TRANSMISSION | 722.9 in MODEL 172 |
Function requirements for transmission control, general
Circuit 87 (connected circuit 15) ON
Function test of fully integrated transmission control unit (Y3/8n4) carried out
Transmission control, general
The transmission control system electronically actuates the actuators (control solenoid valves). The valves are located in the hydraulic valve body assembly which is part of the fully integrated transmission control controller unit (Y3/8) and realizes the hydraulic functionality.
The function of the fully integrated transmission control unit basically consists of the evaluation of various input signals relevant to the transmission function. Based on this, eight electromechanical hydraulic valves are actuated which determine the gear ratio of the transmission (gear selection), the working pressure and the torque converter lockup.
Transmission control function sequence
The function sequence is described in the following steps:
Function sequence for basic shift program
Electronic and hydraulic transmission control function sequence
Function sequence for kickdown
Function sequence for limp-home mode
Function sequence for basic shift program
The basic shift program includes upshift and downshift characteristics for all 7 gears, designed for:
- Empty vehicle
– Operating temperature status
- Smooth road
- Sea level
The basic shift program can be influenced by the driver using the automatic transmission's transmission mode button (S16/12).
The shift programs are adapted by offsetting the characteristics, depending on:
- Vehicle load
- Change in driving resistance
- Accelerator pedal movement
The fully integrated transmission control unit monitors and shifts depending on:
- Vehicle speed
- Selector lever position
- Accelerator pedal position
- Transmission mode selection
– Selected gear, with Manual ("M") transmission mode with code 950 (Sport package)
– Signals on the drive train CAN (CAN C)
Slow accelerator release:
- Undesired downshifts in the upper vehicle speed range are prevented
Rapid accelerator release:
- Upshifting to the next gear is prevented and only permitted again when the vehicle accelerates slowly
The transmission mode "M" is only available with code 950 (Sport package). Vehicles without Sport package do not have transmission mode "M". On these vehicles, the transmission mode "M" is replaced by the "Short-term M mode".
Electronic and hydraulic transmission control function sequence
While the electronic transmission control system is responsible for gear selection and adaptation of pressures to the torque to be transmitted, power supply control for the transmission is performed by the hydraulic elements of the fully integrated transmission controller unit. The oil supply for the hydraulic elements, such as hydrodynamic torque converter, actuators and hydraulic transmission control, is provided by an oil pump connected to the torque converter and/ or through the electrical transmission oil pump (M42). The transmission control system makes possible a precise adaptation of pressures to the respective operating states and to the engine output during the gearshift phase, resulting in a significant improvement in shift quality.
It is possible to reach the engine rpm limit in the individual gears using full throttle and kickdown.
The transmission control system prevents impermissibly high engine speeds with a downshift lockout device. Furthermore, the transmission control system offers the advantage of a flexible adaptation to various vehicle and engine versions.
The electric transmission oil pump safeguards the hydraulic system's oil supply when the internal combustion engine has stopped running. It also supports the primary pump when the combustion engine is operating in the low speed range. This is actuated through the gear oil auxiliary pump control unit (N89).
Function sequence for kickdown
The driver's wish for maximum acceleration is met by the kickdown function.
The kickdown function is activated as of a pedal travel > 90 %.
The "Kick-down" signal is routed from the CDI control unit (N3/9) or ME-SFI [ME] control unit (N3/10) to the fully integrated transmission control unit over the drive CAN.
Based on this information the fully integrated transmission control unit actuates the control solenoid valves so that upshifts and downshifts take place at an increased engine speed in order to achieve maximum performance.
Function sequence for limp-home mode
If the signals are implausible, the transmission control system is only partially operational and switches to the programmed emergency performance map.
The fault is filed along with the assigned fault code in the fault memory and the engine diagnosis indicator lamp (A1e58) in the instrument cluster (A1) is actuated over the drive train CAN and the chassis CAN (CAN E) with the CDI control unit or ME-SFI [ME] control unit interface.
The driver can use the selector lever to select the desired gear ranges "P, R, N, and D".
The driver can choose been forward and reverse travel, engage the park pawl or, when in neutral, interrupt the power flow between torque converter and transmission.
Introduction of short-term M mode, on vehicles as of AEJ 13/1
The gear limitation is replaced by the function "Short-term M mode". The short-term M program is a temporary program that is activated when the lever or the steering wheel gearshift buttons are operated. Pressing the plus (+) direction effects an upshift and pressing in the minus (-) direction effects a downshift, if these are permissible. If not, the current gear is maintained. After a specified time window, this function deactivates again automatically. The activated time can be extended by undershooting a minimum speed (e.g. stopping), upon recognition of a lateral acceleration or if the vehicle is in a deceleration phase. If none of the stated criteria are active anymore, the M mode remains active for a prescribed remaining time. After this remaining time expires, the initial shift mode is returned to. The driver can personally interrupt the short-term M mode prematurely with certain actions. If the driver presses the steering wheel upshift button (S111/1) moves the lever towards "Plus" (+) and holds, changes the gear range or when the driver changes the transmission mode. If one of these preconditions is met, a switch is made back to the initial shift program.
In vehicles as of AEJ 13/1
As of modification year 13/1, a gearshift recommendation is also shown in the instrument cluster. This is part of the short-term M mode that is also integrated.
Referenced documents
- gf2710p9992ahn Overview of system components of automatic transmission (AT)