Driving and operating 149Drive systemsAll-wheel driveThe All-wheel drive system enhancesdriving characteristics and stability,and helps to achieve the bestpossible driveability regardless ofground surface. The system is alwaysactive and cannot be deactivated.The torque is distributed steplesslybetween the wheels of the front andrear axle, depending on the drivingconditions. Additionally the torquebetween the rear wheels is distributeddepending on the surface.For optimum system performance,the vehicle's tyres should not havevarying degrees of wear.If a service message is displayed inthe Driver Information Centre, thesystem may have limited functionality(or be completely disabled in somecases, i.e. the vehicle switches toFront-wheel drive). Seek theassistance of a workshop.Towing the vehicle 3 230.BrakesThe brake system comprises twoindependent brake circuits.If a brake circuit fails, the vehicle canstill be braked using the other brakecircuit. However, braking effect isachieved only when the brake pedalis depressed firmly. Considerablymore force is needed for this. Thebraking distance is extended. Seekthe assistance of a workshop beforecontinuing your journey.When the engine is not running, thesupport of the brake servo unitdisappears once the brake pedal hasbeen depressed once or twice.Braking effect is not reduced, butbraking requires significantly greaterforce. It is especially important to bearthis in mind when being towed.Control indicator R 3 92.Antilock brake systemAntilock brake system (ABS)prevents the wheels from locking.ABS starts to regulate brake pressureas soon as a wheel shows a tendencyto lock. The vehicle remainssteerable, even during hard braking.ABS control is made apparentthrough a pulse in the brake pedaland the noise of the regulationprocess.For optimum braking, keep the brakepedal fully depressed throughout thebraking process, despite the fact thatthe pedal is pulsating. Do not reducethe pressure on the pedal.After starting off, the system performsa self-test which may be audible.Control indicator u 3 93.Adaptive brake lightDuring full braking, all three brakelights flash for the duration of ABScontrol.
