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4WD Transfer Case

A transfer case receives power from the transmission passes it on to all four wheels of the vehicle enabling four wheel drive. The case can be independent off the transmission or "married" were it may even share the same case. The most common transfer case will be independent. An independent transfer case bolts on to the output shaft of the transmission and has a driveshaft for the front and rear axle.

They will be either chain or gear driven. Most manufacturers today use a chain driven transfer cases  because they are lighter and quieter than their gear-based counterpart. Gear driven transfer cases have more strength and are found on heavy applications. Transfer cases usually use ATF but always check with the manufacturer before proceeding.

They are controlled manually or electrically. A manual control is typically a shifter found on the floor of the vehicle and an electric control a series of buttons or slides found on or around the instrument panel. A vacuum controlled unit requires specified vacuum from the engine (17-21Hg). Its also important to check all lines and reservoirs for cracks and wear if low vacuum is found at the case.

The driver may select 2WD or 4WD and a range of high or low. The vehicles transfer case will typically drive the rear wheels in 2WD and engage the fronts if 4WD is desired. This is true in most transfer cases, but all wheel drive is becoming more common. In all wheel drive the vehicle remains in four wheel drive and usually transparent to the driver. These vehicles are designed to handle better in bad weather or light terrain but are not considered off road vehicles.

In an all wheel drive vehicle there is one range or gear ratio. This is typically 1:1 or for every turn of the output shaft there is a turn at each of the axle shafts. This is usually the ratio for the high range on most transfer cases but not for the low range. The low range of a transfer case gear ratio can be 3.1, 3.5:1 or more. This is because torque multiplication is desired in low gear.