The 4wd system has come a long way since the original Jeep.
The U.S. Army Jeep MBs that landed in France in 1944 had three levers on the floor: one to shift the three-speed transmission, one to engage the 4wd transfer case and one to shift between 4wd high and 4wd low. The vehicle had to be stationary to engage the nonsynchronized gears of the transfer case.
Later civilian Jeeps had a fourth control: a dial on the front wheels allowing the driver to disengage the front hubs from the front axle for better fuel economy and longer axle life in 2wd mode.
Today, automakers can choose from a wide range of systems based on the on-road and off-road performance targets for the vehicle, plus the intended price. The systems fall into three major categories:
Part-time. The heart of this system is its two-speed shiftable transfer case, not unlike those in the original U.S. Army Jeeps. At driver command, it routes engine power to a second axle (usually the front) by means of a cockpit lever or button. While ideal for off-road use, these systems are clunky and fuel-guzzling for on-road use and must be deactivated. Examples: Ford F-150, Dodge Ram, Nissan Xterra.
Full-time. With this top-end system, the front and rear axles always receive some percentage of the engine's torque under normal driving conditions. Most full-time systems have a transfer case with a 4wd low gear. A differential in the case biases torque front or rear depending on road conditions, while differentials in each axle can bias torque from side to side. The ideal system can send 100 percent of the engine's torque to one wheel. Examples: Land Rover Discovery, Toyota Land Cruiser.
On-demand. This system runs 2wd until road conditions demand 4wd capability. It requires a torque-splitting device that can react to speed differences between the axles. They commonly take one of two forms: viscous couplings and pump-type couplings. The Oldsmobile Bravada is the exception: Its SmartTrak system uses an electronically controlled multiplate clutch that engages when a prop-shaft sensor detects wheel slip. Examples: Oldsmobile Bravada, Chevrolet Astro, Honda CR-V.