This photograph shows a disassembled viscous coupling that has been overheated. The clear sign is the almost solid black oily mess which can be seen in the housing, the end plate and on the laminate plates pack.

With a normal Viscous coupling (in hot condition) a maximum of 68 Nm to 90 Nm are transferred to the rear wheels, as measured at the propshaft. It is NOT correct to assume that because the output power from the engine has been increased, there will be increased power to the rear wheels. The emphasis is on the term maximum, it will always be the same regardless of the power output from the engine.

The opposite is true in that if the engine power has been increased, the viscous coupling will have more work to do in order to try and deliver the maximum torque to the rear wheels. If, because of the power, the coupling is driving at its maximum and then some, the additional power can only be dissipated as heat. If the coupling gets to hot then the oils will solidify resulting in additional strain on the gearing of the transfer box and its eventual demise.

The transfer box has a safety devise, which should always be fitted and functional. If the transfer box oil temperature gets to around 140 -160 degree's centigrade the ECU will sense it and shut the box down to 2WD. Earlier models (pre-win airbags, circa 1993) a one shot temperature sensor will have the same effect. The sensor then has to be replaced. (£62)

The normal operating temperature of the transfer box is around 50-70 degrees centigrade but will depend on how the car is being driven.