The box can be removed without removing the engine. It is necessary to remove the box when replacing or repairing it. It is not recommended to repair the box at home.
Transmission control is described in detail in Chapter Controls and techniques for safe operation.
The removal and installation procedures are similar to the removal and installation of the manual transmission, a description is given in Section Removal and installation of the gearbox.
Automatic transmission, in the process of starting off, performs the function of an ordinary clutch, and while driving, it performs the work of shifting gears.
The main units of the AT are: a torque converter, a planetary gearbox and a hydraulic or electronic control system. To switch to another gear ratio in the planetary gearbox, hydraulic disc brakes and a disc clutch are used.
The torque converter has the same function as a hydraulic clutch. Its task is to carry out the clutch when starting off and shifting gears.
Switching is controlled by an electronic unit. At the same time, the electronic unit uses a variety of information for proper control, which allows for each mode of vehicle movement and, depending on the desire of the driver, to select the most optimal gear ratio.
Program switches, such as «Economy» or «Sport», are not required, because the control unit automatically, according to the driving mode, sets the switching points. When determining the switching points, driving resistances such as uphill or downhill slopes, trailer towing and headwinds are also taken into account.
The control system can operate in a special safe mode, which it switches to when any malfunction is detected. In this case, the transmission modes are limited, and the car can be delivered under its own power to the repair shop. When the control system enters the safe mode, the control lamp lights up on the dashboard (contact the head Controls and techniques for safe operation).
Analyzing information coming from various sensors (not only sensors directly related to the transmission), the control system selects the optimal one in terms of economy, smoothness, etc. box mode. At a certain throttle position, the control system can lock up the torque converter so that only fourth and fifth gears are engaged. This achieves a significant reduction in fuel consumption.
The only regular maintenance procedure for the box is to check the fluid level (refer to Section Checking the automatic transmission oil level). It is not necessary to change the transmission fluid regularly.
A transmission fluid cooler is installed next to the radiator of the cooling system.
There are several modes of operation of the transmission, each of which can be engaged after moving the selector to one of the following positions: P, R, N, D, S (contact the head Controls and techniques for safe operation).
The engine can only be started if the selector is in position P or N. This is done in order to prevent the car from moving when trying to start (if the selector is set to one of the other positions). Analyzing the information coming from the sensors, the control system allows or prohibits the engine start. The locations of the various relays are indicated in Chapter Onboard electrical equipment.
The Tiptronic 5-speed AT selector next to the automatic shift positions track has a second step-by-step track (consistent) switching. The sports steering wheel can be equipped with corresponding buttons. Such switching is common in sports cars.
With Tiptronic, the driver can force upshift and downshift. Compared to manual shifting, Tiptronic is more comfortable and secure against erroneous operation. For example, switching to a lower gear is performed only if the speed does not go beyond certain limits.
If there are faults in the input signals or solenoid valves, the emergency program is switched over. A fault that occurs at the same time is stored in the fault memory. Information from the memory can be read by a special device.
Multitronic automatic transmission with stepless gearshift
Stepless AT Multitronic
Multitronic is a stepless CTV transmission with continuously variable gear ratio, which ensures not only the efficient use of engine power, but also provides good dynamics and driving comfort.
The main element of the Multitronic system is the V-chain variator. With the help of a variator, the gear ratios between the starting mode and the last gear change smoothly. Thanks to this, the engine, regardless of the driving mode, powerful or economical, operates in the optimal mode corresponding to it. The variator consists of two conical pairs connected to each other by a lamellar chain. Since each pair of discs shifts the conical disc with an electro-hydraulic drive, the working diameter of the chain and, as a result, the gear ratio changes smoothly. In addition, the Multitronic uses an oil-cooled clutch that eliminates not only the typical disadvantages of a torque converter, but also allows for a variety of starting options. The option is selected by the electronic control system in accordance with the desire of the driver, which is recognized by the nature of the movement of the gas pedal. In addition, the electronically controlled clutch realizes a stable creep mode, which is an element of comfort. The electronic control system also provides driver and environmental recognition and works with the dynamic DRP control program. The specified program determines by the nature of the movement of the gas pedal whether the driver wants to move in a dynamic or economical mode.
If the driver gives full throttle (kickdown), the electronic control system immediately switches to sport mode and downshifts to achieve the required high engine speed. In the normal mode of motion, the optimal gear ratio is selected, and all its changes, in contrast to the stepped switching of AT, are carried out smoothly, without jerks and imperceptibly. The electronics also recognizes ups and downs and compensates for the additional load or increases the engine braking torque.
In manual shift mode 6 fixed gear ratios stored in the memory of the control unit (gear) can be smoothly switched by the selector lever or the buttons on the steering wheel.
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