Radiator Cooling Fan Motor: Description and Operation

300D, E & TE & 1989 300CE

The fan has two operating modes. Low speed, which is activated when the refrigerant pressure reaches 20.4 square inches (20 bar). High speed is activated by a temperature switch, once coolant temperature reaches 221°F a relay than the switch is activated.

300SEL 1989-91 300SE

The fan is controlled by the engine coolant temperature sensor. This sensor provides a signal to control fan speed and A/C compressor shutoff. The fan will switch on to high speed if engine coolant temperatures are at or above 225°F turn off if temperatures are 212°F or below. If ambient temperature of 171°F the high speed fan will engage only if there is an open or short circuit in the coolant temperature sensor or the sensor is disconnected.

300SL & 1990-93 300CE

The visco fan clutch is temperature controlled by an internal and external bimetallic strip. Fan speed is dependent on engine RPM, but fan speed will be lower than 1000 RPM. Engagement takes place from about 190-205°F ambient temperature at the outer bimetal strip. The fan safety disconnect occurs above 4000-4500 RPM engine speed. The fan is then engaged when engine speed drops below 4500 RPM.

1992-93 300SE

The visco fan clutch is engaged 166-244°F ambient temperature at the bimetallic strip. The fan safety disconnect occurs at above 4400 RPM engine speed. The fan is then engaged when engine speed drops below 4500 RPM.