GF27.60-P-2002AHT Transmission Control, Function
GF27.60-P-2002AHT Transmission Control, Function
- TRANSMISSION 722.931 in MODEL 221.074 /174, 216.374
Function requirements for transmission control, general
^ Circuit 87 ON
^ Function check performed on fully integrated transmission control unit (VGS) (Y3/8n4)
Transmission control, general
The transmission control system electronically actuates the actuators (control solenoid valves). The control solenoid valves are located in the hydraulic valve body assembly. This implements the hydraulic functions.
The function of the electric controller unit (VGS) (Y3/8) is primarily to evaluate various input signals that are relevant to transmission operation. Based on this, eight electromechanical hydraulic valves are actuated which determine the gear ratio of the transmission (gear selection), the working pressure and the starting clutch.
Transmission control function sequence
The function sequence is subdivided into the following steps:
^ Basic shift program function sequence
^ Electronic and hydraulic transmission control function sequence
^ Kick-down function sequence
^ Limp-home mode function sequence
Basic shift program function sequence
The basic shift program includes upshift and downshift characteristics for all 7 gears, designed for:
- Empty vehicle
- Operating temperature
- Sea level
The basic shift program can be influenced by the driver through the transmission mode button (A40/9s8). The shift programs are adapted by offsetting the characteristics, depending on:
- Load condition of vehicle
- Change in driving resistance
- Pedal movement
The fully integrated transmission control unit (VGS) monitors and shifts depending on:
- Vehicle speed
- Gear selector switch position
- Accelerator pedal position
- Transmission mode selection
- Selected gear, for Manual "M" transmission mode
- 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 up is prevented and only permitted again when the vehicle accelerates more slowly
The transmission mode button is located on the front central operating unit (A40/9) and it sends the request to change the transmission mode over the telematics CAN (CAN A) to the COMAND controller unit (A40/3) and then forwards it over the central CAN (CAN F), chassis CAN (CAN E) and the drive train CAN to the fully integrated transmission control unit (VGS).
Electronic and hydraulic transmission control function sequence
Whilst the electronic transmission control takes over the gear selection and adaptation of pressures to the torque to be transmitted the power supply of the transmission is controlled via the hydraulic elements of the electric controller unit (VGS). The oil supply for the hydraulic elements such as starting clutch, actuators and hydraulic transmission control, is provided through an oil pump (primary pump) connected to the starting clutch and/or through the electric transmission oil pump (M42).
The transmission control system makes possible a precise adaptation of pressures to the respective operating conditions 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.
A lubricating oil volume flows through the housing of the startup clutch.
The lubrication oil volume flow and a special geometry of the clutch linings ensure optimum cooling of the startup clutch.
The electric transmission oil pump provides the hydraulic system's oil supply when the internal combustion engine is switched off. It also supports the primary pump in the internal combustion engine's lower speed range. This is actuated through the gear oil auxiliary pump control unit (N89).
Kick-down function sequence
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 transferred to the fully integrated transmission-control control unit (VGS) by the ME-SFI [ME] control unit (N3/10) over the drive train CAN.
Based on this information the fully integrated transmission control (VGS) control unit actuates the control solenoid valves so that the upshifts and downshifts take place at higher engine speeds in order to achieve maximum performance.
Limp-home mode function sequence
If the signals are implausible, the transmission control system is only partially operational and switches to the programmed emergency performance map.
The driver can engage the desired gear ranges "P, R, N, D" with the DIRECT SELECT gear selector switch (S16/13).
The driver can choose been forward and reverse travel, engage the park pawl or interrupt the power flow between the starting clutch and the transmission when in neutral.
