Transmission control, function - GF27.60-P-2002AHX

TRANSMISSION 722.930 up to 5/31/11, 722.931 as of 6/1/11 in MODEL 212.0/2 

Function requirements for transmission control, general 

• Status of "Circuit 87 ON"
• Function test performed on fully integrated transmission control controller unit (Y3/8)

Transmission control, general 

The transmission control actuates the following actuators electronically:

• Working pressure control solenoid valve (Y3/8y1)
• Clutch control solenoid valve K1 (Y3/8y2)
• Clutch control solenoid valve K2 (Y3/8y3)
• Clutch control solenoid valve K3 (Y3/8y4)
• Multidisk brake control solenoid valve B1 (Y3/8y5)
• Multidisk brake control solenoid valve B2 (Y3/8y6)
• Multidisk brake control solenoid valve B3 (Y3/8y7)
• Wet clutch control solenoid valve (Y3/8y9)

The solenoid valves are positioned on the fully integrated transmission control controller unit. This then realizes the hydraulic functionality.

The function of the fully integrated transmission control controller unit is to evaluate various input signals that are relevant to transmission operation. Based on this, 8 electromechanical hydraulic valves are actuated which determine the gear ratio of the transmission (gear selection), the working pressure and the wet clutch.

Transmission control function sequence 

The function sequence is subdivided into the following steps:

• Function sequence for basic shift program 
• Function sequence for electronic and hydraulic transmission control 
• 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
• Warm operating condition
• Smooth road
• Sea level

The basic shift program can be influenced by the driver using the transmission mode rotary switch (S192).

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 (Y3/8n4) monitors and shifts depending on:

• Vehicle speed
• Drive position
• Accelerator pedal position
• Transmission mode selection
• Engaged gear, with transmission mode Manual "M"

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

Function sequence for electronic and hydraulic transmission control 

While the electronic transmission control is responsible for gear selection and for matching pressures to the torque to be transmitted, the power supply control of the transmission occurs via hydraulic elements of the fully integrated transmission control controller unit. The oil supply for the hydraulic elements (wet clutch, actuators, hydraulic transmission control) is provided through an oil pump connected to the wet clutch and/or over the electrical transmission oil pump (M42) (with code (B03) ECO start/stop function).

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 shift range can be changed while driving in the forward gears. The transmission control system prevents impermissibly high engine speeds with a downshift lockout device. Furthermore, the transmission control offers the advantage of flexible adaptation to various vehicle versions.

With code (B03) ECO start/stop function

The electric transmission oil pump ensures that oil supply continues to the hydraulic system when 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).

The gear limitation is replaced by the function "Short-term M mode". Short-term mode M is a temporary program that is activated when the steering wheel gearshift buttons or the DIRECT SELECT INTERFACE (N150) are operated. Pressing in the positive (+) direction triggers an upshift and pressing in the negative (-) direction triggers a downshift, insofar as this is 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 a minimum speed being dropped below (e.g. stopping), where lateral acceleration is detected by the yaw rate sensor for lateral and longitudinal acceleration (B24/15) 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 prematurely cancel the short-term mode M through specific actions, e.g. if he/she operates and holds the upshift steering wheel gearshift button (S111/1) or the DIRECT SELECT INTERFACE in the "positive" (+) direction, changes the gear range or changes the transmission mode. If one of these preconditions is met, a switch is made back to the initial shift program.

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 to the fully integrated transmission-control control unit by the ME-SFI [ME] control unit (N3/10) over the drive CAN (CAN C). 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.

The forced downshift to the next lower gear occurs when the transmission oil temperature is too high and the vehicle speed is too low.

Function sequence for limp-home mode 

If the signals are implausible, the fully integrated transmission control (VGS) is only partially operational and switches to the programmed emergency performance map. The driver can engage the desired gear range "P", "R", "N", "D" via the DIRECT SELECT INTERFACE (N150). The driver can choose been forward and reverse travel, engage the park pawl or, when in neutral, interrupt the power flow between the wet clutch and the transmission.