The system consists of a silicone filled coupling which connects fan to pulley. Fan drive is operated by a control valve which is governed by temperature sensitive bi-metallic coil or strip. The control valve maintains flow of silicone through drive.
When cooling air temperature is low, fan drive limits fan speed. High air temperature causes bi-metallic coil or strip to allow greater flow of silicone to enter drive. This increases drag between driven member and driving member, resulting in higher fan speed and increased cooling.