The essential character of the Subaru is in its unique symmetrical All Wheel Drive (AWD) system. With its flat boxer engine set back over the front wheels, Subaru have achieved a 50-50 weight distribution, the same balance as a Formula 1 race car. Combine that with power being distributed to all four wheels, and the ability to vary torque output from front to rear and back to compensate for wheel spin, and you have just about the most poised and satisfying production car on the road, below $100,000.
Literally at the center of the AWD system is the viscous coupling, a compact device fitted to the rear of the gearbox in all manual AWD Subarus. A viscous coupling in the simplest terms is a device for transferring torque from a spinning transmission shaft to the front and rear differentials of your Subaru. It serves also to vary the torque between front and rear wheels, depending on driving conditions. Basically the viscous coupling is a sealed steel cylinder containing a number of slotted disks immersed in highly viscous liquid silicon. Power is transferred from the input shaft via the silicon fluid and disks to the output shafts front and rear. When front or rear wheels spin under power, say, on a wet road, the corresponding output shaft will spin faster, generating more heat at its end, which causes the silicon to solidify and prevent torque from being transferred to the spinning wheels. By default, power is redirected proportionately to the other two drive wheels. The silicon is very sensitive to temperature and reliquefies almost instantly once the temperature drops. The system works so well that the driver is usually unaware of it.
Given the power produced by turbo charged engines in Subarus like the WRX, STI, and Liberty GT, it’s clear that this relatively small component has to work very hard, and it seems unsurprising that there have been a high number of viscous failures over the years, sometimes at quite low kilometres. It’s easy to conclude that the viscous coupling must be a weak link in the AWD chain. But that would be a wrong conclusion. Power is not the culprit. In fact there are more failures of viscous couplings in non-turbo models and they are caused by surprisingly minor things, like inconsistencies in tyre sizes and characteristics, extreme off road use, incorrect drive line ratios and poor repair methods.
Imagine for a moment that you fitted 12” wheels to the front of your Subaru and 28” wheels to the rear. It’s easy to see that the smaller front wheels would rotate much faster that the large rear wheels, with corresponding speed differences in the shafts leading into the viscous. The viscous coupling would of course get hot at the fast-spinning front shaft, and remain cool at the rear shaft. This extreme example would see your AWD Subaru turned into a rear drive only Subaru. It would also mean the viscous coupling constantly overheating, which would lead to a collapse in the viscosity of the silicon and failure of the viscous coupling. In real life of course you wouldn’t have such a large variation in wheel sizes--but, significant variations in front and rear tyre pressures, or different tyre brands and diameters, or significant wear differences front to rear, could all lead to the viscous running too hot and to premature failure. Even running the compact spare wheel too long can cause damage. If your Subaru is properly serviced and maintained, and you run the same size and brand of tyres front and rear, keep the pressures even, and rotate the wheels at the correct intervals, the viscous coupling should last for many years without giving any trouble.
Another problem associated with the viscous coupling is the difficulty of accurate diagnosis. A faulty viscous coupling might produce a range of symptoms, such as shuddering in turns, or what sounds like bearing noise, and these are often misinterpreted, even by experienced mechanics. A customer might pay for replacement of items such as the tail shaft, power steering pump, steering rack, one or more wheel bearings, the rear differential assembly and even the gearbox assembly, without fixing the initial problem. The viscous coupling is often the last component to fall under suspicion.
Owners and repairers sometimes think they might fix a problem with a binding viscous coupling by changing the gearbox oil, or putting in additives. Unfortunately, while the viscous coupling does run in the gearbox oil, the problem is either with the liquid silicon or the case hardening inside the coupling shell. These are entirely separate from the gearbox oil.
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These are the longest-serving and most reliable viscous coupling Subaru have produced. Properly maintained these older 4 KG units last a very long time, and they can be disassembled and serviced. Moving parts and bearings can be replaced, but not the discs or the viscous fluid itself. These units are easily recognised by their dark cast iron finish. They were also available in optional competition variants (8 KG or 20KG) designed to handle more aggressive driving styles. These are now obsolete, although the standard 4 KG units are still available new and used.
Found in gearbox number TY752 from 1991 to 1998.
These units were only available to the RA or Spec C models in the Japanese market. These were just as robust and reliable as the early 4 Kg viscous coupling but with the addition of DCCD. As such they have been very popular in the motor sport rally scene, with owners looking to upgrade their viscous coupling and they are hard to source at present. They are obsolete and Subaru are no longer producing them, which has increased the demand for existing stock even more among motoring enthusiasts.
These are found in gearbox number TY754 and TY752 from 1996 to 1998.
Given the complexities it is wise to seek expert advice before buying. Call ADS anytime to discuss your viscous coupling requirements.
This model has been fitted to all 5MT models since 1999 in single and dual range. It is not as robust and reliable as the older 4KG viscous coupling, and tends to fail at the 120-140,000 kilometre mark. This model has also had bearing issues. Units are sealed and non-serviceable so must be removed and replaced when they fail. They are less expensive to buy than the older models however and easier and quicker to remove and replace. These were also available in 8, 12 and 20 KG models, which are now discontinued. This model may be distinguished from earlier models by its stainless steel casing.
They are found in gearbox numbers TY754, 755, 757, and 758 from 1999 to current.
This is one of the more recent viscous couplings and is used in the 6 speed cable shift gearbox fitted to the Impreza, Forester Outback diesel, and now the WRX. These have proven to be more reliable and durable than their predecessors, but due to the cable shift gearbox are more difficult to service and to remove and replace. These use a stainless steel casing and are easily recognised by the larger viscous coupling bearing.
These are found in gearbox numbers TY756 and between the years of 2010 to current.
This is the DCCD viscous coupling, fitted only to the 2005-current WRX STI. The system uses two center differentials, one computer controlled and the other a mechanical unit that reacts faster. The driver is able to switch bias between the two differentials to fine-tune the driving experience. This unit has been highly successful in motorsport applications and very popular among enthusiasts. It is however very expensive, costing from $4000 to $6000 depending on model. This model is recognisable by the two-part housing and the external wiring harness
Found in gearbox number TY856, from 2005 to current.
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