Information compiled from various source across the interweb. Special thanks go to Garrycol of Aulro.

Firstly an overview of what a VCU (Viscous Coupling Unit) is - and what it does...



A viscous coupling is a mechanical device which transfers torque and rotation by the medium of a viscous fluid. It is made of a number of circular plates with tabs or perforations, fitted very close to each other in a sealed drum. Alternate plates are connected to a driving shaft at one end of the assembly and a driven shaft at the other end. The drum is filled with a dilatant fluid, often silicone-based. When the two sets of plates are rotating in unison, the fluid stays cool and remains liquid. When the plates start rotating at different speeds, the shear effect of the tabs or perforations on the fluid will cause it to heat and become nearly solid because the viscosity of dilatant fluids rapidly increases with shear. The fluid in this state will effectively glue the plates together and transmit power from one set of plates to the other. The size of the tabs or perforations, the number of plates, and the fluid used will determine the strength and onset of this mechanical transfer.



The Effect of Heat on the VCU
As the chart below depicts, at 110 degrees centigrade, the VCU silicon fluid viscosity increases dramatically, thus engaging the VCU. Since temperature is a cause and not just an effect of VC engagement, it seems probable that the "engagement temperature" of a "cooked" VCU (a VCU where the fluid has been ruined by prolonged overheating) is lower than for a non-cooked VCU, and that with cooked VC fluid, the VC engages at the higher end of normal VCU operating temperatures, with the result being that the VC is always engaged at the higher normal operating temperatures even if the wheels are turning within the 6% threshold for engagement. This puts incredible strain on the entire drive train.



Below are also graphs showing Torque against Temperature.



Dont forget that the VCU can fail in 2 ways....
1) locked solid (causes drive train failure).
2) no locking capability (doesnt lock front and rear together - only 2WD) [ commonly called "Mondo" mode]

YouTube- FREELANDER cheсk VCU.wmv

First - take one Freelander


Chock the wheels, particularly if you are on a slope like this car is


Put the car in first gear and release the handbrake - and be careful (the handbrake is on the rear wheels and the handbrake has to be released to do the wheel test)

Jack up the drivers rear wheel until it is clear of the ground


Remove the hub cap


Then put a 1 1/4" or 32mm socket with a breaker bar onto the hub nut. Note the position of the breaker bar against the side of the car - marked with white tape.


Turn the wheel clockwise to take up the slack in the drive train - see breaker bar has moved to second white tape position.


Then apply a steady clockwise pressure to the wheel via the breaker bar - the wheel should slowly turn - if it does not then the VC is seized




In this case the VC is OK.

See this clip also

http://s42.photobucket.com/albums/e334/gazzz21/?action=view&current=VCUtest.flv

If you want to do the mark on the tail shaft test - here is where the marks go. Red arrow is the VCU - yellow marks are your aligned marks


Hope all this helps

Garry

good thread, should answer everyones how do i check my vcu questions, now we need a thread on how to do a search of the forum aswell so new members can find it

It should, however, be mentioned that these tests are not universally accepted.

Cribbed from a VCU refurbishing site.......

"If your Freelander feels "tight" or it appears the brakes are binding, especially when reversing on full lock, this means your existing VCU is past its servicable life and you should change it as soon as possible. A damaged VCU put massive strain on the rest of your transmission line, causing the IRD unit, rear diff and gearbox to wear at a hugely accelerated rate, and will cause a very costly failure of these parts. A VCU's servicable life span is usually no more than 70k miles, but it can be a lot less (we have seen cars with failures at 30k miles) depending on how the car has been driven.

It is a common fallacy that VCUs seize. In all our years of experience we have never come across a seized VCU. What actually happens is the silicon viscous fluid gets thicker and thicker with wear and slowly causes the viscous coupling to become stiffer and stiffer to rotate. More and more strain is therefore put on the gear train and failure eventually occurs to the IRD and rear diff.

It has been reported on some internet sites that to test the VCU if you jack the one back wheel of your Freelander without the handbrake applied you should be able to turn the rear wheel and that if you cannot turn the wheel your VCU has seized -THIS IS TOTAL RUBBISH! You would hardly be able to do this by hand as you would need a 2 foot breaker bar and stand on it, the wheel will move very slowly - this still does not indicate whether the VCU is any good or not as all wheels will turn regardless of the condition of the VCU.

Another common fallacy for testing VCUs is the tipex test whereby you put a tipex mark on the shaft next to the front prop shaft and one opposite on the viscous coupling and if these marks have moved out of line after being driven round then the VCU is OK"


Now - you might be cynical and think they have a vested interest in rubbishing VCU tests and in selling new VCU's - I couldnt possibly comment.

It is likely though, that a VCU will not suddenly fail in a sudden and obvious way. It is more likely to be a progressive failure, showing itself in a "stiffening" of the VCU or a lack of locking capability - ie greater slippage.

Others have tried making the "torque wrench" test more clinical, by measuring the torque required to turn the rear wheel. This does, however have built-in problems, ie the torque required in moving the transmission components up to the VCU unit. For example if the rear diff was tight, then this could show up as a stiff VCU.
More information is required to establish whether this is a valid test, particularly when the VCU operates by means of the viscous fluid getting hot and locking up.
One think to be born in mind though - if the VCU fails in its non locking way, it will not damage the other drive train. If it locks up (or becomes significantly stiffer) then it WILL put extra strain on the IRD, the rear diff and the VCU bearings, such that one or all will fail. These tests may not be ideal, but there is no doubt that they WILL identify a locked VCU. It might just save you an IRD.

Other points to bear in mind are that If a VCU has failed it is highly likely that it will have damaged the taper roller thrust bearing in the IRD unit. There will be no obvious sign or noise and IRD failure usually occurs when reversing the vehicle upon which the thrust bearing collapses and total failure occurs normally breaking the teeth off the crown wheel and pinion which causes the banging and grinding noise.

Another cause of IRD failure is due to bad wear in the inner constant velocity joint on the offside drive shaft. This usually results in the outer taper roller bearing (next to the CV joint) collapsing and causing the top gear shaft to break teeth, which in turn usually cracks or smashes the IRD main casing and end casing with subsequent loss of all oil.

IRD and/or VCU failure is also caused by miss matched tyre sizes between the front and rear axles. The difference in tyre diameter, and thus the rolling radius of the wheel causes a constant difference in the rotational speed of the front and rear prop shafts, this difference has to be compensated for by the VCU slipping at a higher rate than which it was designed for. The extra rotations cause the VCU to heat up and become stiffer ,which is what they are designed to do off road if the wheels start to slip and this transfer drive to the the other wheels, but as this is happening while driving in a straight line on a road it has the same effect has an old stiff VCU and transfers extra load to the entire transmission line, eventually resulting in IRD failure. This constant heating also cooks the VCU so that it does not operate correctly, even when correct tyres are fitted. You can measure the diameter the tyres, but this can be slightly misleading, especially on worn tyres as the tyre may not be evenly worn from the inside to outside edge, so the best way to check is to check the external temperature of the VCU after driving it for a few miles on a relatively straight road, it should be virtually cold. If its slightly warm or hotter then the wheels have a ratio problem. A difference of just 5mm in diameter dramatically increases the rotational differences between the front and rear axles, so it is imperative that the tyres are always matched.

Landrover recommend always putting new tyres on the rear, however others recommend changing all four tyres at the same time, to minimise differences in rolling road diameters.

Other items which could affect prop shaft vibration and noise are....

1) VCU Damper
[ame="http://www.youtube.com/watch?v=VPmdsrmOnuc"]YouTube- Freelander Loose Bit[/ame]

These dampers are easily available on E-Bay.

Here is a more "technical" approach if you have actually removed the VCU to test it....
[nomedia="http://www.youtube.com/watch?v=0jkvamLKorc"]YouTube- Freelander VCU test[/nomedia]

Who says MHM is no longer interested in hippos


There are no torque ranges for a working VCU for any of the above tests, so tippex is as good as any

Neither do they give the speed of rotation,

Northern Irelander said:

Who says MHM is no longer interested in hippos

Click to expand...

who sed that?

hello new here can anyone answer my qustion please ,i want to changer my freelander 18 petrol to 2 weel drive i have bought a blanking plate so that i can remove the shaft ,i do know how to do all this ,but my question is what happens at mot will my car still pass iff i change it to 2 wheel drive and whats best remove or leave the main drive shaft on car hope someone can help ronnie

please read http://www.landyzone.co.uk/lz/f38/please-read-before-posting-section-75701.html and then post in the freelander section. remove prop - MOT is OK. Insurance may not be.

see also ..... VCU Torque test results - Land Rover Zone

The Mad Hat Man said:

It should, however, be mentioned that these tests are not universally accepted.

Cribbed from a VCU refurbishing site.......

Now - you might be cynical and think they have a vested interest in rubbishing VCU tests and in selling new VCU's - I couldnt possibly comment.

Click to expand...

I'm sorry, but we are correct, we have never seen a VCU seize solid, but have seen 100's of smashed up transfer boxes and diffs through tight VCU's. If a VCU was locked solid the IRD and or rear diff would last a few miles at best. Your test really doesn't prove anything. My video you have linked to shows how a VCU will behave and can be replicated and repeated on every VCU. I'm not getting into an argument about this, but you really shouldn't be telling people that if you can perform your test the VCU is OK. IRD prices are now through the roof as the crown wheel and pinions now being supplied are silly money so keeping the VCU is good condition is so much more important.

Bring me a seized solid VCU and i will eat my words that they do not seize, but i can catorgorically tell you that just because a VCU slips, it doesn't mean that it is functioning correctly and will not casue your IRD to fail

editted to say:
Other than that this is an excellent thread with lots of really good information and should be read by every hippos owner

I did not say that you were wrong - just that the cynically minded persan MAY think that you have a vested interest in what you published. Please bear in mind that until there is a definitive, easily repeatable test that an owner can carry out, there will always be variation in what people believe.

If you have such a test then please elucidate.

The Mad Hat Man said:

.

If you have such a test then please elucidate.

Click to expand...

Copy what i did in the video, its the and most repeatable and and proved method, although I know its a PITA to make something to do it.

The problem with that, if my memory serves me right, is that it necessitates removing the VCU. What is wanted by owners is something that they can do, with the car fully assembled, that gives them a qualified indication as to whether the VCU is good, bad or in need of further investigation. Maybe something that can be done every month or 1000 miles, rather than a transmission strip down job.

May I also ask you, Austin, as you are in a pretty unique situation to possibly contribute with figures to this thread....

http://www.landyzone.co.uk/lz/f9/vcu-torque-test-results-109486.html

Hippo has recently conducted a revised version of the above torque test which seems to be equivalent of a steady 9.2Nm Torque applied.........

In his own words...
"Freelander 1 2001 v6 viscous coupling unit VCU torque test by jacking up 1 wheel and putting a 1.2meter bar with 8kg of weight on the end. If the wheel turns, then your VCU is not seized. If it doesn't turn, then your VCU is seized and should be removed immediately to reduce the stress in the transmission. Failure to do so will cause either the intermediate reduction drive IRD and/or the rear differential to fail. My VCU had done 26k miles when filmed.

[nomedia]http://www.youtube.com/watch?v=Oh01nQgh7z8[/nomedia]

I did the above test today with a bar and 8kg weight and the wheel turned nice and steady, I carried out this test as when I tried the tippex test and took the FL for a drive for about 4 miles the marks were still lined up which got me a little bit concerned.

do you have a time for a 90 degree movement?

No as I didn't read all the threads until after I had done the test so didn't realise what feedback was needed but I would say my results would be very close to Hippos at least within a few seconds, my FL has 71k but I have no idea if the VCU is original or not.