How VCU works.

Hi Gents,

I try to find some video or reed some article's about VCU working system but no that much luck.
Can anybody explain to me in sort how it work's, what should i expect from him, how can i recognize is it stiffen or is ok(can i do that while driving)??

My friends keep telling me that the FL is in 4x4 mode till 60km/h after that the rear diff is not active. Is that make any sense to the VCU? Is the VCU the unit that turn's off the rear diff and the rear prop??
When i drive faster (over 60km/h) is the rear prop rotating?

I will appreciate is you share your knowledge about this and explain it better.

Thanks
Zoki

multiple plates in silicone type fluid- too much speed difference and it locks up

fanatic said:

multiple plates in silicone type fluid- too much speed difference and it locks up

Click to expand...

Hi m8

Thanks for the replay,
What do you mean by: To much speed difference and it locks up??
Can you explain it more light to me...i'm quite new owner of a FL! :crazy:

Thanks

When the speed on the front wheels turn around faster(spinning) than the rear wheels. The liquid in the VCU gets "thicker" and the rear wheels gain motor power.

JRNORWAY said:

When the speed on the front wheels turn around faster(spinning) than the rear wheels. The liquid in the VCU gets "thicker" and the rear wheels gain motor power.

Click to expand...

Hmmm...more clear for me now.
But what hapend's when all tyres are rotating on a constant speed?
Let's say on a highway, or in the city driving.
Till what speed is the FL in a 4x4mode?
Is that true that above 60km/h the rear prop and diff are off-line ad the FL is in 2wd?
Can i make a test somehow while driving???

Thanks to all reply's once again

Zoki

Its only when the front wheels are slipping/spinning the rear wheel gets engaged. When your are driving in 20km/h(or 100km/h) there is no difference between front & rear wheels.

So 2wd until it slips in other words.

JRNORWAY said:

Its only when the front wheels are slipping/spinning the rear wheel gets engaged. When your are driving in 20km/h(or 100km/h) there is no difference between front & rear wheels.

So 2wd until it slips in other words.

Click to expand...

Hmmm...
So how do you explain this:

https://www.youtube.com/watch?v=87uXoN6B_IM

As you can see in this vid. It says normal driving
The car is in low speed, the front's are probably not slipping and on the video you can see the rear prop. rotating (so it tell's you the rear dif. is in action)??

I'm totally confused now

Gent's i like to apologies to you if i'm boudering to you with this silly questions, but i need to now better for the VCU, so i can be sure about my vehicle.

JRNORWAY said:

Its only when the front wheels are slipping/spinning the rear wheel gets engaged. When your are driving in 20km/h(or 100km/h) there is no difference between front & rear wheels.

So 2wd until it slips in other words.

Click to expand...

not entirely true ,front wheels are driven faster than rears ,and vcu allways transmits some torque to the rear just not full torque till vcu stiffens, the whole point is to have 4wd handling with front wheel drive feel

Land Rovers description on how the VCU works, taken from Rave.......
VCU
The VCU automatically controls the transfer of drive to the rear wheels by limiting the speed differential between the
front and rear propeller shafts. The unit is supported in two propeller shaft bearings attached to the floor cross
member.
The VCU comprises a short cylinder which contains an input shaft supported in a roller bearing race at the front and
a ball bearing race at the rear. Within the cylinder, slotted discs are alternately attached to the outer surface of the
input shaft and the inner surface of the cylinder. An output shaft is welded onto the rear of the cylinder. The input shaft
is attached to the front propeller shaft and the output shaft is attached to the rear propeller shaft.
On models with 1.8 K Series engines, a torsional damper is bolted to a flange on the output shaft.
The cylinder is a sealed unit filled with a silicon jelly. The viscosity of the silicon jelly increases when subjected to
shear. When there is a speed differential between the front and rear propeller shafts, adjacent slotted discs in the VCU
rotate in relation to each other. The shearing action of the rotating slotted discs increases the viscosity and resistance
to rotation of the silicon jelly
The rear wheels are 0.8% under driven, so in most conditions the vehicle is effectively front wheel drive, with the rear
wheels turning the rear propeller shaft slightly faster than the IRD drives the front propeller shaft. Since the speed
differential is low, the increase in viscosity of the silicon jelly is marginal and there is little resistance to relative rotation
of the slotted discs.
When there is a significant speed differential between the front and rear propeller shafts , e.g. the front wheels lose
traction or traversing rough terrain, the viscosity and resistance to rotation of the silicon jelly increases to a level that
slows or stops relative rotation of the slotted discs. With the front and rear propeller shafts locked together, drive is
thus transferred from the IRD to the rear wheels.

jamesmartin said:

not entirely true ,front wheels are driven faster than rears ,and vcu allways transmits some torque to the rear just not full torque till vcu stiffens, the whole point is to have 4wd handling with front wheel drive feel

Click to expand...

Hi James,

I share you info on this.
As i heard...it transfers some torque..but not full power.
I had a situation where i was driving at around 80km/h and released the gas pedal in one moment and pressed it back on little harder and i heard that sound like "BUP" in the back of the FL.

Is that telling me the VCU is stiffen or the VCU is not that flexible?

Thanks
Zoki

it may be just stiffening momentarily ,but one wheel up test would show if its permanently too stiff

jamesmartin said:

not entirely true ,front wheels are driven faster than rears ,and vcu allways transmits some torque to the rear just not full torque till vcu stiffens, the whole point is to have 4wd handling with front wheel drive feel

Click to expand...

Yes i know, but thats just a technicality for faster 4x4 response. For every 400(about) turns on the front propshaft the rear propshaft are turning 401 times. I prefer to leave that out when explaining how the VCU works

There is zero power coming from the IRD -> VCU -> rear wheels when in "2WD". The drivetrain is constructed with that ratio trough the rear wheels-> diff -> vcu output shaft.

why dont yu just read the articles in the CFAQ section - or is that too difficult?

hugle said:

Hmmm...
So how do you explain this:

https://www.youtube.com/watch?v=87uXoN6B_IM

As you can see in this vid. It says normal driving
The car is in low speed, the front's are probably not slipping and on the video you can see the rear prop. rotating (so it tell's you the rear dif. is in action)??

I'm totally confused now

Gent's i like to apologies to you if i'm boudering to you with this silly questions, but i need to now better for the VCU, so i can be sure about my vehicle.

Click to expand...

The rear prop will turn because the rear wheels are turing it because they are being dragged along by the front wheels.

You can't tell if it is working until you feel slip from the front wheels. Then you can feel the car being pushed from the back.

I was off road recently and most of the time it was in 2wd. When the front wheels started to slip, I then felt the rear ones gain traction and push me over the obstacle.

Go drive it on a muddy field.

There is an excellent thread in the FAQ as mad hatter says-go read it.

Epicuser said:

The rear prop will turn because the rear wheels are turing it because they are being dragged along by the front wheels.

You can't tell if it is working until you feel slip from the front wheels. Then you can feel the car being pushed from the back.

I was off road recently and most of the time it was in 2wd. When the front wheels started to slip, I then felt the rear ones gain traction and push me over the obstacle.

Go drive it on a muddy field.

There is an excellent thread in the FAQ as mad hatter says-go read it.

Click to expand...

jack one rear wheel up and try and turn it ,and see if you still think there is no drive at all

jamesmartin said:

jack one rear wheel up and try and turn it ,and see if you still think there is no drive at all

Click to expand...

The VCU will engage both ways. So when one off the rear wheels are "slipping" it will transfer the "power" to the front wheels.

jamesmartin said:

jack one rear wheel up and try and turn it ,and see if you still think there is no drive at all

Click to expand...

When both propshafts are turning at virtually the same speed, the VCU is running free and not transmitting drive.
It will only start to lock up when the two props try to turn at different speeds, when the front wheels lose traction, or you jack a back wheel up an try to turn the rear prop and not the front.
Land Rovers description on how it works is clear enough.

longdog said:

When both propshafts are turning at virtually the same speed, the VCU is running free and not transmitting drive.
It will only start to lock up when the two props try to turn at different speeds, when the front wheels lose traction, or you jack a back wheel up an try to turn the rear prop and not the front.
Land Rovers description on how it works is clear enough.

Click to expand...

try it,even a new vc has enough drag to transmit some drive

Good Morning Gent's

Today i try the one wheel up test.
The results were not that good as Hippo says in his video:
5kg=35sec.
Mine was 70sec.
Can this be a problem?
I used a little bit of a flexible pipe...so maybe that way the time for the test is longer.
The reflex from the weight to the wheel was a delayed.

Tomorrow i will try a different pipe.

But the wheel was turning after all even with hand push.
It wasn't completely stiffen.
I even marked the VC (alignment) and notice the marks were in a different place after the test.
When i revers in full lock...the wheels become hard, but with a little more acceleration, it will pass. The same thine when i turn in full lock i forward. In other cases, while turning it goes quite smooth.

Kindly waiting your answers.

:tea:

jamesmartin said:

try it,even a new vc has enough drag to transmit some drive

Click to expand...

Iin normal driving the rear wheels are turning the rear propshaft and rear of the VCU slightly faster than the IRD is turning the front propshaft and front of the VCU, the silicon gel in the VCU remains fluid.
Doing the one wheel up test makes the rear of the VCU turn and not the front, the silcon gel resists the shearing effect by trying to lock the VCU up.
It does not prove what forces are being appled through the VCU during normal driving.
Land Rovers own description of how it works will do for me.