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What are four-wheel drive parts and where are they located?

Four-wheel drive (commonly referred to as 4WD or 4-wheel drive) systems allow the engine to drive all four wheels of a vehicle simultaneously. The benefit of a four-wheel drive system is that, by being able to drive four wheels instead of two, you have the ability to double the amount of force needed to pull forward. The tires apply to the surface it is driving on, so both traction and acceleration are enhanced. This is particularly useful in low-traction conditions where wheel slippage (when the force applied to a tire is greater than the amount of force that the tire can apply against the ground or the ground can apply against the tire) is much more likely to occur, such as off-roading, wet, or snowy conditions, climbing slippery hills, or on dirt roads. For these reasons a 4WD system is common on trucks and SUVs—including Jeep models. The term four-by-four (4x4) refers to this general class of four-wheeled vehicles that consist of a drivetrain that allows for all four of the wheels to receive torque (i.e. the power that moves your vehicle) from the engine simultaneously. The first number is normally a reference to the total amount of axle ends, and the second number references the number of wheels that are powered. Since running the vehicle in 4WD all the time reduces fuel mileage and is taxing on the various 4WD parts, most 4x4 vehicles run in rear wheel drive most of the time and can be switched into 4WD at the press of a button. There are a number of different parts that make this possible. Different manufacturers all use different systems to achieve this, so we’ll stick primarily to the basics here. 

On some older models the 4WD was activated manually by a lever or shifter in the 4x4 vehicle’s console. More often, recently, the 4WD system is activated when you press the mode selector switch on your dashboard. This sends the electrical signal that activates the necessary systems. Typically, it will show three modes:  two-wheel drive (usually labeled 2HI); normal four-wheel drive (usually labeled 4HI); and a special low gear four-wheel drive mode (usually labeled 4LO). These settings represent different settings on the transfer case. 

The transfer case, attached to the transmission, does the brunt of the work in activating the 4WD system. Through gears or a chain, the transfer case connects the input from the transmission to the rear and front driveshafts. When 4HI is selected, the transfer case shift motor meshes a gear that links the front driveshaft to the rear one. When 4LO is selected, another set of gears is engaged that reduces the output speed of the rear driveshaft (and then, by extension the front one), but increases the torque.  This is useful in tricky off road situations, when high torque is needed and speed is not. The necessary gears are meshed by a selector arm controlled by the manual shift lever or by the transfer case shift motor. 

When the front driveshaft is not active, the front wheels are allowed to spin freely. There must be some mechanism to cause the front wheels to drive the vehicle along with the rear ones. Often this takes the form of locking hubs. These hubs usually spin freely around the axle shafts but can then be locked on to be driven by the shafts. In older 4x4 models, with manual locking hubs, the driver had to get outside of the vehicle and lock the hubs that connect the wheels to the axle shafts. Automatic locking hubs lock into place by the forward motion of the axle shafts. Once the front driveshaft starts to drive the front axle shafts, they activate the automatic locking hub. The automatic locking hubs can be deactivated by driving the vehicle in reverse.

Other 4x4 vehicles may use a vacuum system to connect the axle shafts at the differential. When 4WD is engaged, vacuum hoses are actuated which move a splined collar or a front differential shift fork to engage the axle shafts. Locking hubs are not necessary in this kind of four-wheel drive system. Vacuum actuators control the flow of the vacuum system to activate or deactivate the 4WD system. 

In some vehicles, all these systems are controlled and coordinated by a computer located behind the dash, called the transfer case control module.  

How do I know if my 4WD parts need to be replaced?

In push-button systems, your 4WD mode selector switch will have lights that turn on when the system is in effect. In manual systems, you may have a dash light that serves the same function. The dash light is turned on and off by an electric component called the indicator switch. Normally, if you try to engage 4WD and it doesn’t work, these lights will blink to alert you that there is a problem (unless there is something wrong with the selector switch or the indicator switch). If these warning lights aren’t working, then you may notice that your four-wheel drive system is not engaging when you try to activate it or that it won’t disengage, or that it disengages unexpectedly. 

There are a number of problems that could cause these issues. Any of the above mentioned problems could stem from faulty wiring or deteriorated vacuum hoses. Problems with the transfer case shift motor may keep the transfer case from shifting the vehicle into the selected mode. Problems with the vacuum actuators may keep the front axle shafts from engaging. In this case, you may hear the transfer case engage, but 4WD still will not work. This can eventually damage the transfer case. Conversely, problems with the vacuum actuators may leave the front axles engaged. Dirt or water entering the locking hubs may cause them to corrode, making them more difficult to engage or disengage. They may make a grinding or ratcheting noise if they are worn in this way. Under normal conditions, the hub should make an audible click when activated. Diagnosing which part or parts of your 4WD system are causing your problem may require some testing and experimentation. 

Can I replace four-wheel drive parts myself?

The difficulty of replacing any of the parts of your 4WD system will vary from part to part and from vehicle to vehicle. Generally, many of the 4WD parts will be relatively easy to access. The transfer case control module can sometimes be found behind the glove box. The vacuum actuator can be accessed by removing the battery and battery tray. Locking hubs are, of course, found at the wheels. The transfer case can be found between the two driveshafts, and the transfer case shift motor is mounted to the transfer case.  As mentioned above, it can be difficult to pinpoint the source of a 4WD problem, since problems with any given component tend to result in the same symptoms (i.e. the 4WD system will not engage). For that reason, it would be wise to thoroughly bench-test whatever component it is that you remove to make sure that’s really the part that is not functioning.    

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