Blocking The Central Differential

[nest]

OK, this is the scenario.

I'm driving my RAV, all nice and beautifull, but I get stuck on ice or snow. One wheel is spinning, the others just don't move. I'm alone, nobody to push the car, it's freezing and I have to get home. Pushing the !Removed! diff-blocking button saves my ***** or I really need do make a phonecall?

Theoretically, blocking the differential should distribute the power to all the wheels (60:40), but the question is: it's working?

The VSC block the wheels that don't have the grip, right? I tried to disable the VSC with the method described somewere in the forum with the footbrake and the handbrake, but it didn't worked. (Don't know why...) So, in this situation, blocking the diff does any good?

Did anyone confrunted with this situation?

[bothwell_buyer]

Had a similar debate on rav4world - on the early RAvs there is a diff lock but I think we agreed that it would only lock one wheel front and 1 at back - ie if wheel contact was lost on one wheel at front and 1 wheel at back then car would not move.

now

on later models like the 4.2 / 4.3 I don't know.

[Parts-King]

The diff lock effectively locks the two shafts on one axle together, great for getting you out of snow/mud if stuck, but when engaged you cant steer the car its like welding the two drive shafts together As soon as you are free from the snow, disengage the diff lock

Kingo

[nest]

Why am I asking you that: in the australian Top Gear, when comparing RAV , x-trail and Forester, the guy in the RAV struggles to move the car when he's on ice. From what I understand what he is saying, he lets the computer wizzard do all the job. Now I guess he didn't lock the central diff. Just one front wheel was spinning, and the car didn't moved. (After a while he managed to move the car...). Let's not forget that the winter tyres werer not fitted, though...

[anchorman]

Yes the diff lock button fully engages the drive to the rear axle which has a compact multi plate clutch in the nose and is constantly variable between disengaged and about 5% slip. This slip is built in to protect the final drive against excessive torque - a breakaway limit clutch if you will.

As bothy has suggested you still have the problem of one of the front or rear wheels slipping which will imobilise the vehicle so that is where the VSC/Traction control steps in as it will apply a brake force to the slipping wheel/s and encourage the torque to be transmitted to the stationary wheels. Of course this is not flawless and the computer can get confused especially on very low friction surfaces or when things change quickly. This is mainly because the mechanical components don't react as quickly as the computer and a stuck wheel can soon become a spinning wheel that needs some intervention from the brake! Don't forget if you could imagine a situation where there is zero adhesion at all 4 wheels the VSC computer will knock back engine power to a point where it will not apply any power to the wheels and no amount of pressing the accelerator will bring the engine off idle.

In this case it is sometimes better to try the "untreated" version of 4WD and it is worth trying to disable it. The process does work but needs to be followed to the letter. I have it printed and keep a copy in the car.

With regard to Kingo's example well I hate to contradict but the system takes steering angle into affect and that brings in an important point. In order to prevent "transmission bind up" where attemting to turn the vehicle through tight curves with the front and rear axles locked would make steering difficult and could damage the transmission, the system measures steering angle easily as the 4.3 is equiped with electrical steering gear. In the case of full lock being applied it will reduce the drive to the rear axle to as little as 5% to prevent this situation on a harmful good friction road surface and increase more and more towards a staright ahead position (the system is very clever but the one thing it is completely unaware of is the road surface adhesion - as it cannot see like you can it can only detect the affects of it). This means if you are stuck in the snow and you have the steering turned you have affectively reduced the drive to the rear axle. The VSC will still try to correct a spinning wheel but you have added another parameter/restriction to the equation. In this case it would be better to try to start the vehicle with the wheels staright ahead and then turn the wheel when you have gathered a little momentum.

Have another go at disabling the VSC;

http://toyotaownersclub.com/forums/index.p...c=74864&hl=

Regards

[bothwell_buyer]

great reply!

As far as Kingos, I think that relates more to the 4.1

[dave.m]

Used my diff lock this morning. Should not be smug, but could not help but laugh this morning passing the cars

and vans stuck in the road outside my house. It has a bit of a slope and today was covered by 2" of ice. The

Rav did not miss a beat

[Parts-King]

great reply!

As far as Kingos, I think that relates more to the 4.1

Sorry, I was thinking of a 4.1 and the old diff lock button Must look harder before blundering on

Kingo

[nest]

@ anchorman

Thanks for the comprehensive answer! I had to read it twice, bu finally I got it!:)

I tried once more to disable VSC, and just like in the movies, when the action hero tries to do something and fails at every attempt, he thinks "Just one more time!.." and finally he manage to resolve the problem! :):):) My fault was that I was pushing to hard the footbrake, activating the HAC...But now I'm cool!:):):)

Now I have another one for you: OK, VSC is disabled, but how it affects the other two functions - the power steering and 4wd - considering the Integrated Active Drive System?...

[Hoovie]

Used my diff lock this morning. Should not be smug, but could not help but laugh this morning passing the cars

and vans stuck in the road outside my house. It has a bit of a slope and today was covered by 2" of ice. The

Rav did not miss a beat

When I had a Peugeot 306, I tried to drive from home up an icy road with a gradient of, oh, at least about 1:400 and it couldn't make it one reason why I wanted to go 4WD again

Be interested to see how the RAV4.2 copes (I am expecting a great deal )

[nest]

Just a hint about what our cars would do climbing on ice.

4.2 was great, also the 4.3 after locking the diferential.

[To-yo-ta]

So for those of us with the 4.3 base model XT3, which does not have VSC, do I understand that our vehicles will not have any problems once the diff lock button has been pressed (assuming that we have'nt already crashed that is due to lack of VSC) ?

[anchorman]

Working early but will reply later!

[bothwell_buyer]

So for those of us with the 4.3 base model XT3, which does not have VSC, do I understand that our vehicles will not have any problems once the diff lock button has been pressed (assuming that we have'nt already crashed that is due to lack of VSC) ?

HHHmmmmmmmmmmmmmmmmmmmmmm

how many doors do you have? :lol:

[vivianM]

Working early but will reply later!

Anchorman

Can you make this clear please:

Does the dashboard button on my T180 engage constant four wheel drive or does it lock the rear differential as similar to my previous 2.0L petrol 4.2 which had a limited slip diff. when when fully active that would make the rear axle 'solid'

I am fairly sure that the button just engages 4 wheel drive and does not lock the diff.

In any case my hilly road last night was sheet ice but the the T180 when up as if it was it was a dry road! I only wish the engine did not take so long to warm up.

[nest]

Vivian, that's the trouble with the diesel - you catch a cold until the car gets warm inside!...:) Of course, heated seats resolve the problem. I thank God every chilly day when I get in the car and push that tiny button...:)

[bothwell_buyer]

does diesel ever warm up? Is that not why railway trains that run on diesel are always left running???

My turbo RAV heats up within 3 minutes, or 400 yards :P :P

[anchorman]

Blimey! - So many questions!!

Let me go through them and answer if I can.

Before you consider what the affect of disabling the VSC has lets just go over what it does and how it differs on the 4.3 from normal 2WD vehicles. Break it down :-

A smart 4 wheel drive system

Your car ALWAYS starts in 4WD and will gradually back off depending on throttle load and vehicle speed. If you "poke" the accelerator while you are cruising you will provoke a corresponding pulse of 4WD. For this reason traction control is not necessary to deal with normal weight transfer when starting although the vehicle does have it for low or split surface adhesion (one side dry and the other wet or icy). As stated the 4WD also reduces towards the extremes of steering lock to avoid "tight corner braking". For the most part when the vehicle is cruising and under light throttle loads it has reverted steplessly to a FWD vehicle.

ABS/Traction Control System.

Sensors at each wheel induce tiny voltages as the wheel turns. As long as all wheels are turning all voltages are equal then the system is happy. If the voltages differ when the brake is being pressed then the system knows that one or a number of wheels are locked and the ABS pump will modulate the pressure to the affected wheel/s until the speed is equalised. If the voltages vary while the vehicle is under power then it knows that one or a number of wheels are spinning and the affected brakes are applied to transfer power to a non spinning wheel.

Those vehicles without VSC have a deceleration/acceleration sensor under the left seat so that for the above functions rates of accel'/decel' can be compared with throttle or brake pedal pressure - it allows the system "some" sense of how well it is achieving the wishes of the driver. The sensor under the left seat on those vehicles with VSC can also measure yaw or sideways g force and so adds another dimension. Now we can talk about VSC;

Imagine driving along a dry road with good adhesion. Speed is being monitored at all 4 wheels (Remember that 4.3s do not measure speed at the gearbox. It is vital that the system knows even if only one wheel is turning and which one it is so the speed will be indicated from one turning wheel). If you turn around a curve the vehicle can sense speed and can compare yaw with the amount the driver is turning the steering wheel because the electronic steering is feeding back the steering angle. In other words at any given speed if you turn a corner it knows how much sideways g force to expect and as long as it gets it the system is happy and sleeps. Now consider a low friction road surface. If the vehicle enters a curve and after the driver turns the wheel the motion sensor does not sense the right amount of g force, it knows that the vehicle is drifting sideways. This wakens the sleeping giant and a number of actions are put into motion - as far as possible control is removed from the driver and put into the hands of the various ECU's which will now try to use pre-determined mapping to attempt recover the situation;

the engine power is reduced or cut

the yaw/acceleration/steering angle sensors will determin if the vehicle is in a front or rear wheel skid based upon the information gained up to the event

control is taken of the brake pedal force - although the driver panic reation is to press the brake until he is blue in the face, if the system needs the vehicle to continue in order to correct the situation it will completely release some or all of the brakes. This may seem to conflict with the wishes of the driver but you cannot defeat the law of physics and a vehicle which has turning wheels is "less out of control" than one with locked wheels. So smart is this system that you need do nothing if your instincts will allow, for it will, without your input apply any one or a number of brakes without you touching the pedal in order to bring the car back into a line which matches the speed with the steering angle and the correct g forces and even if you do press the brake in panic it will completely ignore you

it will intervene with the steering assistance and resist (but not stop you) from turning the wheel the wrong way to correct the skid but allow you freely to turn it the correct way

your efforts to intervene with the throttle or brakes are completely ignored and it will only ecourage correct inputs from the steering until such a time that all sensors and computers are happy that the inputs indicate the vehicle is back under control. If this means that instead of turning a curve it sends you with brakes and power off in a straight line (albeit briefly) towards a cliff face then it knows that this is the only chance of recovering the situation and it can think quicker than you can. Of course by this time you may still be pressing the brake and as long as it is able it will begin to apply braking force if adhesion becomes available.

Only when it knows that the vehicle is under control will the various inputs be handed back to the driver in the order that they become available. During intervention the VSC warning light flashes and a buzzer sounds. After intervention a message is shown on the message centre - "here numbskull, you can have it back now but look what you're doing" (just kidding about that bit but the rest is true!!!)

Overwhelming isn't it? Well I haven't finished yet!

in cold weather when you get the "possible ice on road" message the ECU gets it as well. Under these conditions, the mapping is tightened and the VSC is on the lookout for skids.

during a skid if the ECU's believe that control can be regained by laying down some drive to the front or rear wheels it will activate the drive axle clutch accordingly

the system can detect panic braking and even a little old lady can generate twice the brake boost pressure simply by how quickly she hits the pedal (Brake Assist Control).

vehicles without VSC still have access to some left/right brake control. This is achieved by the ABS sensing wheel lock due to sideways weight transfer causing inner wheels to lock and responds accordingly.

The icing on the cake is with the Active Control System which links all these systems together. The trigger is the driver or conditions which created the situation and the affects are astonishing but keep in mind that they cannot defy the laws of physics, only mitigate the outcome.

So!!! If you disable the VSC you take away all this useful help so why do it? The system err's towards caution on the understanding that prevention is better than cure. In extreme conditions, it may be possible to stretch the results of an action a little bit further than the systems would otherwise allow and you might be able to conciously have some fun or even "wriggle out of a hole" if the vehicle becomes stuck. The conditions shown on those rolling road tests above really exceed the design limitations of the RAV. We all like to thing that we have something special and our invincable 4WDs could get us out of anything. However, those rollers present virtually zero adhesion and the way they are configured are to test vehicle that have proper lockable transmissions to all 4 wheels - true off roaders. In reality you can only cheat them by having a bit of a run or hoping that luck and random input from the VSC or TRC or both gives the vehicle a kick of inertia that will see it over the relatively short test.

However, what we have have is essentially a road going vehicle with very acceptable performance and a 4WD system that proffers some limited off road performance and snow capabilities that meet most reasonable requirements. If you want to pull a trailer up a field you need a Land Rover (and you can fix the ruddy thing while you are up there). If you want to wade through a swamp you need a suitably equiped amphibious vehicle with fully lockable transmission. Niether of these two vehicles would allow you to go touring in rain or shine without considerable discomfort or cost so there is a compromise.

As long as you remember your limitations you have the best of both worlds.

So to summarise;

Nest

Disabling the VSC will not stop the power steering motor from working but will prevent any intervention in a skid (as you may want). All of the systems are no longer linked via one system but you might be able to extract slightly higher margins of control (before you lose it altogether!)

To yo ta

You still have the main systems although they were never linked. You could therfor say that you already had the ability to stretch the limitations but you won't get quite as much help if you get it wrong! (I hope that makes sense). To put it into perspective, you still have an array of very capable aids and coupled with your "active" capabilities (the way the vehicle handles and performs) you are still better off than a lot of similar vehicles.

VivianM

Unless a vehicle has been modified they work like this;

4.1 has a conventional FWD transmission. It also has a driven rear axle which is connected to the front gearbox via a transfer box. The transfer box has a third differential which prevents inter axle wind up or "tight corner braking" and resists free rolling motion. The thrid diff is lockable which rigidly connects the front and rear axle but there is no means of preventing left/right wheelspin of either the front or rear axle

4.2 has a permanent 4WD system. It does not have any limited slip differentials but a rather unusual way of utilising the front differential through a permanently engaged transfer box. The rear axle has its own diff' but inter-axle drive is shared at the front diff'. Again there is no means of preventing left/right wheelslip at either axle EXCEPT for later XT5s or XTRs which have VSC in which case see below.

The 4.3 has a conventional FWD transmission. There is a permanently engaged transfer box and a prop' shaft to the rear axle. Whenever the vehicle is in motion all these components turn. However, the rear axle has a constantly variable clutch in the nose. The clutch is electronically controlled via the various ECU's as described above and is constantly varying the amount of drive to the rear axle. This occurs many times during a journey. By pressing the "lock" button you override the ECU's and the clutch is fully engaged (it is prevented from fully locking to reduce the risk of transmission damage so even when engaged the ratio between front and rear axle is around 55/45%). When engaged, only turning the steering towards full lock will reduce this ratio again to prevent transmission bind up and as a final measure the clutch will be de-energised at 25mph or when the vehicle goes into an over-run situation. To reduce the risk of left/right wheelspin, the TRC will apply a braking force to a spinning wheel. When this happens you will hear lots of whirring and bumping from underneath but this is normal.

Earlies again tomorrow so I will look in after work!

[nest]

Thanks, man! You're our mechanical guru! :)

[Robert6920]

Masterclass, Anchorman!!

[bothwell_buyer]

maybe so, but thats why you can't do donuts!!

[vivianM]

Thanks Anchorman for your , as usual, very detailed reply.

I am sure that my previous RAV a 2.0L VX VTT1 did have a limited slip diff (in the rear axle) I clearly recall Culvers now RRG Stockport explaining that on the first service they had to change the rear axle oil because it was a limited slip diff. I had those on my two Jensen Interceptors, and they were needed with a 7.2L engine!

The earlier Rav has been P/E for the T180 but if anyone has bought MC52 OPX they could look up the handbook. It is a new number as I have kept my personalised plate.

Guy

[vivianM]

Re my earlier posting should have given the VIN for my VX

JtheH20V200219833 as some one very clever might be able to get the spec for it and confirm if it did have a limited slip diff or was I conned by the dealer! The handbook went with the car, which had been absolutely trouble free and was only P/Exed as RRG gave me such a good deal on the T180 that I could not turn it down.

Guy

[vivianM]

Working early but will reply later!

for some reason my reply seems not to have appeared on the postings! Sorry if it does come in twice.

I am sure that my my 2.0L RAV a VX VVT1 had a limited slip diff. It has been part/exed for the T180.

If anyone has bought MC52 OPX please look up the handbook! I have had two Jensen Interceptors both with limited slip diffs. My dealer explained that on the first service they had to change the rear diff oil as it was a limited slip one, was I conned?

Re my earlier posting should have given the VIN for my VX

JtheH20V200219833 as some one very clever might be able to get the spec for it and confirm if it did have a limited slip diff or was I conned by the dealer! The handbook went with the car, which had been absolutely trouble free and was only P/Exed as RRG gave me such a good deal on the T180 that I could not turn it down.

Guy

[anchorman]

vM

I have had some very interesting conversations at main dealers. No doubt the are some knowledgeable people in some of them but for others using a generic term like "limited slip diff'" is an easy way of concluding a conversation.

I have all the drawings and maintenance info for a 4.2 including rear axle, transfer box and gearbox and there is not a limited slip diff' to be seen!!! In fact there are some disadvantages in using a limited slip diff which is why they are seldom used on what is essentially a road going vehicle. They are usually the domain of true off roaders or very high performance cars like your Jensen's. They use a spring loaded multi plate clutch or viscous coupling to resist what would otherwise be free to rotate planet wheels. In the case of one wheel spinning in the mud a torque equal to the resistance generated would be applied to the stationary wheel which is sometimes enough to ease the vehicle into motion. The downside is that they generate some "tight corner braking", excessive fuel consumption, tyre wear and drag. By their very nature they are a wearing part (designed to slip) and introduce a maintenance issue.

This explains a bit more;

http://auto.howstuffworks.com/differential3.htm