AN electronic differential lock, standard on Audi A4 quattro models, uses the ABS sensors to determine whether there is any spin on the wheels under acceleration. If necessary, brakes are applied to the affected wheel up to 80km/h to ensure traction on slippery surfaces. The A4 quattro's all-wheel drive system also uses Anti-Slip Regulation (ASR) that reduces engine power to prevent the spinning of individual wheels. With ASR, almost the best possible power transmission to the road is achieved, whatever the road surface.