Transmission control, function

Function requirements for transmission control, general

 Circuit 87 ON

 Function test of ETC control unit (N15/3) completed

Transmission control, general

The transmission control actuates the actuators (solenoid valves and control solenoid valves) electronically. The valves are seated in the hydraulic valve body assembly. This implements the hydraulic functions.

The function of the electric controller unit (Y3/6) basically consists in the evaluation of various input signals relevant to the transmission function. Depending on this eight electromechanical hydraulic valves are actuated which for their part determine the gear ratio of the transmission (gear selection) and torque converter lockup.

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Transmission control function sequence

The function sequence is subdivided into the following steps:

 Basic shift program

 Electronic and hydraulic transmission control

 Kickdown

 Emergency running

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Basic shift program

The basic shift program includes upshift and downshift characteristics for all 5 gears, designed for:

- Empty vehicle

- Warm operating condition

- Level road

- Sea level

The basic shift program can be influenced by the driver via the transmission mode switch (S16/5).

The shift programs are adapted by offsetting the characteristics, depending on:

- Vehicle load

- Change in driving resistance

- Pedal movement

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The ETC control unit monitors and shifts depending on:

- Vehicle speed

- Selector lever position

- Accelerator pedal position

- Transmission mode selection

- Selected gear in transmission mode Manual "M" (except code 494 USA version)

- Signals on engine compartment CAN (CAN-C)

Slow accelerator release:

- Undesired downshifts in the upper vehicle speed range are prevented

Rapid accelerator release:

- Upshift into the next gear is prevented and only permitted again during lower vehicle acceleration

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Electronic and hydraulic transmission control

Whilst the electronic transmission control takes over the gear selection and adaptation of pressures to the torque to be transmitted, the power supply control of the transmission takes place via the hydraulic elements of the electric controller unit. The oil supply to the hydraulic elements, such as the hydrodynamic torque converter, the shift elements and the hydraulic transmission control, is provided by way of an oil pump connected to the torque converter.

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.

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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 lock-out device. Furthermore, the transmission control system offers the advantage of a flexible adaptation to various vehicle and engine versions.

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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 lead from the motor electronics control unit (N3/10) via the engine compartment CAN to the ETC control unit. Based on this information the ETC [EGS] control unit actuates the control solenoid valve so that the upshifts and downshifts take place at increased engine speed in order to achieve maximum performance.

Emergency running

If the signals are implausible, the transmission control system is only partially operational and switches to the programmed emergency performance map. The diver can engage the desired gear ranges "P, R, N, D" using the selector lever.

The driver can choose been forward and reverse travel, engage the park pawl or interrupt the power flow between torque converter and transmission in the neutral position.