QUESTION: I’ve been told that the viscous coupling on my 1999 Freelander 1.8 petrol is bound to fail at some point and that it’ll cost a fortune. Can you tell me what this part does? What are the warning signs when it starts to go wrong?
ANSWER: The Freelander has an intermediate reduction drive (IRD) attached to the gearbox. This supplies drive to the front and rear axles. This rear axle drive goes through a propshaft to the viscous coupling unit (VCU), then through a second in-line propshaft to the rear axle differential. The VCU is midway between the rear propshafts.
In road driving, the front wheels do most of the work and the rear wheels more or less trail behind, with the VCU providing four-wheel drive if needed. In the VCU are two sets of vanes, one linked to the rear propshaft, the other to the forward propshaft. The vanes are immersed in a viscous fluid that stiffens if it is agitated. In normal driving, front wheels rotate at a similar speed to the rears, so the alternate vanes of the VCU are also rotating at similar speeds. If the front wheels lose grip they spin faster, while the rears stay at the same speed. This causes the vanes connected to the front propshaft to spin faster than those connected to the rear shaft, so the fluid inside gets churned up and becomes stiff. That produces a solid drive through the VCU so the front propshaft begins to turn the rear propshaft at the faster speed, thus transmitting drive to the rear wheels. The rear wheels now push the car until the fronts find grip. When grip is regained, all the wheels are rotating at the same speed, so the VCU frees up and you’re back in front-wheel drive.
When using full steering lock at slow speed, you may need more accelerator to overcome the tightening of the VCU. This is normal, but avoiding full lock or keeping the speed as low as possible during such manoeuvres helps prolong the life of the VCU.
The VCU can fail in two ways. It’s rare for a VCU to fail free, (ie, incapable of putting drive to the rear wheels). If this happens, there will be no indication during normal driving, but if the front wheels lose grip you’ll become stuck because drive won’t be transmitted to the rear wheels. Apart from becoming stuck, this condition will not cause further damage to the vehicle.
The most common failure mode is seizure. This means you are in four-wheel drive all the time, and the VCU’s centre differential effect is lost. This shows up as unusual tyre wear, causing the edge blocks of the tyre tread to chamfer or for alternate blocks to wear down. It will also make the steering feel unresponsive or jerky, and the steering’s natural tendency to self-centre will be reduced.
If the VCU seizes, it puts strain on the rear differential and the front IRD, especially the IRD’s pinion and output bearing. It also puts extra load on the VCU support bearings. If you detect a seized VCU, you should have it checked and replaced quickly. Noise from the area of the VCU is likely to be worn support bearings.
Because the IRD exerts a slight gearing effect between front and rear drive, incorrect tyre sizes can alter this to produce the symptoms of a seized VCU, and to load the transmission in a similar way. The critical dimension is the rolling radius of the tyres, measured from the wheel centre to the ground, with the vehicle’s weight on correctly inflated tyres. Because actual sizes vary between tyre manufacturers, it’s worth ensuring the same make of tyre is fitted to front and rear. If differing makes are fitted, or just two tyres need to be renewed, ensure the tyres with the greatest rolling radius are fitted to the rear. This makes life easier for the VCU and the whole transmission. Ed Evans
The above content originally appeared in LRO magazine and is reproduced here with their kind permission. Any advice or opinions are those of LRO magazine and its writers.