The control unit checks the inputs and outputs during cruise control (tempomat) operation.The system is switched off if an implausible status occurs at the inputs or outputs.
Following situations can result in system shut-down:
The minimum speed threshold (vmin threshold) prevents controlled driving operation below a speed of 30 km/h.
If, before exceeding vmin , a signal was already applied to implement a cruise control (tempomat) function, the cruise control (tempomat) system is not switched on after exceeding this threshold.
All tempomat functions are clear again after cancelling operation in the permissible speed range.
If the speed drops below the vmin threshold from controlled driving operation due to extreme driving situations (e.g. gradient), rapid cutout is implemented.
A cruise control (tempomat) function is only carried out if the operating element is in rest position before initiating the function.
The function is implemented with defined significance in order to avoid hazardous situations by simultaneously calling up several cruise control (tempomat) functions.
Significance |
Function |
|---|---|
High |
Brake light switch |
|
Clutch switch |
|
Transmission switch |
|
Tempomat OFF |
|
Decelerate |
|
Accelerate |
Low |
Resume |
The actuator clutch is released if the controlled driving speed is exceeded by more than 8 km/h due to excess thrust (driving downhill) or pressing the accelerator pedal. This threshold can be effective under all tempomat conditions.When the clutch is released, the cruise control (tempomat) remains active in the background.The actuator clutch engages once again when the speed drops below the threshold value.
This override cutout facility cancels cruise control when the set speed is exceeded by more than 16 km/h over a period of more than 30 s.
The permissible controlled maximum speed is limited to a fixed value.This fixed value is defined in the coding data.
Fast cutout is activated if the controlled maximum speed is exceeded by depressing the accelerator pedal.
On reaching the controlled maximum speed by pressing the "accelerate" button, acceleration is ended and the speed is maintained constant.
The cruise control (tempomat) is switched off in the case of excessive vehicle deceleration.In this case, vehicle deceleration is determined with the aid of the speed signal.
For safety reasons, this cutout function is provided in addition to the cutout function via the brake pedal.
Fast cutout of the cruise control takes place after the drive stage selector lever is switched during controlled driving out of the drive stage "D" into "N" or "P".
Fast deactivation of cruise control takes place in the case of overrevving of the engine speed due to the lack of positive connection between the engine and transmission (e.g. by engaging idle during cruise control without operating the clutch).
The measure for this cutout procedure is a distinct change in engine speed without a corresponding change in driving speed.
The reset time of the actuator motor is monitored as soon as a deactivation function has taken place. If the maximum reset time is exceeded, the defect code is stored in the defect code memory and the cruise control (tempomat) cannot be reactivated before the ignition lock is switched off and switched on again.
Maximum reset time for fast cutout: |
0.5 s |
Maximum reset time for comfort cutout: |
2 s |
After applying the supply voltage, the voltage level at the control unit output to the actuator clutch is permanently monitored. The clutch voltage level is read back and compared with the set status in the control units.
If the two statuses do not agree, the cruise control (tempomat) cannot be switched on or, if switched on, it initiates fast deactivation in conjunction with a defect code being stored in the defect code memory.
The stored speed value is deleted when the ignition lock is switched to position 2.
The "toggle test" serves the purpose of monitoring the signal transfer between the operating elements in the multifunction steering wheel (MFL) and the tempomat control unit (GR2).For this purpose, the operating signal contains a regularly changing test bit which is constantly monitored.If no change takes place within 1 second, the fast deactivation function is triggered in conjunction with a defect code being stored in the defect code memory.
From the time the control unit supply voltage is switched on, the supply voltage of the actuator potentiometer is also monitored.In the case of deviations from the nominal value, cruise control is not permissible or fast deactivation is implemented with a defect code being stored in the defect code memory.
The control monitoring function is effective after switching on the cruise control (tempomat).In this function, the set position of the throttle is compared with the current position of the throttle by evaluating the actuator potentiometer.
This monitoring facility makes it possible to detect a defect in the actuator or in its actuation.
If the requirements of the test are not fulfilled, the cruise control is switched off and a defect code is stored in the defect code memory.
A memory test is carried out when the supply voltage is applied.
The correct program run is monitored during operation.
If a defect is found in one of these two tests, the cruise control (tempomat) can no longer be switched on and a defect code is stored in the defect code memory.