Transfer Box - How Does It Work ?

[pgp001]

OK this diagram seems to sum it up nicely.

mnl. I agree with your points about turning the wheels etc, I have sketched out the gear train and it would do as you say.

Not sure about point b. though, I think with both wheels on one side free to turn you should not feel the viscous coupling, but if just one rear was off the ground and the other three planted, then you would feel the viscous coupling as you say. The coupling is effectively looking for difference in rotational speed between front and rear before it locks.

Do you agree or have I got it wrong.

Phil

2004 4WD SYSTEM.pdf

[mnl]

The penny drops,there are effectively 3 concentric shafts the outer 2 are each side of the centre diff, and the viscous applies friction between them all the time, (like the Classic Range rover), so my original idea, of jacking just one front wheel and standing on the wheelbrace in neutral will result in a slow rotation.

Cheers Phil

[anchorman]

I can't find any evidence of this viscous coupling. I can find the front and centre diff (I always said that there is no seperate third diff on a 4.2) but no VC.

Kingo - do you know of such a device on the UK models?

4.2 diff.pdf

[pgp001]

Looks like we are getting to the bottom of it now, cheers Anchorman.

Do you have a UK version of the Transfer Box exploded view, that will show if there is a "control coupling" or not.

Thanks

Phil

[IanXT4]

Now you have me interested, I cannot see it in this diagram its from the Toyota tech website for a 2004 UK model

Components.pdf

[bitman]

The penny drops,there are effectively 3 concentric shafts the outer 2 are each side of the centre diff, and the viscous applies friction between them all the time, (like the Classic Range rover), so my original idea, of jacking just one front wheel and standing on the wheelbrace in neutral will result in a slow rotation.

Cheers Phil

true i wrote that in another tread

[pgp001]

Now you have me interested, I cannot see it in this diagram its from the Toyota tech website for a 2004 UK model

Ian

It's the part described as "Centre Differential Control Coupling" and is between the centre diff input shaft and the crown wheel and pinion.

The only thing that is not 100% clear is if that is Toyota speak for "Viscous Coupling", I dont really see how it can be anything else, based on the other schematics of how the system works.

Phil

[IanXT4]

Not sure I can see what you mean it all looks bolted together ?

By the way I think I have found it half way along the drive shaft is a large circular unit that looks like a viscous coupling see below.

[pgp001]

Ian

You have been looking at too many Freelanders ........On a RAV that is just a universal joint in the propshaft.

The part I keep refering to is circled on the attached pdf. I reckon it is a sealed unit viscous coupling.

Phil

Viscous Coupling.pdf

[jedi134]

Why would it have a vc if it's got a center diff, what is the purpose of the vc?

Does it just dampen the force to the prop or is there some other magical reason for it?

[IanXT4]

Ian

You have been looking at too many Freelanders ........On a RAV that is just a universal joint in the propshaft.

The part I keep refering to is circled on the attached pdf. I reckon it is a sealed unit viscous coupling.

Phil

Hi Phil

I agree with you, I found details of the drive shaft and the picture I took just appears to be a simple coupling.

I think with hindsight that Toyota naming the part "Central DIFFERENTIAL CONTROL coupling" is a pretty good hint

Ian

[jedi134]

Ian

You have been looking at too many Freelanders ........On a RAV that is just a universal joint in the propshaft.

The part I keep refering to is circled on the attached pdf. I reckon it is a sealed unit viscous coupling.

Phil

Hi Phil

I agree with you, I found details of the drive shaft and the picture I took just appears to be a simple coupling.

I think with hindsight that Toyota naming the part "Central DIFFERENTIAL CONTROL coupling" is a pretty good hint

Ian

Must be a tiny vc to fit into the diff casing, compaired to a freelander one. So what does it actually do? Or am I being totally stupid???

[IanXT4]

The more I think about this problem the more my brain aches :ffs:

Looking at all the schematics I believe the RAV has 3 diffs - front, back and centre.

The object of the viscous coupling is to detect speed differences between the front and back axles. I believe its essentially a device that locks up the faster it spins.

The only way a viscous coupling could detect a difference in speed would be if it were in the main transaxle where the front and centre diffs reside ?

If it were in the transfer box (as previously discussed) it would only see power going to the rear axle and therefore would be useless.

There is a one page reference to the viscous coupling page 13 of the brochure

http://www.pentagon-toyota.co.uk/NEW_toyota_brochure/toyota_RAV4.pdf

[pgp001]

I have been looking at a sectional view of the transfer box on my 4.2 2004 model.

I see it has both a front diff and a centre diff for the take off to the rear axle, so three diffs in all.

Just before the output to the rear prop shaft there is a bevel wheel and pinion to turn the drive through 90° and it says there is a viscous coupling in the bevel wheel.

Can someone please explain the exact function of this particular viscous coupling, it says it is for giving a "limited slip" effect.

I know some vehicles have this viscous coupling in the prop shaft (Freelander ?), is this one in the RAV doing exactly the same function, ie only giving drive to the rear wheels when the fronts start to slip.

I understood that the RAV 4.2 had 50/50 front and rear all the time.

Phil

Well it looks like we have finally gone full circle, you have both raised exactly the same question as I started this topic with

Phil

[mnl]

Looking at the diagram 2004 4WD System, the viscous coupling goes between the front and rear axles, effectively short circuiting the centre (3rd differential) In use, if the car is running on tarmac, front and rear axle speeds are similar, and the viscous allows for a slight speed difference by slipping.

If the rear wheels are off the floor, or one is on ice, with an open centre diff (i.e. no diff lock) , all the drive goes to the axle with least grip, and in this case, the front axle stays still, and the rear spins.

This heats up the oil in the viscous unit, and it forms a solid lock between front and rear axles (short circuiting the differential) and all the torque goes to the front axle. (the rear runs at the same speed, but gets no torque, as it doesn't take any torque to make the wheels spin)

After a little while, the fluid cools down, and allows the 2 axles to rotate at different speeds again.

You may wonder why you need a differential in the centre anyway, as the viscous unit allows a slight speed difference, and forms a positive lock when a wheel spins, but so did Landrover, When they built the Freelander, and we know what happens to them. :!Removed!:

[anchorman]

The VC isn't temp sensitive like a fan hub unit. It is a series of plates that are seperated by sticky fluid. Normally it will turn together but if one part tries to slip the plates will bind up with the drag from the fluid. I can see why there is an option on a RAV and that is to stop the lighter rear axle stealing the drive from the front wheels. This would be more pronounced with a heavier engine/transmission.

[jedi134]

Looking at the diagram 2004 4WD System, the viscous coupling goes between the front and rear axles, effectively short circuiting the centre (3rd differential) In use, if the car is running on tarmac, front and rear axle speeds are similar, and the viscous allows for a slight speed difference by slipping.

If the rear wheels are off the floor, or one is on ice, with an open centre diff (i.e. no diff lock) , all the drive goes to the axle with least grip, and in this case, the front axle stays still, and the rear spins.

This heats up the oil in the viscous unit, and it forms a solid lock between front and rear axles (short circuiting the differential) and all the torque goes to the front axle. (the rear runs at the same speed, but gets no torque, as it doesn't take any torque to make the wheels spin)

After a little while, the fluid cools down, and allows the 2 axles to rotate at different speeds again.

You may wonder why you need a differential in the centre anyway, as the viscous unit allows a slight speed difference, and forms a positive lock when a wheel spins, but so did Landrover, When they built the Freelander, and we know what happens to them. :!Removed!:

But surely thats the whole purpose of a center diff............ie to do away with VC`s? so why its in there I have no idea?

[mnl]

But surely thats the whole purpose of a center diff............ie to do away with VC`s? so why its in there I have no idea?

Agreed. In theory, you simply join front and rear axles with a VC, and it copes with the slight speed differences with some slippage, whilst locking up solid when one axle loses traction.

However, as LandRover found out, if the vc is actually transmitting torque all the time* it is slipping, it overheats, and the special liquid** goes solid permanently, seizing the coupling. This then causes the tyre chirping, heavy steering and CV failures.

The torque is transmitted through the gears of the centre diff, and the vc provides a degree of control over the maximum speed difference between the front and rear axles.

If you run different tyre sizes front and rear, or new ones on one axle only, the speed difference produced heats up the vc, and causes premature failure.

(*power = torque x angular velocity, in this case the velocity difference x torque is the power that has to be dissipated in the vc)

(** As Anchorman correctly pointed out, the VC is filled with one of those liquids like custard, that stay liquid with a slow movement, but go solid with high pressures- ever see someone run across a swimming pool full of custard, but sink when he stood still?)

[jedi134]

I understand how a VC works I`ve got a Freelander

But why if the Rav has a center diff is a VC necessary? I thought the center diff does away with VC`s? as the Center diff does all the magical stuff?

A Landrover with a proper center diff (like a Disco) does not have a VC as well........or does it?

[mnl]

I understand how a VC works I`ve got a Freelander

But why if the Rav has a center diff is a VC necessary? I thought the center diff does away with VC`s? as the Center diff does all the magical stuff?

A Landrover with a proper center diff (like a Disco) does not have a VC as well........or does it?

A VC provides a degree of control over the centre diff.

The more agricultural Landrovers have a mechanical diff lock, that jams the centre differential solid. This takes a modicum of skill and mechanical sympathy to operate without breaking something, but doesn't overheat when used extensively in muddy offroad conditions.

The more road oriented products have a vc instead of the difflock, but still have a centre diff, so they are easier to drive and it helps prevent wheelspin on wet roads etc.

BTW the 4.1 and 4.2 RAV do have a "proper" centre diff, it is just implemented in an unusual way being attached to the front differential unit, confusing to those of us familiar with the transfer boxes fitted to LandRover, suzuki Jeep etc.

[pgp001]

mnl has already described the function of the VC perfectly. Its use in the RAV is to act ONLY as an automatic diff lock on the centre diff, the coupling is housed in the transfer case but is connected to the centre diff in the main casing by two tubular shafts. Without the facility to lock the centre diff in some way, you would soon lose traction even if only one wheel was spinning on ice or something, locking the centre diff when wheel spin is detected makes sure you still get drive to the other axle.

Remember the 4.1 had a diff lock button that if not used correctly could damage the transmission, this system was designed to overcome that problem by taking the decision away from the driver, but with the advent of modern electronic systems as on the 4.3 it too soon became outdated.

The Freelander uses the VC in an entirely different way, it is fitted between the two halves of the rear axle prop shaft and only serves to lock the prop shaft and provide rear axle drive once the front axle loses traction.

The Freelander does not have a centre diff, and "when" (not "if" apparently) the VC seizes up, then you get all the problems of transmission wind up.

For a while I too thought the RAV 4.2 VC was just in the drive train to the rear axle, but on closer inspection of the sectioned view you can see the two tubular shafts surrounding the front offside drive shaft.

You will also note that one of the parts shown on the transfer box exploded view is called "Centre Differential Lock Sleeve"

and that is exactly what it is.

I think we have all learned a bit from this discussion, and we can now put it to bed. I am happy that I understand how it works now.

Phil

[kipperwood]

So I was right then Ha ha. Very interesting and whats the next lesson? Can't wait. Kipperwood.