4WD coupling

The rear end of the propeller shaft is equipped with a drive flange attached to the Hardy coupling with 3 bolts and nuts. The Hardy coupling, in turn, is bolted to a drive flange on the all-wheel drive clutch. The all-wheel drive clutch and the rear final-drive unit are directly connected.

The all-wheel drive clutch serves as the actuator that continuously controls the amount of torque being transmitted to the rear axle. The four-wheel drive clutch serves the same function as the clutch used with a manual transmission by providing positive engagement for transmitting power: between the propeller shaft and the rear final-drive unit's input shaft.

The all-wheel drive clutch is controlled by the DSC control unit. The link between the four-wheel drive clutch and the DSC is provided by a 2-pin plug connection.

A twisted-strand wire with shielding is used for the connection to the all-wheel drive clutch. This configuration is necessary for electromagnetic compatibility (EMC) because the all-wheel drive clutch is controlled with a clocked (pulse-width modulated) signal.

Fig 1: Identifying 4WD Coupling, Ventilation Connection And Final Drive

Courtesy of BMW OF NORTH AMERICA, INC.

The all-wheel drive clutch housing is air-tight and sealed against liquids. To provide the necessary pressure compensation when substantial changes in temperature occur, a vent line is provided; this leads to a raised installation location where no sealing against water is required (luggage compartment).