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A transfer case is a device that splits a vehicle's engine power and directs it to the front and rear drive axles of a four-wheel drive vehicle. It is mounted behind the transmission and both front and rear drive shafts connect to it. It contains either gears or a chain drive system in which the power is distributed from the transmission to the axles. The transfer case will typically have the ability to shift between two-wheel drive, four-wheel drive high range, four-wheel drive low range and neutral.
When operating in two-wheel drive mode, the transfer case directs power to the rear axle of the vehicle. The internal components of the case are not being turned under power from the engine. The gears inside thecase are being turned and lubricated by the turning tires and wheels of the vehicle only as it is being driven. In the neutral position, the transfer case prevents the vehicle's transmission from being connected to the drive line while the vehicle is being towed.
In common highway or off-road use, the four-wheel high option allows the front wheels to be driven at the same normal highway speeds as the rear wheels. This provides added traction for increased mobility and control in poor traction conditions while still maintaining adequate speed. The vehicle is able to operate at normal speeds while in this position, but reduced speed is always advised when in the four-wheel drive mode. This option is especially useful in snowy climates where roads can become slick and snow-covered, creating dangerous driving conditions.
Four-wheel low is best used when stuck or in especially poor traction conditions. When in the four-wheel low position, the transfer case causes the vehicle to be operated at extremely reduced speed by driving the wheels at an increased gear ratio. This low range allows the engine to operate at increased revolutions per minute (RPMs) providing substantial power to the tires. The tires are driven at a much reduced speed, thus providing superior traction without spinning and breaking loose.
The transfer case provides the means to drive both axles simultaneously or one axle by itself. This offers the utmost in safety and control to the driver of the vehicle. In most cases, the typical daily driven vehicle will have no cause to utilize this feature under normal driving conditions. It is recommended that the system be engaged and driven at least 25 miles (40.23 kilometers) at least once every six months to lubricate the components and keep them in good working order.
In low-low, the torque delivered to the wheels makes it easy to spin them/break them loose.
4WD has nothing to do with improved safety while driving snow-covered and other slippery roads. Just the opposite. It's when you're stuck that 4WD helps.