In the good old days, when driving was more of an interactive experience, owners of a 4-wheel-drive transmission vehicle would often have to get out and fiddle with a mechanism on the front hub in order to activate or deactivate drive to all wheels. These days, of course, nobody wants to get their hands dirty in this way, and everything is done automatically, but this by itself can present some problems. You may not notice a developing issue until it causes a failure, so how can you keep an eye on the transmission so that you avoid any big issues?
Modern four-wheel-drive systems are designed to be optional, so the front wheels can be disengaged if the driver does not need to worry about traction. Often, these vehicles will only be used in urban environments and will not need this type of support, so the front wheels can be "turned off" most of the time. This will help reduce wear and tear on the front tyres and boost fuel economy as well.
What Can Go Wrong?
Sometimes, however, a buildup of dirt or an accumulation of rust may prevent the system from engaging or disengaging. In other words, the clutch mechanism within each hub on the front axle will remain in its original position and will be unable to move when required.
The driver may not notice that anything is wrong until he or she encounters a situation where they need additional traction. For example, they may have turned down a muddy road, noticed the obstacle ahead, and dutifully engaged the locking front hub. However, they may subsequently find that they are stuck in the mud, as the front wheels are not contributing to the issue. Occasionally, the driver may also notice some grinding noises when trying to shift into four-wheel-drive and may simply put that down to a glitch of some kind.
On the other hand, the hub may remain engaged when the driver switches to two-wheel drive mode. While the result will not be as dramatic as the previous example, the driver may nevertheless notice additional road noise or increased tyre wear.
Checking It Out
The best way to check for an issue is to raise all four wheels off the ground and place the vehicle on a frame stand. When four-wheel-drive mode is engaged you will be able to see if the front wheels are turning or not. Look closely at the axle shafts themselves and if one wheel is turning while the other one is not, then you have a problem with the locking hub on the latter side.
Avoiding a Problem
This problem is more common than you might think, and it's a good reason to take the vehicle in for a regular service so that all components can be checked for functionality.