(dependent on transmission type), and the rear axle (parts code prefix: 017, identification codes: AAT, AET) uses a driver-selectable electro-pneumatic-mechanical switchable  diff lock . Audi's  quattro  four-wheel-drive system was standard on the C4 S4/S6; a first-generation  Torsen -based system which used the "T-1" automatically biasing centre differential (ATB) was used on