[CircusEnvy]

I can't seem to find any clear answers on this, but I was going through my owner's manual and aside from the small blip about how to turn on the AWD lock switch, it doesn't really go into detail when NOT to use it. I safely assume not to use it in areas of high traction, but there are absolutely no warnings about using it in such situations, like you typically see in manuals for the 4Runner or other 4X4 models.

So I'm wondering, is the center differential designed to be idiot proof if you turn it on when driving on pavement? If so, how does this work?

 

[Dr. Dyno]

What year & model are you referring to?

 

[CircusEnvy]

What year & model are you referring to?

Sorry! 2015 XLE with AWD.

 

[Dr. Dyno]

No center differential on 2006-2016 4WD models. There is an electro coupler on the rear diff that automatically engages the rear drive BEFORE it's needed. The LOCK button can virtually be ignored. Many posts on the subject. Button on or not it also automatically disengages when not needed or desired, thus running it FWD most of the time. A totally different system than a traditional 4X4 vehicle such as a 4Runner or F-250 which will "bind up" cornering on pavement.

 

[CircusEnvy]

Interesting. So then really there's no risk of binding under any circumstance?

 

[Dr. Dyno]

Interesting. So then really there's no risk of binding under any circumstance?

No worries at all. It's a pretty smart system. Even with LOCK on the rear drive goes off when you hit the brakes thus allowing ABS to function.

 

[Mikey D.]

Also, when you go over 25 MPH the lock feature disengages.


Sent from AutoGuide.com Free App

 

[mwn]

"No center differential on 2006-2016 4WD models."

The following video confirms there is a central differential at about 5 minutes in. Very hard to run an AWD/4X4 without a central differential sending power to both ends of the vehicle.

"There is an electro coupler on the rear diff that automatically engages the rear drive BEFORE it's needed."

The coupler is used to control the amount of torque transferred to the rear wheels. The ECU tells it how much.

How can the vehicle know AWD is needed before it is needed? The ECU has parameters programed that determine when AWD MAYBE beneficial, ie cornering. The ECU can engage AWD when it is not needed/beneficial ie hard acceleration without loss of traction. If the front wheels do not slip under "hard" acceleration AWD is not needed nor beneficial, in fact it is detrimental as it using power to drive gears that are not needed. RAV4s normally run around in FWD as less power/gas is needed if only one set of gears/wheels are powered.

The ECU can not anticipate loss of traction under normal throttle conditions and thus slippage is needed to tell the ECU that AWD is needed. Thus AWD is not engaged until after it is needed in those cases.

"The LOCK button can virtually be ignored." Actually no, think about it if it was not needed it would not be there. The owner's manual says to use it when stuck in mud and one would assume snow, sand. etc.

"Button on or not it also automatically disengages when not needed or desired" Actually no, switch on it stays locked in until above 25 mph or it is switched off.

 

[katekebo]

OK, so here goes a somewhat lengthy explanation. Toyota uses three types of 4WD / AWD systems/configurations. I will start with the one used in the RAV.

1) Full-time on-demand AWD: This is the system used in current generation RAV. "Full-time" means that it is always "ON" and available, "on-demand" means that it only engages where required (or requested by the driver). It does NOT have a central differential. Under normal driving conditions torque is directed 100% to the front wheels. The rear axle is connected to the transmission through a transfer case, propeller shaft and an electronically controlled clutch, which is installed in front of the rear differential. Under specific conditions the clutch will engage to send part of the available engine torque to the rear axle. This occurs under the following circumstances:

a) Hard acceleration from full stop or very low speed - the clutch will engage as soon as you hit the gas pedal to prevent a spin, delivering up to 50% of torque to the rear axle. As the speed increases, the clutch will gradually release sending less torque to the rear, and disengage completely at 25 mph.

b) Front wheel spin - the clutch will engage automatically if the ECU detects that front wheels are turning faster than rear wheels. The system uses ABS speed sensors to measure the speed of each wheel.

c) Pressing the AWD Lock button - the clutch will engage and stay this way until the vehicle reaches 25 mph. Once above 25 mph it's back to fully automatic operation.

The clutch is designed to limit the amount of torque to the rear axle to 50% of the engine output. If you "push" the system too hard (for example playing too hard on a gravel surface) and the systems senses that too much torque would be send to the rear, the AWD light on your dashboard will start flashing indicating that the clutch is slipping to protect rear differential / axles from damage. The clutch has a temperature sensor that will disable AWD completely if the clutch overheats, and keep it disabled until the temperature has dropped to a reasonable range. This approach allows to have a smaller / lighter rear differential and axles to save weight / cost.

This system will not bind on curves under any circumstances because the clutch will release / reduce pressure as soon as it senses that the steering wheel is off-center (as sensed by the steering wheel position sensor located in the steering column). Other vehicles that use this system are Highlander, Sienna, Venza and Lexus RX. There are slight variations in software hence the system comes under different names (Dynamic Torque Control, Active Torque Control).

2) Part-time 4WD with central transfer case and lock: This system is used, for example, in the Tacoma. The vehicle is essentially rear wheel drive. Front axle can be engaged on-demand by locking the transfer case. Such system should be used off-road / on low-traction surface ONLY. Using it on pavement would result in excessive wear, binding, etc. But it is very strong / robust and each axle is designed to cope with 100% of engine torque, if necessary. It works well in difficult off-road situations such as loose stones / rocks or mud.

3) Full-time 4WD with central differential: This system is used in Land Cruiser, the defunct FJ Cruiser, Lexus GX and premium version of 4Runner. In this system, torque is delivered to both axles at all times through a central differential. Toyota uses Torsen Type III torque-sensing differential. Under normal driving conditions it delivers 70% of the toque to the rear, 30% to the front. If one of the axles starts spinning, the Torsen differential automatically changes the torque bias from 70/30 to anywhere between 50/50 and 90/10. The central differential prevents binding on curves. The central differential can be locked to deliver up to 100% of the torque to one axles. This is, by far, the best 4WD system you can have, but it is more expensive and adds substantial weight to the vehicle as each part of the drivetrain must be designed to withstand 100% of the engine's torque.

The system used in the RAV is an effective traction aid for snow, ice and mild off-roading, but is not designed to compete with "real" 4WD systems like (2) and (3) in off-road or extreme situations. It is a good compromise to provide most of the advantages of AWD/4WD for on-road use, while being 100% "invisible" to the driver (i.e. it's basically fool-proof, and the driver can't screw it up), and reduces the weight and fuel-economy penalties normally associated with other 4WD systems.

Hope this helps.

 

[fishexpo101]

That's a great answer - lengthy but very clear.

 

[mwn]

It does NOT have a central differential. Under normal driving conditions torque is directed 100% to the front wheels. The rear axle is connected to the transmission through a transfer case, propeller shaft and an electronically controlled clutch, which is installed in front of the rear differential.

First, thank you for taking the time to clarify the 3 types of 4X4 systems.

I am a little confused still. The Toyota video I referenced shows/talks about the clutch at the front of the rear differential AND a central differentiation.

I understand how your/the new version works but it does raise a few questions.

1.The transfer case and rear drive shaft must be turning over whenever the vehicle is moving increasing friction and thus wasting power and gasoline. Less than 100% of the torque goes to the front wheels. IE just because 0% torque is getting to the rear wheels it does not mean 100% goes to the front.

2. Under hard [whatever the parameter is] acceleration and no slippage [any dry surface and many wet surfaces?] the rear clutch is being worn/generating heat every time it is engaged again lowering mileage ever so slightly. All this to have the 4X4 traction available before/even when not needed, seems
wasteful.

3. From previous comments and videos it would appear that every time the vehicle is cornering the same wear/heat generation/wasted torque/power is happening.


Thanks again for the clarifications even if it caused "Why did they do that?" questions.

 

[Dr. Dyno]

I watched the video. The narrator was reading a typo when he mentioned a center differential on the RAV4. Earlier he said it was used on other models with the electro-coupler clutch doing the job on the RAV4 4WD.

All driven shafts, bearings and gears have loss but since they aren't transmitting power while in front wheel mode the loss is minimal. The main loss is the added weight of these components. That's in contrast to full time AWD systems where power is being transmitted continuously and even slight differences in wheel diameter due to tread wear cause an additional constant loss.

Yes, there is some loss when 4WD is engaged but again it's the least wasteful system available to have 4X4 traction. The only no-loss option is FWD.

Same answer with cornering except that it is again less wasteful that full time AWD vehicles which typically have some sort of a viscous coupling in their driveshaft that has to slip and waste power on every corner.

 

[docjj]

Here is a video Explainung toyota's AWD system.

http://youtu.be/LVxsoAEIxNQ

 

[katekebo]

First, thank you for taking the time to clarify the 3 types of 4X4 systems.

I am a little confused still. The Toyota video I referenced shows/talks about the clutch at the front of the rear differential AND a central differentiation.

I understand how your/the new version works but it does raise a few questions.

1.The transfer case and rear drive shaft must be turning over whenever the vehicle is moving increasing friction and thus wasting power and gasoline. Less than 100% of the torque goes to the front wheels. IE just because 0% torque is getting to the rear wheels it does not mean 100% goes to the front.

2. Under hard [whatever the parameter is] acceleration and no slippage [any dry surface and many wet surfaces?] the rear clutch is being worn/generating heat every time it is engaged again lowering mileage ever so slightly. All this to have the 4X4 traction available before/even when not needed, seems
wasteful.

3. From previous comments and videos it would appear that every time the vehicle is cornering the same wear/heat generation/wasted torque/power is happening.


Thanks again for the clarifications even if it caused "Why did they do that?" questions.

So, in a few words ...

1) Yes, any 4WD/AWD system is somewhat "wasteful". That's why 4WD/AWD cars usually have worse fuel efficiency than their 2WD counterparts. That's the price you have to pay for the benefits of 4WD/AWD. Don't want to pay the price of it - buy a 2WD model.

2) The clutch actually suffers less wear if engaged preemptively, before slippage occurs. If the clutch engages before the wheels start turning or at very low speed, there is no slippage / wear on the clutch surfaces. If the clutch engages AFTER the drive (front) wheels have started to spin, it will have to deal with a big speed difference between front and rear axles, resulting in wear. That's exactly why Toyota implemented this preemptive logic.

3) No, because the clutch releases (or operates under much reduce pressure) on turns. The ECU senses that the vehicle is turning (using steering wheel input) and reduces the pressure on the clutch. In real life, the speed difference between front and rear axles on gentle curves is minimal.

By the way, the rear clutch is a multiplate wet clutch, so it can take many millions of cycles before is reaches the end of its useful life. In practice, Toyota AWD clutch is designed to last through the entire lifespan of the car (200-300,000 miles). It's not eternal, but it will probably last longer that other parts of the car. Of course you can wear it out if you abuse the system by playing "hard" off-road. But the RAV is not intended for such use - if you want a "hard-core" off-roader or a vehicle for you snow plow, you should buy an appropriate vehicle for such use (Tacoma or 4Runner).

 

[mwn]

Here is a video Explainung toyota's AWD system.

http://youtu.be/LVxsoAEIxNQ

Thank you for the reply, I have watched that Toyota training video before. At about 2:50 he implies that the rear driveshaft is not ALWAYS connected to the the power source at the front as others have said. He says the ECU determines when power WILL be sent to the rear [the rear clutch controls what is passed on to the wheels]. This would indicate a differential/clutch up front.

Question how do the front tires get more than 50% of the power if 50% [45% is used in the other video] of the power is available at the rear clutch? To have 100/0, 90/10, 80/20%, etc available front and rear it would appear that the ECU would need to control the split at the central differential, ie sending the appropriate % either way. The clutch at the rear can only pass on up to the % sent from the front. It can not take more than sent and it can not return unused to the front.

 

[mwn]

"The narrator was reading a typo when he mentioned a center differential on the RAV4."

One would assume that a Toyota produced video would be free of typos but I could be wrong, would not be the first time.

 

[mwn]

katekebo; Thank you again for the reply


1. Rhetorical question: So how can Toyota claim 100% of the torque gets to the front wheels?

2. Good point.

3. Working under less pressure would result in slippage/wear. Remember the one video said this only happens in "hard" cornering.


Still having trouble getting my head around this. Both Toyota videos indicate/imply there is a central differential and that the rear drive shaft is not locked to the front "drive shaft". A differential allows the "shaft" [axle or drive] on either "side" to rotate at different speeds thus the name "differential". If there is no (central) differential between the front and back one must assume both are turning in unison all the time or are disconnected by a clutch until needed.

Comment assuming rear is always engaged, how do the front tires get more than 50% of the power if 50% [45% is used in the other video] of the power is available at the rear clutch? To have 100/0, 90/10, 80/20%, etc available front and rear it would appear that the ECU would need to control the split at the central differential , ie sending the appropriate % either way. The clutch at the rear can only pass on up to the % sent from the front. It can not take more than sent and it can not return unused to the front.

Life was easier in Henry Ford's day; you can have any color you want as long as it is black, AND one could see what one was getting. Interesting that I contacted Toyota in the past about this and they said their contacts were not able to answer my questions.http://www.rav4world.com/forums/editpost.php?do=editpost&p=2103049

 

[Dr. Dyno]

Question for you mwn. What % of torque goes to the front wheels on a FWD vehicle.

REPEAT, REPEAT The 4.3 and 4.4 RAV4s DO NOT have a center differential. Saying/implying so in a video IS AN ERROR. Remember these videos are not mechanic's manuals.

The driveshaft from the front to the rear of the vehicle turns whenever the front wheels do. It spends most of it's time NOT electro coupled to the rear differential. "If there is no (central) differential between the front and back one must assume both are turning in unison all the time or are disconnected by a (rear electro-) clutch until needed." Correct.

To have 100/0, 90/10, 80/20%, etc available front and rear it would appear that the ECU would need to control the split at the central differential , ie sending the appropriate % either way.

NO CENTRAL DIFFERENTIAL. The electro clutch essentially "steals" a variable percentage of power from the front. If it locked up solid you'd have a 50/50 split but apparently Toyota has chosen to call it 55/45. Why I don't know but it's all semantics anyway since it the fronts have no traction but the rears do 100% would go to the rear.

 

[mwn]

Question for you mwn. What % of torque goes to the front wheels on a FWD vehicle.

One would assume 100% of that not lost in friction getting it to the wheels.

REPEAT, REPEAT The 4.3 and 4.4 RAV4s DO NOT have a center differential. Saying/implying so in a video IS AN ERROR. Remember these videos are not mechanic's manuals.

So both videos are wrong, the second one twice, hard to believe that but it is possible. Do you have a link that says no centre differential?

The driveshaft from the front to the rear of the vehicle turns whenever the front wheels do. It spends most of it's time NOT electro coupled to the rear differential. "If there is no (central) differential between the front and back one must assume both are turning in unison all the time or are disconnected by a (rear electro-) clutch until needed." Correct.

The rear electro- does not connect the front and rear drives it connects the rear differential to the rear drive shaft.

NO CENTRAL DIFFERENTIAL. The electro clutch essentially "steals" a variable percentage of power from the front. If it locked up solid you'd have a 50/50 split but apparently Toyota has chosen to call it 55/45.

So another mistake by Toyota re 50 vs 45%? When the split is 90/10 the rear clutch is slipping away wasting 40% of the overall torque available? So you are saying the front and rear drives turn in unison and "some how" the rear clutch takes less than 50% and sends the rest of the torque back to the front? Do you have a link of how the rear clutch manages to do that?

Why I don't know but it's all semantics anyway since it the fronts have no traction but the rears do 100% would go to the rear. /QUOTE]

You are confusing "providing forward motion" with used torque. To look at it in simpler terms consider a conventional 4X4 in locked mode. 50% of the torque goes to each driveshaft irregardless of the amount of traction at each end. So little traction up front the rear wheels prove most of the forward motion yet the torque is still split 50/50.

 

[Dr. Dyno]

One would assume 100% of that not lost in friction getting it to the wheels.

I agree with that statement. You are confused on most of the rest of it.

One source for accurate information on the 4.3 4WD system and it's operation is found in the service manual which I have on DVD. The 4.4 (except hybrid) system is mechanically the same but with more advanced control logic. The basic description is on page CH-66 with more detailed explanations later on.
Hopefully someone who has the time will post or link to it.