70
Design and functional principle
649_145
Brake servo control unit
J539
Push rod
(brake pedal connection)
Tandem master brake cylinder Lower pinion shaft
Upper pinion shaft
Brake pedal position sender
G100
Booster sleeveEngine-gearbox unitPiston rod
The brake pressure applied by the driver is boosted by the engine-
gearbox unit. An AC motor drives 2 pinion shafts through a reduc-
tion ratio. The splines of the pinion shafts are in mesh with the
spline on the booster sleeve. The rotary motion of the pinion shafts
is translated into a longitudinal motion of the booster sleeve.
To increase the brake pressure, the booster sleeve is moved
towards the tandem master brake cylinder (to the left in the
diagram). The electric motor is activated by the brake servo control
unit J539. The control unit receives information on the position of
the brake pedal and the push rod (= driver input) from the inte-
grated brake pedal position sender G100.
The position of the electric motor rotor and hence, indirectly, the
position of the booster sleeve is monitored by a rotor position
sensor (Hall sender) in the electric motor.
The sliding assembly of the booster sleeve on the push rod, and
the resultant separation of both components, ensures that the
driver can apply brake pressure even if the boost function fails.
Control unit J539 provides for terminal 15 run-on. If the vehicle is
stationary and the driver is not applying the service brake, the
run-on time is approximately 60 seconds. If the vehicle is actively
braked by the driver after terminal 15 is switched o, brake boost
is maintained for up to approx. 360 seconds.
After approx. 180 seconds and after 360 seconds, the driver is
instructed to secure the vehicle against rolling and informed that
brake boost is about to be deactivated.
The signals from the brake pedal position sender G100 of the
electro-mechanical brake servo are used for activating the brake
light.
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