an anti - lock braking system is specifically designed for a motor vehicle having four - wheel drive and a device for shifting between two - wheel and four - wheel drive incorporating a lockable central differential . additional differentials at the front and rear axles may also be locked to avoid wheel spin . the anti - lock braking system produces a brake pressure reduction signal when the wheel speed falls below a reference value . in accordance with the invention the reference value is varied in dependence upon whether the vehicle is shifted into two - wheel or four - wheel drive , in dependence upon the engagement of the central differential lock , and / or in dependence upon the engagement of the other differential lock .