Gear Shift, Function - GF27.60-P-2000ANI

Transmission 725.0 in model 253 

Function requirements, general 

• Engine running
• Transmission not in limp-home mode
• Drive train operational

Shift operation, general 

Use of the fully integrated transmission control means that an additional mechanical or electronic gear range adjustment system is no longer necessary. The fully integrated transmission control initiates all shift operations and gear range changes by means of electrohydraulic actuation. Gear ranges "P", "R", "N", "D" are selected using the DIRECT SELECT lever (S16/13) located on the right of the steering column tube module. The shift positions of the DIRECT SELECT lever are read in by the steering column tube module control unit (N80) and sent via chassis FlexRay (Flex E), powertrain control unit (N127) and drive train CAN (CAN C1) to the fully integrated transmission control unit (Y3/8n4).

Messages and displays in instrument cluster (A1) 

Information on the selected gear, the engaged gear range and the transmission mode is sent by the fully integrated transmission control unit via the drive train CAN, powertrain control unit, chassis FlexRay, electronic ignition lock control unit (N73) and user interface CAN (CAN HMI) to the instrument cluster. This then actuates the relevant display. The CDI control unit (N3/9) or the ME-SFI control unit (N3/10) sends the status of the engine management, the signal for "operational drivetrain" and the signal for "engine running" via the drive train CAN to the fully integrated transmission control unit.

The fully integrated transmission control unit processes the signals for the following components:

• Pressure sensor (Y3/8b5)
• Turbine wheel rpm sensor (Y3/8n1)
• Rpm sensor for internal input speed (Y3/8n2)
• Output shaft rpm sensor (Y3/8n3)
• Transmission oil temperature sensor (Y3/8s2)
• Park pawl position sensor (Y3/8s4)

The following components are switched on by the fully integrated transmission control unit:

• Park pawl lift solenoid (Y3/8I1)
• Lubrication pressure solenoid (Y3/8y12)
• K81 clutch control solenoid (Y3/8y13)
• K38 clutch control solenoid (Y3/8y14)
• K27 clutch control solenoid (Y3/8y15)
• B08 multidisk brake control solenoid (Y3/8y16)
• B05 multidisk brake control solenoid (Y3/8y17)
• B06 multidisk brake control solenoid (Y3/8y18)
• Working pressure solenoid (Y3/8y19)
• Torque converter lockup clutch solenoid (Y3/8y20) (except transmission 725.096)
• Wet clutch solenoid (Y3/8y22) (transmission 725.096)

The following data is exchanged between the fully integrated transmission control unit and the CDI control unit or ME-SFI [ME] control unit via the drive train CAN:

• Engine speed
• Request for upshift delay
• Request for reduction in engine torque
• Status of transmission control

Shift 

The function sequence is described in the following steps:

• Data transfer during shift operations 
• Shift operation 1 to 2 

Data transfer during shift operations 

Compared to the Sport "S" transmission mode, the Economy "E" transmission mode has a shorter shift range (shifting at low rpm of the combustion engine or the electric machine) and therefore a lower drive torque is available when starting off forwards and starting off in reverse. Reverse gear, park position and neutral can only be shifted via the DIRECT SELECT lever.

The basic shifting program can be influenced by the driver via the DYNAMIC SELECT switch (N72/4s11), which is located in the area of the center console in the left lower control panel (N72/4). The left lower control panel sends the signal over the LCP-LIN (LIN A3), Audio/COMAND control panel (A40/9), telematics CAN (CAN A), head unit (A26/17), user interface CAN, electronic ignition lock control unit and the suspension FlexRay to the Electronic Stability Program control unit (N30/4). The Electronic Stability Program control unit sends the information for the selected transmission mode via the chassis FlexRay, the powertrain control unit and the drive train CAN to the fully integrated transmission control unit.

In drive programs RACE (model 253.389/989) Sport Plus "S+", Sport "S" and Economy "E", the driver can operate the steering wheel downshift button (S110/1) and steering wheel upshift button (S111/1) to select the following function:

• Activate gear limitation
• Shift to optimum gear
• Increase gearshift limitation
• Cancel gear limitation

Activate gear limitation 

Pressing the steering wheel downshift button once activates the gear limitation function (if the speed of the internal combustion engine permits, a downshift is made to the gear immediately below the currently selected one).

Shift to optimum gear 

The function "Shift into optimum gear" is selected by pressing and holding the downshift steering wheel gearshift button. The gear that allows optimum vehicle acceleration is selected.

Increase gearshift limitation 

Pressing the steering wheel upshift button once with gear limitation selected increases gearshift limitation by one gear (e.g. from 3 to 4).

Cancel gear limitation 

Pressing and holding down the upshift steering wheel gearshift button cancels the gear limitation function completely (gearshift to "D"). The steering wheel electronics (N135) reads in the signals of steering wheel gearshift button directly and sends them to the fully integrated transmission control unit via steering LIN (LIN E1), steering column tube module control unit, chassis FlexRay, powertrain control unit and drive train CAN. This control unit converts the request into hydraulic signals which are executed by the respective regulating valve.

Shift operation 1 to 2 

Multidisk brake B05, multidisk brake B06 and multidisk clutch K38 are engaged in 1st gear. When shifting from 1st gear to 2nd gear, multidisk brake B05 is disengaged and multidisk clutch K81 is engaged. The shift operation can be started by the fully integrated transmission control unit. Here, the multidisk brake B08 is filled and the pressure in multidisk brake B08 is increased continuously. Simultaneously the pressure in multidisk brake B06 is reduced. This involves a flowing transition. When the 2nd gear torque is reached, the speed of the internal combustion engine must be adapted to the 2nd gear level (elevation phase). This speed adjustment is achieved by increasing the pressure in the multidisk brake B08 and, if necessary, reducing the engine torque through the CDI control unit or ME-SFI [ME] control unit.

A forced downshift occurs at excessively low rpm and a forced upshift occurs at an excessively high rpm. When the vehicle is stationary the transmission automatically shifts into 1st gear. It is possible to start off both in 1st or 2nd gear.

Transmission 725.0 (except 725.096) in model 253

Short-term M program 

The short-term M mode is a temporary mode that is activated when the steering wheel gearshift buttons are operated. When one of the steering wheel gearshift buttons is operated, the transmission control switches up or down a gear, if it is allowed. If not, the current gear is maintained. After a specified time window, the short-term M mode is deactivated. The time window can be extended by undershooting a minimum speed (e.g. stopping), upon recognition of a lateral acceleration or if the vehicle is in overrun mode. After this time window has elapsed, a switch is made back to the initial shift program. The driver can prematurely end the short-term M program through specific actions, e.g. if he presses and holds the steering wheel upshift button, changes the drive range or changes the drive program. In this case, a switch is made to the initial shift program.

Transmission 725.096 in model 253

Permanent M-program setting 

The M-program is activated when the manual shifting mode button (N72/4s12) is operated. Upon pressing the steering wheel gearshift buttons, the transmission control upshifts or downshifts if this is allowed. If not, the current gear is maintained.

If the permanent setting is activated, the automatic transmission does not automatically perform an upshift even when the engine's maximum governed rpm is reached.