After replacing my radiator which I was told had developed a mysterious weld failure ($1200) the all wheel drive system was engaging and disengaging on dry pavement. Inspection determined failure of the transmission. It was stated by the dealership that only a few droplets of radiator coolant accidently mixing with transmission fluid would cause this failure. The fact that both these lines had been handled when they had to be removed from the failed radiator was chalked up to a "coincidence" by the service people at the Mercedes dealership. Even though car was out of warranty in years, Mercedes agreed to pay all but $500 to replace the transmission. Of course this was only one of many problems to beset a vehicle with relatively low mileage.