Four-wheel-drive systems can be grouped in two categories:
Part-Time: operates in two-wheel drive unless its all-wheel-drive mode is engaged manually by a lever or button. Some part-time systems can engage four-wheel drive automatically when wheel slippage occurs in two-wheel drive.
Some part-time systems lock the front and rear wheels together so that the front wheels cannot rotate faster in turns as they should. As a result, these part-time four-wheel-drive systems should be operated only on loose surfaces such as dirt, gravel or snow.
Full-Time: operates in all-wheel drive all the time. The term "full-time" is used interchangeably with "continuous" and "permanent," although some auto companies attach more specific meanings to these terms in an attempt to differentiate their systems.
While the term "four-wheel drive" applies to any system powering all four wheels, the term "all-wheel drive" generally refers to passenger car 4WD systems. Most full-time, all-wheel-drive layouts (such as the Mercedes-Benz 4MATIC system) employ a center differential to allow the front wheels to rotate faster in turns, allowing the vehicle to be driven on paved roads without binding the drivetrain and forcing the wheels to slip or skip. Full-time systems provide on-road handling benefits since all four wheels are driven at the right speed.
A number of Mercedes-Benz models are available with the 4MATIC all-wheel-drive system: 4MATIC is standard on all five families of Mercedes-Benz sport utility vehicles: the G-, GL-, GLK-, R- and M-Class (although the GLK350 and ML350 are also available with RWD), as well as the CL550 coupe, and it's an option on certain C-, E- and S- Class sedans.
The Mercedes-Benz 4MATIC system uses a mechanical center differential that distributes engine power to the front and rear wheels, while differential units at the front and rear distribute power to the left and right wheels. 4MATIC features an innovative electronic four-wheel traction control system to limit wheel spin when conditions are slippery, thus allowing power to go to the wheels with more grip.
Working in conjunction with four-wheel traction control, the center differential in 4MATIC passenger cars is equipped with a clutch that provides the traction benefits of a limited-slip differential while allowing the front wheels to rotate faster in turns. S-Class, E-Class and C-Class 4MATIC models now come equipped with a multi-plate "breakaway" clutch, and other models use a single-plate clutch.
Rear-Biased Torque Distribution
4MATIC passenger cars feature a planetary-type center differential that provides rear-biased power distribution – approximately 45:55 front-rear power distribution that makes for more car-like handling. The R-, M-, G- and GL-Class have 50:50 torque distribution.
Dual-Range Transfer Case
Controlled electronically by a button on the center console, a dual-range transfer case is standard on the G-Class. A "high" range of 1.00:1 is used for most conditions, while a low range of 2.16 can be engaged for better control in off-road conditions such as steep inclines and descents. Other Mercedes- Benz SUVs and passenger cars with 4MATIC have a single-range system, so there's no button on the console.
FOUR-WHEEL TRACTION CONTROL
Differential Disadvantages For Traction
When turning any vehicle, automotive differential units (usually for the front or rear drive wheels) provide a major benefit by allowing the outside wheel to turn faster than the inside wheel. Many four-wheel drive vehicles also use a third differential in the center of the drivetrain that allows the front wheels to rotate faster than the rear wheels in a turn. In summary, three differential units – front, center and rear – are used in many full-time 4WD vehicles since all four wheels are rotating at different speeds in a turn.
However, differentials inherently distribute more engine power to the wheel that's going faster, which, in slippery conditions, is also the wheel with less resistance. Transferring more power to any wheel with less resistance can mean that more engine torque actually goes to a slipping wheel – not a good thing, especially for a four-wheel-drive vehicle. To compensate for this inherent behavior of differentials, many full-time four-wheel-drive systems make use of mechanical differential locks or limited-slip clutches.
Four-Wheel Traction Control – The Ideal Solution
All Mercedes-Benz vehicles with 4MATIC all-wheel drive use an innovative four-wheel electronic traction control system that, in effect, provides the traction effect of locked differentials. Whenever one wheel begins to rotate faster than the other three, the traction control system momentarily applies the brake on that one wheel. Since applying the brake increases resistance on that wheel, the differentials naturally shift more power to another wheel.
Even One Wheel Keeps It Going
As a result of this split-second interplay between the differentials and the traction-control braking action, Mercedes-Benz all-wheel-drive systems are among the very few that keep the vehicle going even when three wheels have lost traction! Mercedes-Benz all-wheel-drive traction control is also lightweight, mechanically simple and provides excellent handling.
The all-wheel traction control system developed by Mercedes-Benz performs so well and yet is so elegantly simple – making use of existing electronics and drivetrain technology in an innovative way – that it sets a standard for efficient, practical full-time all-wheel-drive systems.
G-Class – The Best of Both 4WD Worlds
The Mercedes-Benz G-Class comes with four-wheel electronic traction control and differential locks. The standard G-Class differential locks were originally designed to meet tough military specifications, and they're perfect for off-road driving enthusiasts who want to explore the full extremes of off-road traction. Each of the three locks can be engaged electronically by dash-mounted rocker switches that control mechanically operated sleeves on each differential.
In short, each differential lock provides an additional margin of off-road traction. Most off-roaders engage the center diff lock first. If traction worsens or the ascent gets steeper, the rear diff lock comes next. On the G-Class, its front diff lock can also be engaged when the going gets really tough.
Mercedes-Benz Electronic Safety and Traction Systems
A number of the safety and traction systems available on Mercedes-Benz passenger vehicles are integrated electronically, making common use of sensors, notably for wheel speed:
ABS anti-lock brakes: Releases the brakes momentarily whenever wheel speed sensors indicate a locked wheel during braking. ABS does not "pump" the brakes; ABS action is actually like "pumping in reverse." The key purpose of ABS is to preserve directional stability by allowing the driver to continue steering during emergency braking, but it does not necessarily provide shorter stopping distances. All Mercedes-Benz models come with standard ABS anti-lock brakes.
Traction control: Applies one of the brakes momentarily during acceleration whenever the wheel speed sensors indicate that a drive wheel is going faster (slipping) than the others. The system can also reduce the throttle electronically if necessary to restore traction. Traction control is standard on all models.
4MATIC all-wheel drive: 4MATIC four-wheel traction control applies the brakes to any of the four drive wheels whenever the wheel speed sensors indicate the wheel is going faster (slipping) than the others. This brake action works hand in hand with front, center and rear "open" differentials of the Mercedes-Benz all-wheel-drive systems to control torque distribution to the four wheels. As a result, Mercedes-Benz has one of the few systems that will get the vehicle going even when three wheels are slipping. 4MATIC is standard on the GLK-, GL-, G-, R- and ML-Class sport utility vehicles, as well as on the CL550 coupe. 4MATIC is an option on the C-, E- and S-Class passenger cars.
ESP (Electronic Stability Program): Applies one or more of the brakes momentarily to any wheel to correct oversteer or understeer, based on signals from several sensors, including those for steering angle, vehicle yaw and wheel speed. Designed to restore vehicle stability in the event of incipient skids, ESP works under all conditions: during acceleration, coasting or braking. Pioneered by Mercedes-Benz, ESP is standard on all of its models and has been mandated by the Federal government on all future cars.
Brake Assist: Applies full braking power automatically during emergency braking whenever the driver's foot is on the pedal. Brake Assist uses a pedal speed sensor and a computer to determine emergency situations and operates a valve on the brake booster unit that can provide full brake boost. Although it's a totally separate system from ABS, Brake Assist operation does rely on ABS to prevent wheel lockup. Brake Assist is standard on all Mercedes-Benz model.