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Accelerator pedal position
The current ratio is selected by the transmission control unit either in accordance with the requirements for low fuel consumption or in order to meet the required driving performance.
The gearshift timing is selected in accordance with a gearshift map which is programmed into the control unit as a function of the accelerator pedal position and the vehicle speed.
For kick-down (accelerator pedal pressed down at least 95 %) the next lowest gear is selected, as long as the engine speed allows it. All gearshift timings are set by the transmission control unit to the upper shift characteristic.
The driver can feel the kickdown point in the accelerator pedal as a slight increase in resistance to the operation of the pedal.
Hill climb mode
When the vehicle is in hill climb mode or towing a trailer, the transmission control delays gearshifts to prevent frequent shifting back and forth between the gears.
The transmission control unit determines hill climb mode or trailer operation from the change in throttle valve position supplied by the PCM in relation to the acceleration of the vehicle.
Depending on the driving resistance, the transmission control unit chooses between two gearshift maps which have been specially programmed for this purpose and in which the shift timings are again chosen as a function of the accelerator pedal position and the vehicle speed..
Hill descent mode
The hill descent mode is used to take better advantage of engine braking during hill descents.
A 'hill descent' situation is recognised by the transmission control unit if the vehicle is accelerating without operation of the accelerator pedal.
If in addition the brake is also depressed then the transmission control unit automatically shifts back from 4th into 3rd gear.
Cold start mode
In order to reach operating temperature as quickly as possible under low ambient temperatures, gearshifts into 4th gear and engagement of the torque converter clutch (TCC) are suppressed if one of the following conditions is met:
- Transmission fluid temperature below +20 °C
- Coolant temperature below - 40°C
Overheating protection mode
The overheating protection mode serves to protect the transmission against overheating and the serious damage that this can cause.
If the transmission fluid temperature reaches a temperature of around 135 °C, the transmission control employs a shift pattern designed to prevent a further increase in the transmission fluid temperature.
When the transmission fluid temperature drops back below approx. 125 °C, the transmission control exits the overheating protection mode.
The MIL warning lamp is actuated by the transmission control unit if the transmission fluid temperature reaches approximately 140 °C.
The MIL warning lamp goes out again when the transmission fluid temperature drops back to below around 130 °C.
Main line pressure control
In order to ensure the highest possible efficiency of the automatic transmission and to limit the power losses from the fluid pump, the main line pressure is adapted accordingly by the main regulating valve as a function of the accelerator pedal position (driver torque demand) and the selector lever position.
Engagement of the TCC
The TCC is engaged in 3rd and 4th gear depending on the current driving situation.
Engagement is controlled in accordance with the shift map stored in the control unit.
Torque reduction during gearshifts
In order to improve the quality of gearshifts and to avoid gearshift judder, the engine torque is reduced by the PCM in response to a request from the transmission control unit during gearshifts.
The engine torque is also reduced during engagement and disengagement of the TCC.
Pressure control during gearshifts
In order to ensure that the engagement of the clutches and brakes is as judder-free as possible, the main line pressure is reduced during gearshifts.
Shift timing adaptation during upshifts
In order to ensure that the engagement of the clutches and brakes is as judder-free as possible, the timing of the gearshift processes is monitored during upshifts.
In the event of any discrepancy from the target values, the main line pressure is adapted accordingly during the next gearshift.
The shift timing adaptation during upshifts is only active at transmission fluid temperatures between 50 °C and 120 °C.
Shift timing adaptation during shifts from 3rd to 4th gear
The shift from 3rd gear into 4th gear is the only gearshift during which a clutch is disengaged and a brake is engaged simultaneously.
In order to control the synchronized switching of the two components as exactly as possible, the shift timing solenoid valve is actuated accordingly.
In order to ensure that this particular gearshift is performed as judder-free as possible throughout the service life of the transmission, the gearshift is monitored by the two rotational speed sensors and the actuation of the shift timing solenoid valve is adapted accordingly.
The shift timing adaptation for gearshifts from 3rd gear into 4th gear is only active at transmission fluid temperatures between 30 °C and 120 °C.
Shift timing adaptation during shifts from 4th to 3rd gear
The shift from 4th gear into 3rd gear is the only gearshift during which a brake is disengaged and a clutch is engaged simultaneously.
In order to control the engagement of the clutch as precisely as possible, the main regulating valve is actuated accordingly in order to build up the actuating pressure at exactly the right time.
In order to ensure that this particular gearshift is performed as judder-free as possible throughout the service life of the transmission, the gearshift is monitored by the two rotational speed sensors and the actuation of the main regulating valve is adapted accordingly.
The shift timing adaptation for gearshifts from 4th gear into 3rd gear is only active at transmission fluid temperatures between 20 °C and 120 °C.
Reverse gear safety strategy
The transmission control unit prevents shifts into reverse gear while the vehicle is driving forwards, as otherwise serious transmission damage can be caused.
The reverse gear safety strategy is active when reverse gear is engaged with the selector lever at vehicle speeds in excess of 11 km/h.
In this case the transmission control unit actuates the shift timing solenoid valve, as a result of which the actuating pressure does not reach the reverse gear clutch or the reverse gear brake.
The reverse gear safety strategy is deactivated when the vehicle speed is below 9 km/h. The actuating pressure can then reach the reverse gear clutch and the reverse gear brake and the gearshift into reverse gear is performed.
Avoidance of gearshift judder during engagement of a transmission range
In order to prevent gearshift judder when moving the selector lever from N to D, the transmission control unit initially shifts into 2nd gear instead of 1st gear, and then immediately shifts back into 1st gear before the gearshift process is finished.
This reduces the amount of gearshift judder during engagement of a forward drive range.
Torque reduction when pulling away
In order to protect the clutches against excessively high torque when pulling away, the PCM reduces the engine torque in response to a request from the transmission control.
If the accelerator pedal is depressed while the vehicle is pulling away, then the transmission control unit sends a corresponding signal requesting a torque reduction via the CAN databus.
Selector lever positions
Selector lever position "P"
No gear is engaged in selector lever position "P".
The parking pawl is engaged manually via the selector lever cable and the selector shaft.
Selector lever position "R"
The reverse gear is engaged in selector lever position "R".
Selector lever position "N"
No gear is engaged in selector lever position "N".
The powertrain is not blocked.
Selector lever position "D"
In selector lever position "D" the transmission control will allow all gears to be engaged provided the overdrive switch is not operated.
If the overdrive switch is pressed then the transmission control prevents gearshifts into 4th gear or shifts back into 3rd gear.
Selector lever position "2"
Only the second gear is engaged when the selector lever is in position "2". Gearshifts into 1st gear are not permitted.
If the selector lever is moved to position "2" at higher speeds than are permitted for second gear, then the transmission control delays the downshift until the vehicle speed has slowed down enough.
In selector lever position "2" engine braking is effective when the vehicle is coasting.
Selector lever position "1"
Only the first gear is engaged when the selector lever is in position "1".
If the selector lever is moved to position "1" at higher speeds than are permitted for first gear, then the transmission control delays the downshift until the vehicle speed has slowed down enough.
In selector lever position "1" engine braking is effective when the vehicle is coasting.
Signal transmission on the CAN databus
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