Abstract: There is disclosed a detachable lock mechanism which can be installed on automotive-type cooling fans having a conventional fan clutch connection between the fan and the fan drive shaft. The lock mechanism functions by providing a positive mechanical lock between the drive shaft and the cooling fan, thereby bypassing the drive clutch and its function. The locking mechanism provides a desirable positive lock to insure maximum fan speed when operating a vehicle in low gears and low forward speeds. The device is of particular value for off-road service of recreational vehicles and the like which are often operated for extended periods in low gears, conducive to overheating. In these applications, the operation of the fan clutch is detrimental to adequate engine cooling and a mechanical locking mechanism thereby serves a useful function in avoiding overheating of these vehicles.