Transmission control, function

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 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 torque converter lockup.

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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

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Basic shift program function sequence

The basic shift program includes upshift and downshift characteristics for all 7 gears, designed for:

- Empty vehicle

- Operating temperature

- Level road

- 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

- Accelerator pedal movement

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The fully integrated transmission control (VGS) control unit (Y3/8n4) 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 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

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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 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 (primary pump) connected to the torque converter or by means of the electric transmission oil pump (M42) (model 216 as of model year. 2011 /YoM 10 model refinement package / model 221 as of model year 2011 /YoM 10, with code (B03) ECO start/stop function). 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 lockout device. Furthermore, the transmission control system offers the advantage of a flexible adaptation to various vehicle and engine versions.

 On model 216 as of model year 2011 /YoM 10 model refinement package / model 221 as of model year 2011 /YoM 10, with code (B03) ECO start/stop function.

The electric transmission oil pump ensures oil supply to the hydraulic system when the engine is switched off. It also supports the primary pump during at low engine speeds . This is actuated through the gear oil auxiliary pump control unit (N89).

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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 "Kickdown" signal is transferred to the fully integrated transmission control unit (VGS) by the CDI control unit (N3/9) or ME-SFI [ME] control unit (N3/10) via 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 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 interrupt the power flow between torque converter and transmission in the neutral position.