10.4 Transfer Cases

Transfer cases are used in off-road vehicles to divide engine torque between the front and rear driving axles. The transfer case also allows the front driving axle to be disengaged, which is necessary to prevent undue drive line component wear during highway use. Another purpose of the transfer case is to move the drive shaft for the front driving axle off to the side so that it can clear the engine. This arrangement is necessary to allow adequate ground clearance and to allow the body of the vehicle to remain at a practical height. Figure 10-22 shows a typical drive line arrangement with a transfer case.


Figure 10-22 — Typical drive line arrangement with a transfer case.

A conventional transfer case is constructed similar to a transmission in that it uses shift forks, splines, gears, shims, bearings, and other components found in manual and automatic transmissions. The transfer case has an outer case made of either cast iron or aluminum that is filled with a lubricant that cuts friction on all moving parts. Seals hold the lubricant in the case and prevent leakage from around the shafts and yokes. Shims are used to set up the proper clearances between the internal components and the case.

Conventional transfer cases in heavier vehicles have two-speed positions and a de- clutching device for disconnecting the front-driving wheels. Some light-duty vehicles use a chain to transmit torque to the front-driving axle.

Gear Type Transfer Case

Gear-driven transfer cases provide a high and low final drive gear range in the same manner as an auxiliary transmission (Figure 11-23). In most cases, the shifting is accomplished through a sliding dog clutch, and shifting must be done while the vehicle is not moving. Typical operation of a conventional two-speed transfer case is as follows:

High Range—When an operator is driving the front and rear axles in the high range (1:1 gear ratio), the external teeth of the sliding gear (splined to the transmission main shaft) are in mesh with the internal teeth on the constant mesh gear mounted on the transmission main shaft. Likewise, the external teeth of the front axle sliding gear are in mesh with the internal teeth of the constant mesh gear, or the sliding clutches are engaged. Disengagement of the drive to the front axle is accomplished by shifting the sliding gear on the front axle main shaft out of mesh with the constant mesh gear, permitting the latter to roll free on the shaft or sliding the clutches out of mesh.


Figure 11-23 — Gear type transfer case. \

Low Range—When an operator is using the low range in the transfer case, the sliding gear on the transmission main shaft is disengaged from the constant mesh gear and engaged with the idler gear on the idler shaft. This design reduces the speed by having the sliding gear mesh with the larger idler gear. The shifting linkage on some vehicles is arranged so shifting into low range is possible only when the drive to the front axle is engaged. This design prevents the operator from applying maximum torque to the rear drive only, which can cause damage.

Chain Type Transfer Case

Most new light-duty four-wheel drive systems today use a chain drive transfer case (Figure 10-24). Chain drives can be used in combination with many other transfer case components such as planetary gears, differential assemblies, and electronically controlled clutch packs. Chain drive systems can be controlled manually by the driver or by other systems such as pneumatic, hydraulic, electrical, or electronic devices.


Figure 10-24 — Chain type transfer case.

Types of chain drive transfer cases vary greatly depending on the manufacturer. All chain drive transfer cases use a planetary unit for changing ratios.

When the vehicle is in two-wheel drive and high gear, power flows into the transfer case from the transmission output shaft and through the planetary gearset. The planetary gearset is in the high position and turns as a unit. This creates a one-to-one ratio. The output shaft rotates inside the upper drive chain sprocket but does not transfer power.

When shifted into four-wheel drive and high gear, power continues to flow from the transmission output shaft through the planetary gearset at the one-to-one ratio. At this point, the gear selector connects the output shaft to the upper drive chain sprocket.

Engine power continues to flow through the output shaft to the rear axle. Power also flows through the upper drive train sprocket, through the drive chain, and into the lower sprocket. The lower sprocket is splined to the front output shaft so power will flow to the front axle.

When four-wheel low is selected, power flows through the planetary gearset that has been shifted into the low gear position, and the action of the gears reduces speed to a lower ratio. The lower ratio causes the input speed to be reduced while increasing the amount of torque.

Transfer Case Maintenance and Service

The fluid level in a transfer case should be checked at recommended intervals. To check the lubricant level, remove the transfer case fill plug, which is normally located on the side or rear of the case. The lubricant should be almost even with the fill hole. If required, add the recommended type and amount.

The first indication of trouble within a transfer case, as with other components of the power train, is usually noisy operation. If an operator reports trouble, make a visual inspection before removing the unit from the vehicle. Check for such things as oil level, oil leakage, and water in the oil.

Make sure the shift lever linkages are not bent or improperly lubricated. This will make it hard to shift or, in some cases, impossible to shift. Make sure other possible troubles, such as clutch slippage, damaged drive shaft, and damaged axles, have been eliminated.

Worn or broken gears, worn bearings, and excessive end play in the shafts can cause noisy operation. When transfer case service is required, follow the procedures outlined in the service manual. It will give directions for repairing the particular make and model.

Test your Knowledge

4. In a chain type transfer case, the lower sprocket is splined to what component?

A. Front output shaft
B. Rear output shaft
C. Synchronizing gear
D. Planetary gear set