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 4wheels.
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.