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 serviceable life and you should change it as soon as possible. A damaged VCU puts massive strain on the rest of your transmission line, causing the IRD unit, rear differential and gearbox to wear at a hugely accelerated rate, and will cause a very costly failure of all these parts. A VCU's serviceable 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.

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 of 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. We have seen various examples of miss matched tyre sizes such as a garage fitting different size profile tyres to the front of the car compared to the rear, putting new tyres on one end of the car but leaving the old worn tyres on the other, significantly more tyre wear on one axle, low tyre pressures and cars that have done a lot of towing (rolling radius of tyre is less due to the weight of the trailer compressing the tyres). 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. Always change all 4 tyres at the same time, rather than trying to save a few pounds by changing the fronts and getting a few thousand more out of the rears. The variations between makes of tyres with the same profile can be even more than this, so tyre makes should also be matched.

Freelander rear diff failures are not as common as IRD failures, but are still a weak point. Usually the root cause of this is a faulty viscous coupling unit (VCU), which should therefore be changed at the same time as the differential.

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Dispelling Some Myths

There is nothing we havent seen or heard before, so we are happy to correct some myths you may have heard :

• Myth 1 - It is a common fallacy that VCU's 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 differential.

• Myth 2 - It has been reported on some internet sites that it is possible to test the VCU by jacking up one back wheel of your Freelander without the handbrake applied, and that you should be able to turn the rear wheel - with the reasoning that if you cannot turn the wheel, your VCU has seized - this is a complete myth! You would hardly be able to do this by hand as you would need a 2 foot breaker bar and have to stand on it. The wheel will move very slowly, however 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.

• Myth 3 - Another common fallacy for testing VCU's is the 'Tipp-ex test' whereby you place a Tipp-ex mark on the shaft next to the front prop shaft, and one opposite on the viscous coupling, to observe if these marks have moved out of line after being driven round.