after) factor so the front will always receive the majority of engine torque.
And even my 2001 AWD RX300 has engine dethrottling if I persist on revving the engine in slippery conditions. The engine MUST be dethrottled to prevent brake rotor warpage from overheating due to continuous TC braking.
TC, Traction Control, will delay the onset of engine dethrottling for a few hundred milliseconds on RWD or AWD vehicles whereas FWD vehicles, due to their hazardous nature, will have the engine INSTANTLY dethrottled upon driven wheel slip/spin.
And as a general rule TC will NEVER be used to automatically implement a "virtual" LSD on a FWD vehicle due to the possibility of yanking the stearing wheel right out of an unwary driver's hands. So TC will apply braking to BOTH front wheels and dethrottle the engine even though only one wheel is slipping/spinning.
The exception is the new RAV4 "auto" LSD function/mode which should not be used before reading the CAUTION note in the owners manual.
Four-cylinder and V6 Highlander models are available with a four-wheel drive system that distributes engine power 50:50 front-to-rear, with four-wheel traction control (TRAC) controlling potential slippage at any wheel.
I don't know who told you trac brakes both front wheels at the same time, but that is the craziest thing I have ever heard. It will brake each front individually when they begin to slip. Trac uses gradual brake fluid force to keep stability.
My Tundra Has the same warning about the Auto LSD option. And that only works in rear wheel drive.
Having read the posts that I have, including one driver who apparently kept the 4WD lock feature on during all in-town driving (thus explaining his reported 13 MPG) I really wonder why they even included the locking feature at all.
I think this should be repeated for those who may have the toyota trac system... there's no need that I can think of to ever lock the center diff while on road.
We don't need 4WD at all and have never had it. Mechanical solution?
Thanks.
If you want better mileage, trade it on a sedan, hatchback, or wagon. The large frontal area of an SUV/CUV increases wind resistance, greatly reducing mileage.
THX ....... love the Sequoia accept for this ridiculous intrusion in my driving ability and safety.
On my 2001 RX300 if I unplug the MAF/IAT sensor module while the engine is idling CEL/VSC Trac faults will result, the engine will die, but can be restarted after the sensor is reconnected.
Thereafter for the next few drive cycles the VSC/Trac system will be disabled.
There are remote wireless relay systems available in the marketplace that I'm sure could be rigged up to do this from the driver's seat.
I use one, wireless relay, to open the door of the GPS/Nav DVD player so as to get rid of the "I agree" and moving mao display at times, MOST of the time, that I have no use for GPS.
How do you know?
If not you will soon be looking at some pretty severe charges to repair the DAMAGED parts of the driveline.
That being said the indication could be the result of a misaligned sensor switch mounted on/at the spline clutch actuator. The spline, "dog" clutch is disengaging but the switch remains "tripped". Same as like your backup lights always remaining on.
I'm surprised that your dealer treated this issue so lightly, unless the clutch really is disengaging this can result in some damn serious driveline damage.
I don't know of any reason why the turning radius would change in 4-hi. Perhaps(???) you have been feeling the stearing wheel "kick-back" while turning on dry pavement and thinking that kick-back was a limit to how hard you could turn the stearing wheel...
I don't know if your Tundra uses a spline clutch to engage/disenage the 4WD mode, lock the center diff'l, but if so those clutches are notorious for becoming reluctant to disenage as they are typically not "driven" into the disenaged position. The least amount of friction or binding, or even "binding" due to driveline torque, will tend to keep them from disenaging.
So I'm sure it is going in and out of 4wd just fine.
Can that sensor switch that you mentioned earlier be replaced easily?
2001 Toyota Tacoma 3.4L Manual transmission
Thanks for all your help.