It is no secret that Mercedes-Benz cars are over-engineered. But high tech engineering is of essence in the making of a vehicle that performs like a car on tarmac and as an off-road vehicle off it. The age of the crossover vehicle threw up a whole new array of challenges and who was better prepared than Mercedes-Benz to pick up the gauntlet?

The resultant SUV, the M-class, embodies a host of technologies which make its job as both an on-road/off-road vehicle much simpler.

The M-class has continuous 4-wheel drive. A dual-range transfer box, directly flange-mounted on the auto 'box, apportions the input drive between the front and rear axles whilst a central differential balances the speed differences between the two axles. What really makes this driveline technology work is the 4-wheel electronic traction support, or 4-ETS. This ingenious system renders conventional locking differentials superfluous. Instead, a microcomputer controls the drive torque distribution by means of brief brake pulses on the spinning wheels. This process involves 4-ETS making use of the anti-lock braking system (ABS) wheel sensors. As soon as these speed sensors indicate that one or more wheels have lost traction, these wheels are automatically braked. The torque distribution is altered almost immediately: the drive power at the wheels that still have good traction rises by the amount by which the spinning wheels are braked, thus allowing continued driving on loose or slippery surfaces.

Added to that is the standard electronic stability program, ESP for short, that Mercedes-Benz uses on all its vehicles. Thanks to measured control of the braking power available at the front or rear wheels, ESP reduces the risk of skidding and helps maintain better control in critical situations. Thanks to a series of additional sensors, predominantly required for calculating the extent of vehicle skidding, the system perfectly complements the technology at work in the anti-lock braking and acceleration skid control systems. While the vehicle is being driven, the ESP computer continuously compares actual vehicle behaviour with calculated set points. If the vehicle departs from its safe ideal line, the system calls upon a specially developed logic circuit and intervenes at lightning speed to put the vehicle back on track. This process is two-fold: precisely modulated brake pulses on one or more wheels and/or a reduction in the engine torque. This process also enables ESP to correct both driving errors and vehicle skidding caused by ice, rain or other adverse road conditions under which the motorist normally has little chance of keeping the vehicle on track by steering or braking alone.

The transfer box is equipped with two planetary gear groups which enable the engagement of a special low-range gear. At the push of the low-range button on the dashboard, the transfer box shifts from the usual on-road ratio of 1:1 to a lower ratio of 2.64:1, allowing even better vehicle progress on rough terrain. The reason behind this is that the drive shafts transmit the high engine torque at reduced rpm, meaning that the off-roader can achieve a slope climbing ability of 60 per cent or more, depending on the terrain. Pressing the low-range button automatically activates the off-road ABS which partially locks the wheels when the vehicle is braked from a low speed (below 20kmph), allowing the vehicle to obtain better grip on loose terrain when driving down steep inclines. As a consequence, the stopping distance is shortened. The five-speed automatic transmission also activates a special off-road shift program in low range.

The electronically controlled five-speed automatic transmission adapts the shift characteristics to suit the situation and individual driver requirements. When cornering, or driving uphill or downhill, the transmission ECU automatically alters the shift points and, in so doing, prevents any sudden gear changes. Thanks to an ongoing process involving the comparison of information provided by the sensors with stored set points, the transmission is able to adapt to the situation on the road at lightning speed. In addition to this, the ECU records the movements of the accelerator pedal and, using this data, deduces the driver's acceleration requirements. Meanwhile, the torque converter lock-up is activated at low road speeds to reduce fuel consumption. The torque converter lock-up clutch works on the basis of computer-controlled continuous slip which decouples the engine from the transmission to such an extent that vibrations no longer occur.

Added to these are some other standard features common to other Mercedes-Benz models like the brake assist system, which was developed by the company for shortening emergency stopping distances and is activated when the driver is too slow or not hard enough on the brakes in critical situations. In such instances the system automatically provides maximum braking within a fraction of a second, reducing braking distances by up to 45 per cent. Also featured on the M-class is the ASSYST active service system which individually records the various operating loads and processes the information for the purpose of engine maintenance. A microcomputer calculates the actual state of the engine oil based on the data it receives from sensors - including details of oil level, oil temperature, coolant temperature, rpm, road speed and engine load, and then comes up with the optimum maintenance schedule for the vehicle. A display in the instrument panel tells the driver how many kilometres there are to go before the next service is due. The active service system has an oil level sensor that not only sends a warning when the oil level gets too high or too low and indicates how much oil needs to be added, but also detects when the driver tops up with fresh oil and extends the oil change interval accordingly.

The 6-cylinder engine is also a technological marvel and uses offset-phase dual ignition. It employs two spark plugs per cylinder to ensure the benefits of short flame propagation and optimum combustion control. The spark plugs are located near the cylinder walls and are in a good position to ignite virtually all of the mixture. Depending on the engine load and speed, the spark plugs are activated one after the other, rather than both together, with the spark plug activation sequence changing after each combustion cycle. This brings about a substantial reduction in pressure build-up inside the cylinders, meaning much less combustion noise and a balancing of thermal stresses on the piston. All this leads to a more efficient and environment-friendly engine. These high displacement six-cylinder engines also feature three-valves per cylinder instead of four. This has been done to reduce temperature losses of the exhaust gases while going from the cylinder to the catalytic convertor. The high-displacement engine phenomenon, caused by the operating principle involved, is effectively avoided by further reducing the amount of heat lost between the combustion chambers and the catalytic convertor. Compared to a four-valve engine, the heat loss in the exhaust flow is reduced by 70 degrees CELSIUS if a complete exhaust duct is dispensed with. This means that the catalyst reaches its operating temperature in considerably less time after a cold start, reducing cold-start emissions by about 40 per cent.