Last post on Aug 21, 2013 at 5:16 PM
You are in the Toyota Sequoia
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Toyota Sequoia, Toyota Highlander, Toyota 4Runner, Toyota Land Cruiser, Toyota RAV4, Toyota Tacoma, Truck, SUV
#1986 of 2493 Re: 4Runner Rear and Center Differentials ? [2toyotas]
Jul 07, 2006 (5:19 am)
I have a 2002 4R. When I lock the center diff in 4WD, VSC (yaw control) is off--no argument there. There is some confusion about whether the TRAC function stays on or not. Toyota has used versions of this system since (I think) 1999 or at least 2001 with the 4Runner. With the new FJ, Toyota has clearly spelled out that there are two versions of the same traction system, TRAC and ATRAC. I believe this is a newer version (3rd generation system?)of the same system Toyota has been using for a number of years now, including the 4R and Sequoia.
TRAC (traction control) works in 2WD-Hi or 4WD-Hi with open center diff (essentially AWD or full-time 4WD-Hi mode where you can drive an any surface). ATRAC works in 4WD-Hi or 4WD-Low with center diff locked (off-road mode if you will). I believe there is some difference in how it works based on whether you are on or off-road. In my experience, TRAC (on road) will brake all four wheels if slippage occurs, whereas ATRAC (off-road) I believe will allow continuous wheelspin to at least one wheel (while braking the opposing wheel across the axle) on the front and rear axles to allow the truck to regain traction in off-road conditions. IE, if the right-front and left-rear wheels are off the ground, ATRAC will apply braking force to those wheels, but allow the left-front and right-rear to spin freely in the dirt/mud/sand/rocks to gain traction.
Oh, and BTW, the 4R up to 2000 and the current FJ Cruiser has a rear locked diff option. The old 4R had no VSC/TRAC/ATRAC though, whereas the new FJ does have that too. But, only the 6 speed FJ has the full-time 4WD (Hi, Hi w/ center diff locked, and Low w/ center diff locked) with center diff (rear diff lock is standard), whereas the 5-speed auto version has only a part-time 4WD with no center diff (2WD-Hi, and 4WD-Hi and 4WD-Low for off-road conditinos with the rear diff lock being optional).
This is my understanding of the system. Everyone got all that?
#1987 of 2493 Re: 4Runner Rear and Center Differentials ? [chiefjojo]
Jul 07, 2006 (6:57 am)
I think we need to get the guy who designed these systems to give us a lecture by posting.
Okay, um, I know that 4Runner has two different transfer case options. In the V6 the transfer case can be set to rear wheel drive (2WD). Toyota for some bizarre reason calls this a part-time system. This is very confusing for the reason that older 4wd systems in many SUV's and pickups had no center differential at all and thus could only be used on slippery surfaces to avoid axle bind-up and tire scrubbing. That no center differential system was called part-time 4wd because it could only be used part of the time (snow days and mud festivals). Toyota's part time definition simply means that you can use rear wheel drive if you choose to, not because you have to. Toyota's part time system does have a center differential.
The V8 4Runner has a full time system which toyota means to say that you cannot operate in rear wheel drive or front wheel drive alone. Everyone else in the industry calls this (AWD) All Wheel Drive.
Now chiefjojo are you saying that Trac is for the "part time" transfer case and ATrac is for that "full time" transfer case ???
#1988 of 2493 Part-time....
Jul 07, 2006 (8:38 am)
Anytime the center diff'l or transfer case is locked the vehicle should NEVER be driven on a tractive surface. That's where, and only where, the term "part-time" comes from.
It gets really confusing when vehicles like the 4runner are equipped with both full-time AWD and part-time 4WD/4X4 modes.
AWD systems seem to come in three "flavors" currently, a fully open center differential with the brakes being used to prevent wheelspin/slip and thereby apportion engine torque, ue of a VC to "partially" lock the center diff'l with wheelspin/slip, and of course the Torsen center differential.
4WD/4X4 systems, in general, always use a locked center differential and therefore constitute Part-Time systems since they cannot be used, should NEVER be engaged, on tractive surfaces.
My own personal differentiation of 4WD versus 4X4 is that the 4X4 uses a transfer case, planetary gearset, so a low gear range can be provided.
ABS functionality is ALWAYS disabled in Part-time, locked center differential, mode. We all know that during braking the weight "shifts" toward the front and therefore the front brakes take on the "brunt" of any braking effort.
That means that during braking the front wheels almost always turn slower than the front. If the center differtial is locked then ABS activity at the rear would result, by default, in the same activity on the front.
Therefore I would be VERY surprised if traction control isn't also disabled in 4WD/4X4 mode. With the center differential locked traction control braking at the front would also result in braking control of the rear driveline.
Insofar as VSC is involved some aspects may be disabled (rear braking to aid in recovering from understearing comes immediately to mind). While front differential braking, or "unbraking" to prevent over-stearing might remain enabled.
#1990 of 2493 Re: 4Runner Rear and Center Differentials ? [chiefjojo]
Jul 07, 2006 (7:25 pm)
Sequoia, 4Runner, and Land Cruiser all use ATrac and Highlander, and Rav4 use Trac. The only difference is Low Range. ATrac changes from gradual fluid pressure control for stability-priority in High Range on all 5 that I mentioned, to sudden fluid pressure control for drivability-priority in Low Range. The second mode is for rugged offroad driving. The Highlander and Rav4 do not have a Low Range so they get plain Trac. When the center differential is locked in either High or Low engine output is not reduced as it is in the unlocked mode. Also when in Low range and in 1st gear there is gradual fluid pressure to the front wheels to keep control going down hill.
On the FJ Cruiser Trac operates in 2WD or 4WD high, but shuts off in Low Range. In Low Range you have an option of both ATrac and a locking rear differential, both are engaged by a switch. When you lock the rear differential, ABS and ATrac are disabled. Two interesting things, when you lock the rear diff. ATrac does not work, so the front has only an open differential. Second you can now shut off ATrac, will we see this on 4Runner, Sequoia, and Land Cruiser? The rear diff and ATrac are an option on the FJ, but come together. So you either have both or nothing.
#1991 of 2493 Re: Part-time.... [wwest]
Jul 07, 2006 (7:35 pm)
ABS always functions with the center differential locked on all Toyota vehicles. Traction Control also works when the center differential locked. VSC is disabled when locked. The only time ABS and Trac turn off is if the rear differential is locked, and that is only on the FJ Cruiser and the Tacoma.
#1992 of 2493 Re: 4Runner Rear and Center Differentials ? [2toyotas]
Jul 08, 2006 (8:09 am)
2toyotas, you write about fluid pressure, but the center differential in the 4Runner is a Torsen type which is all mechanical with worm gears and worm wheels. There are no viscous limited slip units on any of the 4Runner differentials, nor are there any clutch-pak limited slip differentials on the 4Runner.
So, are you writing about the brake fluid being pumped by the abs pump as controlled by the ATrac system ???
Toyota should have just used Torsen diffs on the front and rear to minimize confusion on this forum
#1993 of 2493 Re: 4Runner Rear and Center Differentials ? [ccarofknowldge]
Jul 08, 2006 (5:28 pm)
Yes brake fluid being pumped by the ABS.
I had a Hummer H2 that I borrowed from a friend for a week, and it has Torsen diffs on the front and rear, and it was horrible on ice. I was driving it one night and it was slipping all over the place. When I got home I had a very hard time getting it up my steep driveway. When I finally did I was curious so I backed my 4Runner down and it went up with no problem at all. The traction control kicked in, did its thing and I didn't break stride. A torsen diff is good in the center because it keeps power going to both axles all the time without it being locked, but I prefer traction control controlling side to side traction especially for snow and ice. The Sequoia had all open diffs until 2005. I had an 03 and an 05, and the 05 with the Torsen center diff made a big difference. Also the Highlander had a Viscous center diff from 01 till 04. In 04 they took the Viscous out of the center and used an open diff with Trac. For 07 they are going back to a Viscous center diff, so it must be better to have a limited slip unit in the center, this way both axles are always getting power.
#1994 of 2493 Re: 4Runner Rear and Center Differentials ? [2toyotas]
Jul 09, 2006 (7:18 am)
Thanks for the reply 2toyotas. Seems like Toyota does its homework well . During the time when I had a vehicle with a viscous slip limiter working in conjunction with the center diff and a viscous slip limiter working in conjunction with the rear, I drove in snow and ice and never got stuck or fish tailed. When I first got it I tried taking off hard on snow and ice in a deserted area (with nothing to hit ) to see what would happen. Absolutely nothing other than the vehicle just went where I wanted it to. Safe to the point it seemed like I was on dry pavement.
From what I have read, the "Trick" with the Hummer and other vehicles that have Torsen's on front and/or rear that are not "pre-loaded" (a designed in drag to compensate for one of the design anomolies during a zero traction event) is that when you are trying to get up an icy driveway or out of an icy parking spot, you have to two foot the brake and gas simultaneously. The drag of a small amount of braking action while accelerating causes the Torsens to avoid that zero traction anomoly and do their thing. I have never had the pleasure to try this personally with a Torsen diff, but I need to do it on one of my clutch pack diff vehicles and it does seem to work as described.
Next time your friend gives you a turn at the wheel, give it a try and let us know if the theory from the Hummer instruction manual actually works in practice on the vehicle.
Incidently in this month's Four Wheeler mag they put Torsens into a project vehicle for mud and dirt and their review was that it was better than a "locker" for those purposes.
Thanks for all the helpful info.
#1995 of 2493 Re: Part-time.... [2toyotas]
Jul 09, 2006 (9:29 am)
For the aforementioned reasons NONE of the braking related functions, ABS, VSC, Atrac, Traction, etc, will be enabled with the center differential locked.
I have now read almost all, if not actually all, owners manuals of related vehicles (4runner, Sequoia, GX470, etc,)and in no place did it relate that these functions would be active with the center diff'l locked.
Being well aware that doesn't cover the issue well I did find one statement saying that if the traction control's engine dethrottling mode interfered with vehicle manuverablity then the driver could lock the center diff'l to avoid engine dethrottling.