Controlling the differential clutch
The control module controls torque transfer to the rear wheels by using the pressure control valve to control the hydraulic pressure to the differential clutch.
The amount of torque to be transferred to the rear wheels depends on a number of different factors. The control module takes care of all the calculations required for all-wheel drive and retrieves the information it needs on the bus. Other systems such as ESP may, under certain conditions, request that the differential clutch is opened.
When driving off, the differential clutch is applied to provide optimum acceleration for the prevailing road conditions. As vehicle speed increases the torque distribution to the rear wheels decreases to around 5-10% for motorway driving at constant speed.
During acceleration from a constant speed the differential clutch will be applied further, i.e. immediately transfer a greater proportion of torque to the rear wheels. This takes place very quickly as clutch reaction time is around 80 ms, which is a considerably shorter time than it takes for the engine to build up the torque. Consequently the clutch is ready to handle the increased torque from the engine before the engine delivers the increased torque.