Automatic Hold: Notes
This function has been around since the E65, on which it was called "Auto-P". The Automatic Hold function was also used on the E70/E71.
Although the Automatic Hold function is computed on the DSC control unit, it can not be put into effect without an electromechanical parking brake (EMF). The EMF is always required whenever the DSC hydraulic modulator is unable to permanently hold the vehicle stationary. In particular, that is the case when the engine is switched off.
When the Automatic Hold function is active, the driver first of all brakes the vehicle to a standstill. It is then held stationary by the DSC hydraulic modulator. That is achieved by maintaining the final brake pressure applied by the driver. If the vehicle starts to roll on an incline, the DSC hydraulic modulator actively generates brake pressure.
Pressing the accelerator causes the brake pressure to be released and the vehicle starts to move again. Automatic holding and releasing of the brakes in that way makes driving in easier in conditions such as urban traffic and stopping at traffic lights or stop-and-go driving in traffic tailbacks.
After the engine is started, the function can be activated until the next time the engine is switched off. To do so, the driver's door must be closed and the driver's seatbelt fastened. The function can, of course, also be manually deactivated before the engine is switched.
The footwell module reads the signal from the door switch. The ACSM control unit analyses the signal from the seat belt buckle contact. The two signals are transmitted to the DSC control unit via the bus systems. One signal that is not analyzed for the Automatic Hold function on the F01/F02 is the driver's seat occupancy signal.
Conversely, the Automatic Hold function is automatically deactivated if the driver's door is opened and the driver's seatbelt unfastened. To prevent the vehicle rolling away in that situation, the EMF parking mode is activated. As long as the engine is running, the parking mode is effected by means of the DSC hydraulic modulator. If the driver switches the engine off, the function is taken over by the EMF actuator unit.
The Automatic Hold function is activated and deactivated by means of the button marked "AUTO H" on the center console. Activation of the function is acknowledged by the function indicator lamp (green LED in the button). Whenever the green LED is lit, the Automatic Hold function is active. The status of the Automatic Hold function is also indicated on the instrument cluster.
The various function statuses and how they are indicated are summarized below.
| Status of Automatic Hold function | Function indicator lamps in buttons | Display on instrument cluster |
|---|---|---|
| Switched off | ||
| Switched on and on standby (e. g. when vehicle is moving) | ||
| Switched on and active (vehicle is being held stationary) | ||
| Deactivated by driver getting out of vehicle or switching engine off |
When Automatic Hold is holding the vehicle stationary, two additional internal DSC subfunctions are activated: roll-away monitoring and slide detection.
The roll-away monitoring function is described in the section "EMF functions".
The slide detection function is designed to intervene if the vehicle starts to slide after stopping, i. e. if all four stationary wheels start to slip. That can happen on a steep slope when the road is slippery, for instance. If the driver were holding the vehicle stationary and became aware of such a situation, he/she would release the brake. In that way the vehicle can at least be steered as it rolls down the slope.
The slide detection function is based on exactly the same principle. When the vehicle is being held stationary by Automatic Hold, the slide detection function monitors the signals from the wheel-speed sensors. The DSC releases the pressure on one of the brakes in alternation while keeping the others under pressure. If the wheel-speed sensor registers a movement from the wheel on which the brake is released, then obviously the entire vehicle must be moving. That means that the other wheels, on which the brakes are applied, must be sliding while locked. Under those circumstances, the condition "sliding" would be detected.
The response to detection of sliding is progressive release of brake pressure so that the vehicle becomes steerable. The driver is made aware of the critical situation by a Check Control message and an audible warning signal.