Your Guide To Automotive Features and Options

Just Bought Your First 4WD? Mini-Guide To Your Transfer Case & Differentials

If you recently bought your first 4WD vehicle, you may be itching to get out there and find the gnarliest, off-roading trails around. But before you do, there are a few important things to understand about the transfer case and differentials. Here's what you need to know so you will be able to properly maintain your 4WD vehicle and identify problems if they happen.

Transfer Case

The transfer case is what takes the power from the transmission and divides it between the two drive shafts of your vehicle, one for the front axle and one for the rear. The way the power is divided depends on which 4WD setting your vehicle is in. The transfer case is what makes your 4 wheels work together with equal power when the vehicle is switched into 4WD and going straight.

Transfer cases use either chains or gears to transfer the power of the transmission to the front and rear drive shafts. Chains are typical in most 4WD vehicles. Transfer cases with gears are mostly installed in trucks that are designed for heavy hauling, such as tow trucks.

Given all the moving parts, transfer cases have fluid inside them. If a transfer case leaks, it could cause the chains or gears to bind up, which can cause your wheels to also bind up. A dry transfer case can also damage your transmission.

However, the number one cause of transfer cases going bad has to do with the tires. If the tires are not the same sizes they will rotate at different speeds when driving straight, which is not normal. This can confuse the computer, which monitors wheel speeds and activates clutch pads and differentials as if the vehicle is turning. The confusion between all the moving parts can cause an increase in the wear and tear of your entire 4WD system.

Therefore, your spare tire should be the same size as your other tires. Do not drive with small spare tires that are often called donuts, especially not in 4WD mode. Always make sure your tires are properly and equally inflated before putting your vehicle into 4WD.


The differentials take the power from their drive shafts and divide the power according to the wheel rotation and turning radius. There are two differentials: one for the front and one for the rear.

First, understand that during normal operation and with normal tires, the wheels should rotate at the same speeds except when turning. While turning, the inside tire will rotate slower than the outside tire. The differentials are what control the speeds that each wheel turns.

Differentials also have fluid inside them. And just like with the transfer case, the differentials can leak and cause the gears inside them to bind up. Should this happen while you are driving, you will lose control of your steering and you may lose power to the affected axle.

A differential that binds up could cause the gears to blow through the casing. If this happens, the axle may break apart. As you can imagine, this can be extremely dangerous on any roads, especially the gnarliest, off-roading trails you are interested in.

Since the transfer case and the differentials work together with the transmission system to get your 4WD from here to there, if any one of them is faulty it could cause damage to either of the other parts. Don't skip out on maintenance of these essential truck parts of the 4WD system. Refer to your owner's manual for how often the fluids should be changed.

And don't forget about the importance of the tires and how one tire in a different size can throw everything off. Happy trails!