Last post on Aug 21, 2013 at 5:16 PM
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Toyota Sequoia, Toyota Highlander, Toyota 4Runner, Toyota Land Cruiser, Toyota RAV4, Toyota Tacoma, Truck, SUV
#2027 of 2493 Re: 4Runner Rear and Center Differentials ? [bwhaler]
Dec 27, 2006 (9:38 pm)
The VC is a part of, mounted within, the PTO, Power Take Off, to drive the rear wheels. It operates in gear oil and the PTO has metal to metal contact with the center/rear diff'l case which uses ATF.
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.
#2028 of 2493 Re: 4Runner Rear and Center Differentials ? [wwest]
Dec 27, 2006 (10:36 pm)
Sorry but the 07 Highlander does not have a VC. It will after the 08 redesign. The Lexus RX350 which is new for 07 now has the VC back in the center differential. I beleive the system will be better with the VC back. It will keep power to both axles while TRAC sends power left and right. Without the VC if both front wheels were on ice and the left rear wheel was on ice TRAC would brake both front wheels and the left rear wheel and all power would go to the right rear wheel. With a VC in the center differential, it would keep the front wheels spinning together and brake only the left rear wheel, and power would go to the right rear wheel. I think without the front wheels being braked it will help the vehicle keep momentum.
#2029 of 2493 Re: 4Runner Rear and Center Differentials ? [2toyotas]
Dec 28, 2006 (10:22 am)
It may be that the reason the VC was dropped was because the traction control system, at least in my 01, activated so quickly and in such a way that made the VC useless.
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....
#2030 of 2493 1997 Toyota 4 Runner 4 WD Issue
Dec 30, 2006 (6:40 pm)
I live here in Colorado and with all the snow lately, the 4WD doesn't engage when you push the button. This has been an intermittent problem and was wondering if anyone has had this problem or have any suggestions as to what is going on. Thanks
#2031 of 2493 FJ Cruiser/Part Time 4WD ...need illustration
Jan 05, 2007 (1:04 pm)
Just curious....When my FJ (5speed AT/part time 4wd) is in 2wd motion, does the front drive shaft spin, and if so is the front axle spinning also? This is my first part-time 4wd vehicle and just trying to understand if the transfer case is not engaged what is spinning (besides front wheels obviously).
#2032 of 2493 Re: FJ Cruiser/Part Time 4WD ...need illustration [tommyg12]
Jan 05, 2007 (8:05 pm)
Tommyg, I was having dings in my windshield (Camry) fixed last week and a guy with an FJ was there getting his second windshield in six months (since new). He said the windshield is at such an angle (almost 90 deg) that rocks don't glance off, but cause lots of damage. I know it probably is a great vehicle but beware.
#2034 of 2493 2005 4runner SE - VSC & 4WD
Feb 18, 2007 (5:33 am)
I have a 2005 4runner SE and I was wondering when you should turn off the VSC. If the road conditions are icy should I be driving in 4WD with the VSC off or on? I am confused. Please somebody explain.
#2035 of 2493 Re: 2005 4runner SE - VSC & 4WD [jmorabito25]
Feb 18, 2007 (11:29 am)
Check your owners manual to be sure but I would bet that in 4WD, locked center diff'l, ABS and TC, traction control, are automatically turned off. There is good reason to suspect VSC is also off.
#2036 of 2493 Re: 2005 4runner SE - VSC & 4WD [wwest]
Feb 19, 2007 (8:38 pm)
ABS and Traction Control never turn off. Only VSC when you lock the center Differential. When the center diff is unlocked Trac will brake the spinning wheels and control engine output. When the center diff is locked it does not control engine output, it will just brake the spinning wheels. Unlocked will keep the vehicle under better control most of the time. Although I live west of Philadelphia, and during our last snowfall, which was a very heavy and icy snow, I went through an unplowed lot and unlocked center diff was very NANNYish. It would barely keep the truck moving, and almost stopped the truck. Very slow forward progress.