Wheel size adaptation
In order that correct torque should be transferred to the rear wheels the application of the differential clutch must be compensated for in terms of the existing difference in rolling radius between front and rear wheels.
If the rolling radius of the rear wheels is greater than that of the front wheels then a certain given application provides more torque to the rear wheels than if the rolling radius of the front and rear wheels was the same. Consequently the current to the pressure control valve must be reduced slightly (lower current results in lower hydraulic pressure = less torque to the rear wheels).
By measuring wheel speed the control module can detect whether any wheel has a deviating rolling radius. Adaptation of wheel size takes several minutes, the value is stored in the control module. If wheels have been changed to another rolling radius, e.g. swapped front/rear wheels then the drive torque to the rear wheels will be incorrect before adaptation has taken place. This may be perceived a small jerk/tug when driving off.
If one wheel deviates more than a certain given value then the control module assumes that a compact model spare wheel has been fitted and switches off rear-wheel drive.