Abstract: A fan clutch lock for temporarily forcing the cooling fan associated with an internal combustion engine to turn without slippage, comprising a shaft member adapted to be driven in rotation by the engine. Mounting means are provided for mounting a fan to be turned by the shaft, with the mounting means involving a clutch arranged normally to permit a controlled amount of fan slippage as generally regulated by the engine operating temperature, with an increased amount of fan slippage ordinarily occurring at higher engine operating speeds. A novel locking means provided in accordance with this invention enables the fan clutch to be selectively circumvented such that upon operation of said locking means, the fan is required to rotate at the same speed as said shaft member at all engine operating speeds, thus enhancing the cooling capacity of the fan. This additional cooling is particularly desirable when the engine is heavily loaded.