Automatic Transmission Control, Function - GF27.60-P-2002ANI

Transmission 725.0 in model 253 

Function requirements, general 

• Terminal 15 ON

Transmission control, general 

The function of the fully integrated transmission control unit (Y3/8n4) consists of the evaluation of various input signals (relevant for the transmission function). Depending on these, the electromechanical hydraulic valves (solenoids) are actuated which in turn determine the transmission ratio of the automatic transmission (gear selection), the gear ranges ("P", "N" "R", and "D") and the working pressure. The solenoids are positioned on the fully integrated transmission control controller unit (Y3/8) and they ensure the hydraulic usability is given. The fully integrated transmission control unit and the CDI control unit (N3/9) (diesel engine) or ME-SFI control unit (N3/10) (gasoline engine) read in the status of circuit 15 to check the function requirements. The status of circuit 15 is sent by the electronic ignition lock control unit (N73) over the chassis FlexRay (Flex E), powertrain control unit (N127) and the drive train CAN (CAN C1).

Gear ranges "P", "R", "N", "D" are selected using the DIRECT SELECT lever (S16/13) located on the right of the steering column tube modules control unit (N80). The steering column tube module control unit reads in the DIRECT SELECT lever signals directly and sends them over the chassis FlexRay, powertrain control unit and 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 fully integrated transmission control unit actuates the following assembly parts directly:

• Electric auxiliary oil pump (M42)
• 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 fully integrated transmission control unit and the CDI control unit or the ME-SFI control unit exchange, among other things, the following data via the drive train CAN:

• Status of engine management
• Electric auxiliary oil pump ON request
• Electric auxiliary oil pump status
• Status of transmission control
• Instrument cluster (A1) message, request

Messages and displays in instrument cluster 

Messages and requests are sent by the fully integrated transmission control unit via the drive train CAN, powertrain control unit, chassis FlexRay, electronic ignition lock control unit and user interface CAN (CAN HMI) to the instrument cluster.

Transmission control 

The function sequence is grouped into the following steps:

• Basic shift program 
• Electronic and hydraulic transmission control 
• Kickdown 
• Limp-home mode 

Basic shift program 

The basic shift program includes upshift and downshift characteristics for all 9 gears, designed for:

• Vehicle unladen
• Warm operating condition
• Smooth road
• Sea level

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

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

Transmission 725.0 in model 253

with code 550 (Trailer coupling)

The trailer recognition control unit (N28/1) sends the status of the trailer recognition over the interior CAN (CAN B), electronic ignition lock control unit, suspension FlexRay, powertrain control unit and the drive CAN to the fully integrated transmission control unit.

The fully integrated transmission control unit monitors and shifts depending on:

• Vehicle speed
• Selector lever position
• Accelerator pedal position

The Electronic Stability Program control unit (N30/4) receives the sensor signal from the left front axle rpm sensor (L6/1), right front axle rpm sensor (L6/2), left rear axle rpm sensor (L6/3), right rear axle rpm sensor (L6/4) and forwards the vehicle speed via the chassis FlexRay, powertrain control unit and drive train CAN to the fully integrated transmission control unit and the CDI control unit or ME-SFI control unit.

The powertrain control unit reads in the signals from the accelerator pedal sensor (B37) directly and sends them via the drive train CAN to the fully integrated transmission control unit.

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 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 status 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, suspension FlexRay, powertrain control unit and the drive CAN to the fully integrated transmission control unit.

Electronic and hydraulic transmission control 

While the electronic transmission control is responsible for gear selection and adaptation of pressures to the torque to be transmitted, power supply control for the automatic transmission is performed by the hydraulic elements of the fully integrated transmission control controller unit. The oil supply for the hydraulic elements (hydrodynamic torque converter (transmission 725.004/008/011/041/044/048/054/058) or the wet clutch (transmission 725.096), shift elements, hydraulic transmission control) is provided through an oil pump (primary pump) or the electric auxiliary oil pump connected to the drive shaft. 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 transmission control system prevents impermissible high engine speeds with a downshift lockout function. Furthermore, the transmission control system offers the advantage of a flexible adaptation to various vehicle and engine 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 (diesel engine) or the ME-SFI control unit (gasoline engine) sends the request "Electric auxiliary oil pump ON" via the drive train CAN to the fully integrated transmission control unit. The fully integrated transmission control unit actuates the electric auxiliary oil pump directly and sends the status via the drive train CAN to the CDI control unit or the ME-SFI control unit.

Kickdown 

The driver's wish for maximum acceleration is met by the kickdown function. The kickdown function is activated as from a pedal travel of more than 90 %. The "kick-down" signal from the accelerator pedal sensor is read in by the powertrain control unit. The powertrain control unit sends the signals from the accelerator pedal sensor via the powertrain CAN to the fully integrated transmission control unit. Based on this information the fully integrated transmission control unit actuates the solenoid valves so that upshifts and downshifts take place at an increased engine speed in order to achieve maximum performance.

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 gear range "P", "R", "N" or "D" using the DIRECT SELECT selector lever. The driver can choose between forward and reverse travel, engage the park pawl or interrupt the power flow in neutral between the torque converter (transmission 725.004/008/011/041/044/048/054/058) or the wet clutch (transmission 725.096) and the automatic transmission.