General Description
FIVE SPEED ELECTRONIC AUTOMATIC TRANSMISSIONMore Fuel Efficient And Responsive
All Mercedes Benz Automobiles Sold In North America feature the driver adaptive five speed electronic automatic transmission.
The new electronically controlled transmission has five forward gears for better fuel economy and acceleration, as well as state of the art computer controls for smooth, flexible operation. It even has a "winter" mode which features an additional "higher" (numerically lower) reverse gear ratio, second gear forward start and lower overall shift points, for maximum stability and traction on snow and ice.
In spite of being a five speed, the gearbox uses only three planetary gear sets and six shift elements instead of the usual four gear sets and seven shift elements. It has no conventional brake bands, only multiplate clutches for both driving and braking the planetary gears. As a result the new 722.6xx transmission unit weighs 176 pounds and is only two feet in length.
Smart Shifting
The heart of the fully electronic transmission is a high performance microprocessor which is designed so that the transmission shifts as the driver expects and wishes it to react. Continuous electronic networking with the engine and chassis management systems allow the transmission computer to adapt to a wide variety of driver situations and even to the drivers "style."
By comparing road speed changes and "load" or throttle opening, the transmission computer can instantly discern uphill and downhill grades and adjust the shifting as a result. It avoids annoying shifts back and forth between two gears on a long incline, and delays upshifts on decents (for engine breaking), as a driver is likely to do with a manual transmission. It even considers data from the cruise control system and engine temperature sensor in the shifting decision.
The computer also measures how fast the driver presses and releases the accelerator pedal. If the pedal is pressed relatively quickly, the transmission is more likely to downshift , for instance during sporty more spirited driving, while upshift are less likely when the pedal is released quickly. With "normal" gas pedal operation, the transmission shifts according to a basic program aimed at getting the best fuel mileage.
Safe, Smooth Shifting
The transmission is also networked to the ABS and traction control systems (and to yaw and steering sensors on vehicles equipped with the Electronic Stability Program (ESP) system) so that it can recognize cornering and low traction conditions. Based on these data, the transmission may delay or hasten a gear shift to ensure there is no negative effect on traction control.
Not only does the transmission computer adjust shift characteristics according to engine speed and load (very smooth shifts during leisurely driving, crisper shifting with more spirited driving), but it's even able to reduce engine power for a split second (by retarding engine timing) to ensure smooth gear changes.
This fourth generation of Mercedes transmissions uses a refined, proven hydrodynamic torque converter with a new design lock-up clutch inside the converter for maximum fuel efficiency. The lock-up clutch engages in third, fourth and fifth gears (not just in top gear, as with many other cars) - but still with slight computer controlled slippage (60-80 rpm difference), to prevent drivetrain noise and vibration. Designed to last the life of the vehicle, the lock-up clutch is submerged in transmission oil and uses special long life friction material.
Fail Safe Shifting
If the electronic system ever fails, the new transmission is designed to revert to basic hydraulic operation. It stays in the selected gear, with the torque converter lock-up clutch open for maximum torque multiplacation. On restarting, it operates in second gear, allowing it to be driven to a service location.