190 Driving and Driving Safety
Function
PSM intervenes and corrects the course if the
actual direction of motion deviates from the
desired course (steering-wheel position):
It brakes individual wheels as needed.
If necessary, PSM also influences the engine
power or the gear-changing characteristic of
Tiptronic in order to stabilize the vehicle.
The events below inform the driver of PSM control
operations and warn him to adapt his driving style
to the road conditions:
– The PSM warning light on the instrument panel
flashes.
– Hydraulic noises can be heard.
– The vehicle decelerates and steering-wheel
forces are altered as PSM controls the brakes.
– Reduced engine power.
– The brake pedal pulsates and its position is
changed during braking.
In order to achieve full vehicle deceleration,
foot pressure must be increased after the
brake pedal has begun vibrating.
Examples of PSM control operations
– If the “front wheels of the vehicle drift” on
a corner, the engine power is reduced and the
rear wheel on the inside of the corner is braked
if necessary.
– If the rear of the vehicle swings out on
a corner, the front wheel on the outside of the
corner is braked.
– Brake system prefilling:
The brake system is prepared for possible
subsequent emergency braking if the
accelerator pedal is released suddenly and
quickly. The brake system is prefilled and the
brake pads are already applied gently to the
brake disks.
– Brake booster:
In the event of an emergency braking operation
where the pedal force is insufficient, a brake
booster provides the braking pressure
necessary for maximum deceleration at all
4 wheels.
Information
When PSM is switched off, wheel-specific brake
interventions and the anti-slip regulation system
(ASR) are also inactive.
The automatic brake differential (ABD) remains on.
Off-road PSM
(PSM in the off-road driving program)
When the off-road driving program is active, an off-
road PSM specially matched to off-road driving is
activated to improve traction.
In order to improve driveability on difficult terrain,
the response of the off-road PSM is less sensitive
in the low speed range when the vehicle is set to
the off-road driving program.
If the brakes are highly stressed, the automatic
brake differential (ABD) switches itself off to
protect the brakes. It remains deactivated until the
brake system has cooled sufficiently again.
Automatic brake differential (ABD)
The ABD system controls the front and rear axles
separately. If one wheel of an axle starts to spin,
it is braked so that the other wheel on the same
axle can be driven.
ABD recognizes different driving states, and it
features control strategies adapted to these
states. In situations in which little propulsive power
is required, such as when the vehicle moves off on
a level gravel surface, traction control already
becomes active at low engine speeds. If a large
amount of propulsive power is required, such as
when moving off on an uphill slope or for rapid
acceleration, the ABD system is adapted
accordingly.
Anti-slip regulation (ASR)
The anti-slip regulation system prevents the
wheels from spinning by adjusting the engine
power, thereby ensuring good lane-holding ability
and stable handling.
Engine drag torque control (MSR)
In conditions of excessive slip, the engine drag
torque control system prevents all driven wheels
from locking up when the vehicle is overrunning.
This is also the case for downshifts on a slippery
road.
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