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Toyota 4WD systems explained

2487 messages,  Last post on Apr 28, 2012 at 12:48 PM

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What is this discussion about? Toyota Sequoia, Toyota Highlander, Toyota 4Runner, Toyota Land Cruiser, Toyota RAV4, Toyota Tacoma, Truck, SUV

Messages Page 207 of 250

#2052 of 2487 Re: Found this explaination - How does Highlander 4WD work? [my001] by wwest

Mar 06, 2007 (3:07 pm)

Replying to: my001 (Mar 06, 2007 11:13 am)
Sorry, but the VC, viscous coupling, was dropped across the entire Toyota/Lexus FWD based product line as of the 04 model year. It has now been adopted again for the RX350 models but insofar as I can learn not for the others.
 
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.

#2053 of 2487 RE: 2007 HL IS 50:50 at least by my001

Mar 06, 2007 (3:52 pm)

Get this from 2007 HL Buyer's guide:
 
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.

#2054 of 2487 Re: Found this explaination - How does Highlander 4WD work? [wwest] by 2toyotas

Mar 08, 2007 (6:37 pm)

Replying to: wwest (Mar 06, 2007 3:07 pm)
First the rear is not overdriven by the front. It would be if the transfer unit ratio was 1:1, but it is not. The transfer unit ratio is .34:1, and that makes up for the different ratios front and rear. Power is 50/50 until a wheel or wheels slip and then trac steps in.
  
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.

#2055 of 2487 Re: Found this explaination - How does Highlander 4WD work? [2toyotas] by wwest

Mar 09, 2007 (8:21 am)

Replying to: 2toyotas (Mar 08, 2007 6:37 pm)
If TC will brake each front wheel individually why is an "auto" LSD function/mode required?

#2056 of 2487 Re: Found this explaination - How does Highlander 4WD work? [wwest] by 2toyotas

Mar 09, 2007 (12:05 pm)

Replying to: wwest (Mar 09, 2007 8:21 am)
It uses less engine dethrottling. It will act like a mechanical LSD. When I use it on my Tundra it will allow more wheel spin. The reason toyota uses it is to help get a vehicle unstuck by allowing a lot more wheel spin, unlike Trac which when in 2WD can really bog forward progress down.

#2057 of 2487 Re: Found this explaination - How does Highlander 4WD work? [2toyotas] by wwest

Mar 09, 2007 (7:50 pm)

Replying to: 2toyotas (Mar 09, 2007 12:05 pm)
Yes, if TC, in "auto" LSD, is allowed to brake only a single slipping wheel then engine dethrottling serves no purpose. It's only in normal traction mode that both front wheels are braked with only one slipping therefore the engine MUST be dethrottled.

#2058 of 2487 Re: Found this explaination - How does Highlander 4WD work? [wwest] by 2toyotas

Mar 10, 2007 (7:18 pm)

Replying to: wwest (Mar 09, 2007 7:50 pm)
Agree to disagree!!

#2059 of 2487 RAV4 4WD/AWD by leeharvey418

Jun 18, 2007 (4:15 am)

Can somebody please explain to me why the RAV4's system is called a "4WD" when it is not selectable and is (almost) completely automatic in its functioning? If I had to guess, I would apeculate that the locking function being user selectable makes Toyota's legal department loath to putting an AWD label on the system.
 
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.

#2060 of 2487 Re: Locking Center Differential [nedzel] by chiefjojo

Jun 27, 2007 (8:17 pm)

Replying to: nedzel (Mar 05, 2007 8:54 am)
"in normal driving, even on snowy roads, leave the center diff unlocked. If you are going offroad, then lock the center diff."
 
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.

#2061 of 2487 Disabling 4WD on 2005 RAV4 by zang

Jul 29, 2007 (10:18 am)

My wife just purchased a 2005 RAV4 after 12 years of owning a Camry. We've noticed poor gas mileage and wondered if the rear wheels can be disabled of the 'always-on' 4WD?
We don't need 4WD at all and have never had it. Mechanical solution?
Thanks.

Messages Page 207 of 250

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