While sport-utility vehicles and 4x4 pickups are extremely popular these days, few understand the principle behind four wheel drive. The diagram to the left illustrates the layout on most 4x4 trucks and truck-based SUVs.

The transmission on most 4x4s are no different than those found on rear-wheel driven vehicles. It is after the transmission where the differences begin.

After the transmission, power is then sent to the transfer case (shown in red). This gearbox contains a set of gears that can be shifted to send power exclusively to the rear wheels (2WD) or power to all wheels (4WD). While most transfer cases can operate in either mode, many new vehicles come equipped with full-time transfer cases.

The transfer case is a critical link in your vehicle's four wheel drive system. It must be monitored for adequate lubrication and changed regularly following the manufacturer's guidelines.

Power from the transfer case is then delivered to the wheels by a differential. The differential housing contains a ring and pinion gear set which turns the axles. A major role of the differential is to allow each wheel to travel at different velocities.

The differential is an important part of your vehicles drive train. Thus, it is important to make sure that the differential always contains sufficient gear lubricant. It is also imperative to replace the fluid at intervals stated by your vehicle's manufacturer.