Acceleration/Deceleration Sensor: Description and Operation

PURPOSE
The BMW 325iX use all wheel drive with constantly switched central and rear axle locks and also a continuously effective Antilock Brake System (ABS). So that all three systems can be effective simultaneously, a sensor is required to provide information on the road condition (higher or lower traction). The acceleration pickup performs this duty.

CONSTRUCTION
In the acceleration pickup there are two mercury switches, one each for forward and reverse drive, which are installed in longitudinal direction of car at an angle to the horizontal.

The series connected mercury switches provide closed contact in horizontal position (resistance less than 1 k-ohm).

OPERATION
The glass tube is inclined in such a manner, that braking deceleration of 35 to 45° is necessary to open the contact.

If an open switch condition (resistance greater than 2.5 M-ohms) is reported to the ABS control unit over a period of time longer than 20 seconds, the ABS switches "Off" and reports this to the driver with a warning lamp in the instrument cluster.

The braking deceleration value for forward drive can be simulated by inclining the acceleration pick-up from 19.5 to 24° (with Special Tool 345 150 this is equal to 26.5 to 33.0 mm (1.043 to 1.299")).

The switching point for reverse drive does not have to be adjusted.