Vehicles with BMS46 have no viscous fan but are only equipped with an electric fan.
The electric fan is activated by means of a power output stage directly on the fan motor. The motor control unit activates this power output stage by means of a square-wave signal with duty factors (variable pulse width) between 10 and 90 % thus controlling the various speeds of the electric fan. Pulse duty factors less than 5 % and greater than 95 % do not trigger activation but rather they are used for fault detection purposes. The power output stage features its own positive and ground supply.
The fan speed is influenced by the coolant temperature at the radiator outlet and the pressure in the air conditioning system. The fan speed is reduced as the vehicle speed increases.
The electric fan is driven at maximum speed in the event of a fault in the coolant temperature sensor at the radiator outlet.
With the engine running, stage I of the electric fan is activated by the engine control unit via a relay. Activation takes place in response to the request by the air conditioning control unit or when the thermostat switch closes (due to the temperature threshold being exceeded at the radiator outlet).
Stage II is activated directly by the thermostat switch on exceeding a higher temperature threshold as from ignition lock position 2.