Transmission Control, Function - GF27.60-P-2002ANR

Transmission 725.0 (except 725.096) in model 257, 290 

up to model year 2021 

Function requirements for transmission control, general 

• Function test conducted on control unit for fully integrated transmission control electric controller unit (Y3/8n4)

Transmission control, general 

The function of the control unit for the fully integrated transmission control electric controller unit consists of evaluating various input signals (relevant for transmission function). Depending on this, the electromechanical hydraulic valves (solenoids) are actuated, which in turn determine the transmission ratio of the automatic transmission (gear selection) and the working pressure.

The solenoids are positioned on the fully integrated transmission control electric controller unit (Y3/8). This then realizes the hydraulic functionality.

The drive authorization system enable is sent by the electronic ignition lock control unit (N73) over the suspension FlexRay (Flex E), powertrain control unit (N127) and the drive CAN (CAN C1) to the control unit for the fully integrated transmission control electric controller unit.

The control unit for the fully integrated transmission control electric controller unit actuates the following component parts electronically:

• Electric auxiliary oil pump (M42)
• Park pawl lift solenoid (Y3/8l1)
• 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)

The following signals are processed by the control unit for the electric controller unit of the fully integrated transmission control:

• Pressure sensor (Y3/8b5)
• Turbine wheel RPM sensor (Y3/8n1)
• Input speed RPM sensor (Y3/8n2)
• Output shaft RPM sensor (Y3/8n3)
• Transmission oil temperature sensor (Y3/8s2)
• Park pawl position sensor (Y3/8s4)
• Electronic ignition switch control unit

Transmission control function sequence 

The function sequence is described in 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 9 gears, designed for:

• Empty vehicle
• Operating temperature
• Smooth road
• Sea level

Slow accelerator release:

• Undesired downshifts in the upper vehicle speed range are prevented.

Rapid accelerator release:

• Upshifting to next gear is prevented and only permitted again when the vehicle accelerates more slowly.

The basic shifting program can be influenced by the driver through the DYNAMIC SELECT switch (N72/4s1), which is located in the area of the center console in the left lower control panel (N72/4).

Except model 257.361 

The left lower control panel sends the signal over the LCP-LIN (LIN A3), Audio/COMAND control panel (A40/9), telematics CAN (CAN A), the head unit (A26/17), user interface CAN (CAN HMI), electronic ignition lock control unit and the suspension FlexRay to the Electronic Stability Program control unit (N30/4). The Electronic Stability Program stores the information for the chosen transmission mode on the chassis FlexRay. The powertrain control unit receives this information and sends it over the drive CAN to the control unit for the electric controller unit of the fully integrated transmission control.

Model 257.361 

The left lower control panel sends the signal via the LCP-LIN, Audio/COMAND control panel, telematics CAN, head unit, user interface CAN, electronic ignition lock control unit and suspension FlexRay to the Drive Unit control unit (N145/1). The Drive Unit control unit puts the information for the selected drive program on the suspension FlexRay.

The powertrain control unit receives this information and sends it over the drive CAN to the control unit for the electric controller unit of the fully integrated transmission control.

The manual shift mode button (N72/4s12) can be used to select the "manual shift mode".

For vehicles with code 550 (Trailer hitch) the trailer recognition control unit (N28/1) sends the status of the trailer recognition to the control unit for the fully integrated transmission control electric controller unit over the interior CAN (CAN B), electronic ignition lock control unit, suspension FlexRay, powertrain control unit and the drive CAN.

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

• Load condition of vehicle
• Change in driving resistance
• Accelerator pedal movement

The control unit for the fully integrated transmission control electric controller unit monitors and switches to match:

• Vehicle speed
• Drive range selected at DIRECT SELECT selector lever (S16/13)
• Accelerator pedal position
• Transmission mode selection
• Selected gear, for short-term M mode
• Signals on the drive train CAN

The vehicle speed is calculated using the wheel speed signals.

The calculated vehicle speed is sent by the instrument cluster (A1) over the user interface CAN, electronic ignition lock control unit, suspension FlexRay, powertrain control unit and the drive CAN to the control unit for the fully integrated transmission control electric controller unit.

Messages and displays in instrument cluster 

Information about the selected gear, drive range or the drive program are sent by the control unit for the electric controller unit of the fully integrated transmission control to the drive CAN. The powertrain control unit receives and forwards it via the suspension FlexRay with the electronic ignition lock interface via the user interface CAN to the instrument cluster. This then actuates the relevant display via the following signals:

Instrument cluster 

• Gear indicator, actuation
• Transmission mode display, actuation

Function sequence for electronic and hydraulic transmission control 

While the electronic transmission control is responsible for gear selection and adaptation of pressures to the torque to be transferred, power supply control for the automatic transmission is performed by the hydraulic elements of the fully integrated transmission control electric controller unit. The oil supply for the hydraulic elements (hydrodynamic torque converter, shift elements, hydraulic transmission control) is provided through an oil pump (primary pump) connected to the drive shaft and/or over the electrical auxiliary oil pump. The transmission control system makes possible a precise adaptation of the pressure 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 any impermissible high engine speeds with a downshift lockout device.

Furthermore, the transmission control offers the advantage of flexible adaptation to various vehicle versions.

The electrical auxiliary oil pump ensures the oil supply of the hydraulic system when the combustion engine is switched off. It also supports the primary pump in the combustion engine's lower speed range. The CDI control unit (N3/9) (with diesel engines) or the ME-SFI [ME] control unit (N3/10) (with gasoline engine) sends the "electric auxiliary oil pump ON" request to the fully integrated transmission control electric controller unit. The electric auxiliary oil pump is actuated directly through the control unit for the fully integrated transmission control electric controller unit. The fully integrated transmission control electric controller unit sends the status of the electric auxiliary oil pump back to the CDI control unit or the ME-SFI [ME] control unit.

Function sequence for kickdown 

The driver's wish for maximum acceleration is met by the kickdown function.

The kickdown function is activated from a pedal travel of > 90 % at the accelerator pedal.

The "kickdown" signal is sent by the CDI control unit or the ME-SFI [ME] control unit, which then sends the status of the engine management over the drive CAN to the control unit for the fully integrated transmission control electric controller unit.

Based on this information the control unit for the fully integrated transmission control electric controller unit actuates the solenoid valves so that upshifts and downshifts take place at an increased engine speed in order to achieve maximum performance.

The forced downshift into the next lower gear is performed when transmission temperatures are too high and vehicle speed is too low.

Function sequence for limp-home mode 

In case of a defect in one of the solenoids, the corresponding gear is blocked. This gear can no longer be selected. The driver can engage the desired drive range "P", "R", "N" or "D" using the DIRECT SELECT lever. The driver can choose been forward and reverse travel, engage the park pawl or, when in neutral, interrupt the power flow between torque converter and automatic transmission.

The fault codes are stored in the fault memory. These can be read out using XENTRY Diagnostics. The diagnosis status of the transmission control is received by the diagnostic connector (X11/4) from the control unit for the fully integrated transmission control electric controller unit over the drive train CAN, powertrain control unit, chassis FlexRay, electronic ignition lock control unit and the diagnostic CAN (CAN D).

Short-term M program 

The short-term M mode is a temporary mode that is activated when the steering wheel gearshift buttons 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 undershooting a minimum speed (e.g. stopping), upon recognition of a lateral acceleration or if the vehicle is in deceleration mode. If none of the stated criteria are active anymore, the short-term M mode remains active for a specified period of time. After this remaining time has elapsed, a switch is made back to the initial shift program. In addition, the driver is shown a recommended shift operation in the instrument cluster. This is part of the gearshift recommendation that is also integrated. The driver can prematurely cancel the short-term M program through specific actions, e.g. by pressing and holding the steering wheel upshift button (S111/1) in the "positive" direction (+), by changing the gear range or by changing the transmission mode. If one of these preconditions is met, a switch is made back to the initial shift program.

Shifts, which could result in non-permissible high or low engine speeds, are not performed.

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