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The Mercedes-Benz 722-623 Automatic Transmission
The U.S. spec Mercedes-Benz CLK430 features an electronically controlled, driver-adaptive, five-speed automatic transmission as standard equipment. This transmission has a "r mode " (engaged with a console-mounted button) that causes the car to start in second-gear, as well as lowering overall shift points for maximum stability and traction on snow and ice.
Intelligent Gear Selection
At the center of this fully electronic transmission is a high-performance microprocessor, which is designed so the transmission shifts as the driver would with a manual gearbox. The transmission processor has continuous electronic networking with the engine and chassis management systems. This allows the transmission computer to adapt to a wide variety of driving situations, as well as make alterations to shift points based on the driver's style.
By comparing road speed changes, load, and throttle opening, the transmission computer can instantly recognize both uphill and downhill grades, and adjust shifting as a result. This transmission avoids those annoying shifts back and forth between two gears on a long incline. It also delays upshifts on descents, using engine braking exactly as a driver is likely to do with a manual transmission. This transmission even considers data from the cruise control system and engine temperature sensor in the shifting decision.
The computer also measures how fast the accelerator pedal is presses and released. The more aggressively the pedal is depressed, the more likely the transmission is to downshift. If the pedal is abruptly lifted, upshifts are much less likely. When the pedal is applied in a smooth, normal fashion, the transmission shifts according to a basic program aimed toward obtaining the best fuel economy.
Smooth Shifts
The transmission computer is also networked to the ABS, traction control systems, yaw sensors, and steering sensors from the standard ESP stability control system so that it can recognize cornering and low-traction conditions. Based on this data, the transmission may delay or hasten a gearshift with the goal being that the shift not upset vehicle stability or cause a lack of traction.
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 also reduce engine power to ensure smooth gear changes at wide open throttle. It does this by sending a signal to the engine computer requesting that timing be retarded for the split second that it takes to change gears.
This is the fourth generation of Mercedes-Benz automatic transmissions. This transmission uses a hydrodynamic torque converter design, that features a locking clutch mechanism for maximum fuel economy. This lockup clutch doesn't simply lock in top gear as is so common in the industry, it locks in third, fourth, and fifth gears. The locking system has a computer controlled degree of slippage to reduce noise, vibration, and harshness.
TouchShift
This transmission offers Touch Shift. Touch Shift allows the driver to manually select gears without the need to manage a clutch. With touch Shift, the driver can pull the gear lever slightly to the left to downshift, or push it slightly to the right to upshift. This eliminates the need to clumsily move the shifter across gates as was the case in prior designs. Rather than looking at the shit gate to determine what gear they're in, the CLK430 has an in-dash gear indicator conveniently located in the gauge cluster. Although Touch Shift allows the driver to manually select gears, the transmission computer prevents downshifts that would result in the engine being revved above redline. If desired, Touch Shift need not be used at all. The driver can simply put the car in "D ", and drive in a normal fashion.
SpeedShift? Not Quite
Touch Shift should not be confused with SpeedShift. SpeedShift transmission are a special high performance version of the five-speed automatic from the AMG division of Mercedes-Benz. SpeedShift transmissions shift 35 percent faster than and have much stronger internals than the standard 722-623.
If All Else Fails
Lastly, if the electronic system ever fails, the automatic 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 multiplication. On restarting, it operates in second gear, allowing it to be driven to a Mercedes-Benz dealer for service.
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© Marcus B. Fitzhugh 2005
Mercedes-Benz Specifications